210 | Piston to Cylinder Wall Clearance
The clearance between the piston skirt and the cylinder wall is one of the most critical clearances in any engine. Get this wrong and you risk a piston seizing in the bore, or a piston that rocks in the bore which creates unnecessary noise and is detrimental to ring seal. In this webinar we’ll find out what factors influence piston to cylinder wall clearance and how to decide on the correct clearance for your application.
- Welcome along to today's webinar. Now we're going to be talking today about piston to cylinder wall clearance. We're going to find out what that term is, what it means and why it is one of the most critical measurements in your engine. If you want to build an engine that's going to be nice and quiet, produce good power and torque, minimal oil consumption and most importantly of course, provide a long service life. Now this is kind of one of those hot topics when it comes to performance engine building and like most things in the aftermarket performance industry, there's quite a lot of myth and misunderstanding around piston to cylinder wall clearance so hopefully today we're going to try and break down some of those barriers, get rid of some of those myths.
So you've got a really solid understanding of exactly what it is and what you need to know about it. As usual we will be having a set of questions and answers so if there's anything that I talk about today on this topic, please feel free to ask those in the chat, the team will transfer those through to me and we'll get stuck into those at the end. So I guess we'll start by talking about what it is and really the clue is in the name. It is simply the clearance between the skirt of the piston and the inside of our cylinder bore. So if we look at one of the sample pistons we've got here, we've got our piston skirt and in order for that piston to run properly in our bores, we need a specific amount of clearance between the outside of the skirt and the inside of our bore.
Although to be fair I don't think I've got much chance of fitting an LS piston into our Subaru FA20, that's a problem we'll deal with a little bit later. Now the idea behind getting our clearance right is that if our clearance is correct, it's going to make sure that the piston runs nice and stable in the bore. So as you could probably imagine, if the piston to cylinder wall clearance is excessive, we're going to get a situation where the piston can rock backwards and forwards in the bore. Now there's a couple of problems with this. First of all it's going to create noise.
Now that's not something we really want from our freshly built performance engine. Because the piston's going to be rocking at quite high speed as well, if it's gonna be doing that a lot, clearly this is going to end up causing more wear than if the piston is nice and stable in the bore. The other important aspect that is often overlooked here is that we need to make sure that our piston is nice and stable in the bore because in turn that stabilises the ring pack against the bore and it's the seal between the ring and the bore that's so critical to make sure that our combustion pressure stays on the top of the piston, reduces the blow by past the ring set, down into the crank case, and as an added up shot of all of that, if we can keep our ring pack nice and stable, we're also going to get an advantage because we're going to end up with less oil consumption. So that's what we're trying to achieve there, we're trying to get a specific amount of clearance to stabilise the piston. The tricky part here though is that when we're building our engines, we're obviously building them at room temperature.
We've got a lot of heat being produced in the combustion chamber that's then being fed down into the crown of the piston. Of course our pistons are made up from an aluminium alloy so that's going to expand as it heats up. So what we're trying to do here at room temperature is choose a piston to cylinder wall clearance that's essentially going to give us a nice tight minimal running clearance when that piston has expanded and is up to operating temperature. And this is where things start to get a little bit tricky. The first aspect is that different pistons will have different thermal expansion coefficients or in other words they're going to expand more or less as they heat up.
So of course this is a big factor that we need to take into consideration. We need to consider the type of piston that we are running in our engine. And really when it comes down to it there are three types of piston broadly that we're going to encounter. And I just want to go through those so we've got some understanding of the pistons and what the properties of that piston mean to us, where their advantages and disadvantages lie and of course how that affects their thermal expansion coefficient as well. So really in terms of the thermal expansion or in other words how much the piston expands as it gets up to operating temperature, it really comes down to the silicon content that is contained in the aluminium alloy of the piston.
So if we're dealing with a factory piston or a factory engine, almost inevitably that factory piston is going to be a cast piston. This is done because it is cost effective, it's very cheap for the manufacturer to produce a cast piston, and to be honest these days cast pistons actually are pretty damn good compared to what we dealt with years and years ago. And they can actually take a reasonably large level of abuse these days so just because you've got a cast piston, depending on your application, doesn't necessarily mean that it's going to be problematic. So most of these cast pistons we're using are what is referred to as a hypereutectic piston and this refers to the content of silicon contained in the aluminium alloy. So a hypereutectic piston is generally going to be somewhere in the 16% to 19% range in terms of its silicon content.
So the more silicon content we have in here, what we're going to end up with is a harder more brittle piston construction and we're also going to end up as our silicon content increases, with a piston that expands less for a given temperature. Now from the cast piston perspective we've got a cast piston, I'm not gonna get into the manufacturing process but the casting process essentially we're just pouring the molten alloy into a mould. We end up with a piston that has no real grain structure. So this is one of the downsides. Because of that, coupled with the high silicon content, it does tend to be quite brittle and this is really where the cast piston doesn't stand up too well to very high cylinder pressures and in particular it is very very intolerant of any level of detonation.
When we get detonation occurring we've got sharp spikes in pressure occurring inside the combustion chamber and the easiest way to liken this to is someone actually slamming down on the crown of the piston with a hammer. And with a cast piston with a high silicon content, we're very likely, with high levels of detonation to end up breaking sections out of our ring lands. And that's one of the most common failure modes we see with a cast piston. Particularly one that's been tuned with too much ignition timing, aggressive ignition timing, where it's actually been suffering from knock. Now the hard aspect of that high silicon content, hypereutectic piston, actually from the OE manufacturer's perspective is an advantage though.
What it means is that the piston is less likely to wear. So that's great for an OE manufacturer, it's going to give a piston that provides a really long service life, it's going to maintain its shape and it's also going to maintain accurate control of our ring grooves, which again is really important to make sure that our rings are sealing properly. The integrity of those ring grooves is really important, so you've got a good solid, stable surface for our rings to seal against. The other aspect is that in terms of the clearance that we're likely to see, now I can't give you specifics because the actual clearance is going to depend on the application, as well as, to a degree, the bore diameter. But we're likely to see our cast factory hypereutectic piston cylinder wall to wall clearance somewhere in the region of perhaps one, 1.5 thou, sometimes even less than one thou.
So very very tight cylinder to piston clearances and what this does as well, right from a cold start that piston is very very stable in the bore, there's absolutley no chance of that piston rocking backwards and forwards because we're very tight to the cylinder wall, and of course as the piston expands, it is supported even more. So that's our factory cast piston. The next step up the rung if we're looking at performance pistons would be the 4032 forged alloy. So these are available from just about every aftermarket piston manufacturer. And they're not very common and to be perfectly honest, they're a piston that's probably actually really well suited to a large majority of the builds we see out there.
Particularly street cars that are going to see or going to want long service lives between rebuilds, and even moderately powerful race engines, the 4032 forging's actually not a bad option. So first of all we're moving now from a cast piston into a forging. So this is again to do with the manufacturing process of the blank that the piston is machined from and essentially we're now forcing the alloy material into the shape of a piston under high heat and pressure. And the advantage with this is it gives us a nice grain structure which gives us more strength in the piston. Again a little aside from our actual topic but worth talking about.
So with the 4032 the silicon content drops from the 16% to 19% we see in a hypereutectic cast piston to down around 10% to 12% silcon content. So 12% is around the eutectic point, basically above that we're getting to a point where we've got more silicon than can be absorbed or dissolved into the aluminium alloy we're dealing with. So we've still got a reasonable silicon content so with that 4032 forging, that silicon content helps again make the piston relatively hard. We do have some aspect of the piston also increasing in terms of its brittleness there, obviously that can be a disadvantage, this is really a sort of, there's pros and cons along that line there. With the cylinder to piston clearance, cylinder wall to piston clearance there on a 4032 forging, we might jump up to somewhere in the region of maybe two to three thousandths of an inch.
So we're significantly more than what we'd see for a comparable cast piston. But still two to three thou, that's relatively tight and we're still going to maintain good control of that piston when the engine is cold. Now because the 4032 piston has less silicon content than a casting, plus it is forged, we've got a piston that is less brittle and more flexible than a cast piston. So it's going to be stronger but because it still has that reasonable silicon content, we're still going to end up with good wear characteristics from that 4032 piston. So as I've said, kind of a really good middle of the road choice if you're looking for a piston that's going to need to see a long service lift, at power levels that a cast piston probably isn't going to be suitable for.
And then the, probably the most popular forgind we see in aftermarket forged pistons is 2618. And this really refers again to the silicon content in the piston and here we drop from 10% to 12% that we saw in the 4032 forging, down to about 0.25%. Obviously this is going to vary a little bit from one manufacturer to another. So because we've now got very very little silicon content in that forging, it's going to expand a lot more as it heats up. So the downside of this is we need a lot more cylinder to piston wall clearance than we do with our 4032 and our cast piston.
We're likely to be in the range of maybe 3.5 through to five thou piston to cylinder wall clearance. It could easily be considerably more than that, particularly for large bore diameters on heavily boosted motors or engines running a lot of boost or a lot of nitrous for that matter, we're gonna need a lot more clearance than that because we're going to be producing a lot more heat. So the upside or the advantage with the 2618 forging and why people go for this is that in comparison to the 4032, the material is more malleable, it's a technical term, basically it's slightly softer. And I'm talking softer in terms of it is still a metal, you're not going to be bending it with your bare hands or anything, don't think it's anything like that. It also has a higher tensile strength and it has an improved fatigue resistance over 4032.
So for very high power applications, it's the no brainer, it is definitely the go to piston. And if you can break a 2618 piston, chances are there's something probably pretty wrong with either your construction of the engine or your tuning. So to again put some numbers around this, compared to a 4032 piston, a 2618 forging is going to expand around about 15% more at operating temperature. So that's the range of pistons we're dealing with and a rough guide to anything from about 0.5 to one thou for a cast hypereutectic piston through to five thou or more for our 2618 forged piston. Now none of that really matters when our engine is up to operating temperature and we're going to end up of course with the piston expanded and it's going to be nice and stable in the bore.
Where we see these problems come in though is particularly with the 2618 forging, where the piston is cold, particularly in very cold climates, when we cold start the engine we can end up with that characteristic piston rattle. And that is literally the piston rocking backwards and forwards as it moves up and down the bore. Doesn't sound very nice, it's got all of those problems I've already discussed previously. Fortunately it should go away as soon as our engine has a little bit of heat into it. So this is something that to a degree can be slightly unavoidable.
We do wanna take care with this because obviously driving the engine hard or using a lot of power with the engine when it is cold, when our piston's still expanding up to operating temperature, is not really advisable, we're gonna end up with more blow by, we're going to end up with more oil consumption with that as well. So we needed to consider that. Now one of the things I have noted as well over my career, I am not an engine machinist. So I have always worked with engine machinists in order to get our engine machining work done, so our piston, our blocks are bored and honed to suit specifications. And what I've kind of found and I think this is fair to say probably goes for a lot of the mainstream machine shops is that when they are selecting piston to cylinder wall clearance, they tend to air on the side of caution.
Now within reason, going a little bit looser in terms of our piston to cylinder wall clearance is always going to be safer than going to a little bit tighter. If we've got a little bit of excess piston to cylinder wall clearance, we're going to end up with an engine that's a little bit noisy when it's cold. Perhaps an engine that is down a little bit on power, perhaps an engine that breathes a little bit more and doesn't quite make as much power as it could, sorry uses a little bit more oil than it should, compared to if we go a little bit tight on our piston to cylinder wall clearance, of course as our piston expands up, if we don't have enough clearance we get to a situation where the piston siezes in the bore. Now if that happens, that's not going to end well and you're going to end up with the requirement to tear the engine down and replace the component. So when faced with those choices and somewhat understandable the machinist tends to air on the side of caution but sometimes I find that they go a little bit further than they really need to.
So it's all about education and understanding what we can get away with, it's also about understanding your specific application. And what we'll find is that all of our pistons come with some recommendations for the piston to cylinder wall clearance. So what I'm going to do, I think my iPhone camera's actually died which isn't too helpful. But that's OK, we've got another alternative here. We've got our piston specification sheet that comes from JE with the pistons that we're using for our Subaru FA20.
So what I'll try doing here is just get these under our overhead shot and hopefully, hopefully I'm gonna be able to get this in a way that you're going to be able to see it. Yep I think that'll work. So what we're looking at here is important information for our clearance. And we can see here that for this particular set of pistons, set the clearance to 0.0040 inches or four thousandths of an inch, if you wanna work in metric, 0.1016 millimetres. Note here as well, some applications may require more clearance.
So this is really important to understand, I'm gonna come back to that. While we're on this overhead shot though I also wanna mention that it's really important where we actually measure this clearance and we've got this nice little drawing here to show us where to do this. So this is called the gauge point and if you can't quite see this at home here, our gauge point is 0.275 of an inch or seven millimetres from the bottom of the skirt. So this is some of the important information that we need. There is actually also a suggested finished bore dimension on this specification sheet that the engine machinist should be able to finish the bores to, and if they do that, that should by definition give us our requested clearance.
OK so I just wanna come back and talk about the fact that some applications may require more clearance. And with engine building there aren't a lot of absolutes unfortunately. We do need to understand what will affect our ideal running clearances. And this is no different for example to gapping a set of piston rings. There isn't a one size fits all answer here, there's a guideline that we can use and then we need to adapt that guideline based on what we're doing.
Hopefully by now you can understand that really all of this comes down to how much heat the piston is being exposed to. So for example if we're running a naturally aspirated engine and then we take that same engine and we add a turbocharger or a supercharger, our engine is now producing more power, it's doing that because it's combusting more fuel and air in the combustion chamber. Understandably the larger combustion event creates more heat which is what the crown of the piston is exposed to. So all things being equal we're likely to need a little bit more cylinder to piston, piston to cylinder wall clearance for a forced induction engine than a naturally aspirated one. This could also come down to the use of the engine.
For an endurance race engine compared to a street engine, same sort of thing, we're likely to see the piston exposed to more temperature for a longer period of time and obviously at the extremes is some of our very very high boost race engines that are used for drag racing where we might be running 60 to 80 psi of boost. Perhaps an engine that is producing somewhere in the region of maybe 1500 horsepower from only two litres and four cylinders. So at that range we need to kind of take that into account and be a little bit more generous with our piston to cylinder wall clearance. Now the gauge point that I just showed you as well, the point on the skirt that we need to measure, this is really important and this is again something that's often overlooked. Now while it may, on face value look like our piston is parallel or the two sides of the piston are parallel to each other, if we actually measured them with a micrometer, we'd find that that's not the case.
And generally what we end up with is a slight barrel shape to our piston skirt. And the reason we have this barrel shape when the piston is here at room temperature is because the crown of the piston is going to expand more than the bottom of the skirt. It's simply going to be exposed to more heat. So the manufacturer takes this into account and the skirt diameter is machined to cope with that. And this is the situation that I've seen in a few Facebook groups, people talk about the condition of their engine and they'll basically have the engine sitting on a stand, even if it's a fresh build, the piston will be at top dead centre and they'll be rocking it backwards and forwards in the bore.
And on face value if you don't know what you're looking at, it might seem that the clearance is crazy excessive and that's simply because we do have so much more clearance right up at the ring pack than we do down at the bottom of the skirt, that's fine, that's exactly how it should be. But the important point to note with that is if we are going to get sensible data, when we are measuring our piston to cylinder wall clearance, it's critical to measure the skirt of the piston at that gauge point. Alright so what we're going to do is, I'll just talk about skirt coatings and then we'll go through a quick demonstration of how we can measure that. So this is another aspect that can affect our piston to cylinder wall clearance, so it's important to understand what coating you've got applied, if any, and what effect that could have. So I've got three examples here, obviously we've got this LS piston that I've been playing around with and clearly this has got no skirt coating on it at all.
So pretty straightforward, we're just gonna measure the skirt with our micrometer and that's all we need to consider. I've got another piston here, these next two pistons are both Subaru FA20 from JE. This particular piston has got what is a relatively common anti friction coating that's applied, this is available from just about every aftermarket piston manufacturer. So the idea is that it's a dry film lubricant, is designed to wear. It is a relatively soft and a very very thin coating.
So with this style of coating, we don't need to make any allowance for this in terms of our piston to cylinder wall clearance. So this would be stated quite clearly if you are asking for a coating on your piston skirt, so you need to just take that into account. There are other coatings which we'll look at next, where we do need to consider that. So this coating, particularly after the engine's done a fair bit of running, when we take it out we're gonna see that there will be a wear pattern on this, and particularly where the skirt has been contacting the cylinder wall, we're going to end up with basically that coating just about worn away in some patches. The next coating that we're going to look at, so this is relatively new from JE, they're calling it their Perfect Skirt coating.
And I think at the moment, I think it might be Wiseco are also offering their own version of it which I believe they're calling ArmorGlide. So essentially same thing but what it's designed to do is actually fix some of those downsides with the 2618 forging where we need excessive piston to cylinder wall clearance when the engine is assembled. So this coating is quite a lot thicker and it's not really designed to wear away like the coating we've just looked at. So this is actually designed into the clearance that we specify, or JE specify for the piston and what it's designed to do is take up some of that, what we could call excessive clearance and make sure that the piston is nice and stable, particularly when it's cold. But it can deform, basically to the shape of our cylinder to really bed into the cylinder and make sure that we don't have any problems with our piston siezing.
So with this sort of coating, you'll actually see hopefully, let's go to our overhead camera actually. You can see on the skirt of the piston we actually have a little window that is in that skirt coating. So that goes through the skirt coating right to the aluminium and this is the area where we need to take our measurements. So if we actually take the measurement on that skirt coating itself, because it is relatively thick, that's gonna give us a completely inaccurate result. So the idea with the Perfect Skirt coating is that we can get down to clearances in the range of maybe a thou or thereabouts so quite comparable really to a traditional cast piston, gonna get that nice quiet engine operation.
The actual clearance we're getting is still sufficient for the 2618 forging. But that perfect skirt coating is there to take up the slack. So we haven't actually used that set of pistons yet so I am only talking essentially from JE sales propaganda so I will add that in there. But while this is relatively new, they have been out in operation and pretty well proven by this point as well. Alright we are gonna move into some questions and answers pretty shortly so if you do have any, this is the perfect chance to ask those questions and we're gonna jump into those shortly.
So I just wanna go through a rough operation of how we measure our piston to cylinder wall clearance. And regardless if you are relying on a machine shop, which 99.9% of you probably are, if you have your own machine shop you probably don't need to be attending this webinar and learning about piston to cylinder wall clearance. So yeah you're probably gonna be relying on an external engine machinist. The thing I've found is that it's always a good idea to follow up and just check and see what your clearances are. And I think this is probably one of the golden rules with engine building in general, just never make assumptions.
Always measure and check absolutely everything. And even if you've got a quality machinist who's turning out exceptional work, you've got to also understand that these people are still human and at the end of the day they are still able to make mistakes. It's gonna be much cheaper and much easier to pick that up during the dummy assembly and the build stage, rather than just blindly assembling your engine, assuming that everything is on point and then finding out the hard way that something really wasn't correct. The other aspect that's a little bit more subtle I think is that if you've got a relationship with your engine machinist and they know that you are going to be checking up on their work and measuring, and you don't need to put this in a nasty way, but if they do have an understanding that you're smart, you know what you're doing, and you are making your own measurements to check their work, I generally find that they tend to go the extra distance to make sure that you're not gonna show them up. So I just find that checking on the clearances tends to actually give you, by default, a better finish in the first place.
Alright so what we wanna do is start by measuring our piston, we'll get rid of our LS piston here and we'll take one of our JE forged pistons. And we've got our gauge point which we found was seven millimetres up from the top of the skirt and just to make this really simple, I actually find a good way is to start by just marking that with a sharpie marker. So I've got a little pin point here that I've already placed on both sides of that piston at the point where we will be making our measurement. So in this case all we need to do is use a conventional micrometer and we're going to basically use our micrometer here and make a measurement right on those points, always a good idea to just follow usual protocol when we are using our micrometer, making sure that we are getting a nice accurate measurement there. I'll also mention that when we are making these measurements, because aluminium does have a relatively high rate of termal expansion compared to cast iron for example, it is advised, you'll probably find in your spec sheet that these measurements are taken at 20 degrees C.
And we just recently had a question about this on our forum, how much difference does it make? And the reality is that across the normal temperatures that we're likely to see, the normal ambient temperatures, we're likely to see in our engine building rooms, it's probably not the biggest consideration. But if you are operating at very very high ambient temperatures, then it might be worth considering investing in an air conditioning unit. If for no better reason than making your own life a little bit more comfortable. So we've taken our measurement there with our micrometer and just before we move on, obviously this is our conventional skirt coating here. I just want to show you just a little trick with the new Perfect Skirt coating that JE have started applying and this actually does add another element of complexity because we can't use a conventional micrometer to measure that.
As you can see again on our overhead shot, the little window that we've got there is really quite small and when we compare that to the size of the anvils on our conventional micrometer, we're going to end up just spanning across that little window and we're going to be ending up measuring the skirt coating itself. So what we're going to need, if we are using one of those skirt coatings and we want to measure the piston accurately, is we're going to need what's referred to as a blade micrometer. Now again I think our iPhone's dead so I can't show you this in more detail but no rocket science here, it's just exactly as its name implies. Rather than anvils here we've got little blades so it just makes it easy to get right into the windows on the piston skirt and get a nice accurate measurement. We do need to consider though I think we paid somewhere in the region of about maybe USD$300-350 for a blade micrometer.
So a little bit pricey if you are only looking at doing one engine with these sorts of pistons. Alright so once we've got our micrometer reading on our skirt of our piston, it's always a good idea to also confirm or check off the reading that we've got in our micrometer and compare that to our spec sheet. And it's just a nice sanity check that first of all, we actually have measured the piston correctly. Also a good sanity check to make sure that if we've measured the piston correctly, we're getting repeatable answers that don't line up with our spec sheet. We may have a problem.
It's always easier to assume that our pistons are in good condition but it's really easy for a piston to take a tumble off a work bench, particularly if that's happened prior to you getting the pistons or at your engine machine shop, it can be something that's really really easy to overlook so always a good idea to just confirm everything makes sense there. The next step is that we are going to need another piece of measuring equipment here which is our dial bore gauge. And the dial bore gauge essentially doesn't do much in the way of measurement on its own, it's really a comparative device. So we've got a dial indicator at the top. We've got a range of just different extenders that are available here.
Actually I'll just bring up our full kit here so you can see what we're getting. So this is available to suit a range of bore sizes and basically we just replace or change over the little extension depending on what bore size we're working with. So this one here will go all the way from 50 millimetres through to 105 millimetres plus we've got a 55 millimetre extension. So in other words up to 160 millimetres. So once we've zeroed, oh so as I was saying, it's a comparative tool so we've got a little plunger at the bottom and that will move our dial indicator.
And what we're going to be doing here is basically locating that between the anvils of our micrometer. Pretty fiddly I'll be honest. We're gonna rock that backwards and forwards, I'm not gonna go through this process here. But rocking it backwards and forwards, finding the smallest point and we're going to zero our dial indicator. So at that point what we've done is we have zeroed our dial bore gauge on the outside diameter of our piston skirt.
So essentially then if we see a zero on our dial bore gauge, we know that we've got exactly the same bore diameter as our piston skirt. We're gonna hope that that's not the case. The next step there of course is to insert our dial bore gauge into our bore. Little bit difficult to show you this on camera but what we do is just rock it backwards and forwards here and we're looking for where abouts our needle is coming in relation to our zero mark and that's going to show us our clearance. The other aspect with our dial bore gauge, and again even if you're not checking piston to cylinder wall clearance you can use this to check the condition of your bores.
So what I mean by this is if we move our dial bore gauge and check in three locations, so we want to check the top the middle and the bottom of the bores, and then we can repeat that process perpendicular to our first reading. That's a really good way of checking our cylinders for any belling, any taper or any out of round. And in the perfect world we'd like to see everything there absolutely perfectly round, perfectly parallel from top to bottom. An important point to note here, if you are dealing with a performance engine build, in general, most of the builds we go through, we use a torque plate that's used for the boring and honing process. So the torque plate bolts to the top of the engine block and it replicates the distortion that will be put into the engine block once the cylinder head is bolted down.
So what we're trying to do is basically bore and hone the block so that it's perfectly round when the cylinder head's bolted down. It's not much point if we just go and get our block bored and honed like it's sitting here right now, everything might be absolutely perfect sitting here at room temperature on the bench but then we go and bolt the cylinder head on it and if that distorts the block and all of a sudden our bores are no longer perfectly round, that's the actual condition that the engine's going to be running under and that's obviously where we want our cylinders to be round. If we are using a torque plate of course it should go without saying that we also want to use that torque plate or have it bolted down when we are checking our piston to cylinder wall clearance, otherwise that in itself can be misleading. Alrighty we'll jump in and we'll have a quick look and see what questions we've got, if you've got any more, please feel free to keep asking. Brendan Banks has asked, do piston coatings actually work to build up the piston skirt to get a perfect interference fit or does it just keep wearing away and you'll be right back to excess clearance? OK so I think I've probably covered a little bit of that inside of the webinar.
There's very different types of coatings so it's going to depend on what you are using. So the friction reducing coating, the dry film coating, you're going to probably find that it's pretty futile trying to apply that in a thicker coating to take up excessive piston to cylinder wall clearance, it really is designed to be a coating that will wear away so it's a very short term solution. On the other hand, that Perfect Skirt coating or the ArmorGlide as Wiseco are referring to a similar technology, very different coating so again, not one that I've personally had experience with yet but in general I would say if you are talking about an engine that is high mileage and you're trying to recover excessive piston to cylinder wall clearance, trying to do this by adding an aftermarket coating's probably not going to work for you. Robin has asked, if you buy an aftermarket piston for a specific standard bore, let's say JE pistons for an R32 Golf, bore diameter 84 millimetres, is the piston then manufactured to fit the standard bore? OK that's actually a good question Robin and that's probably something that I should have covered in this webinar. This is problematic so what we find is that obviously over the life of an engine, it's going to wear.
We've got the pistons running against the cylinder walls, we've got the piston rings running against the cylinder walls, there is inherently some amount of wear is going to occur, particularly in a high mileage motor. So this is the tricky part when it comes to choosing a piston for an aftermarket build. Now in some instances we may not actually be able to bore and hone the block to what's referred to as a first over size. This would be our normal go to, we would bore and hone the block by 20 thousandths of an inch or half a millimetre and that allows the wear to be removed from the bore, allows a fresh hone finish to be put on the bore and then we're good to go again. Now there is a disadvantage with this of course because we are removing some material from the bores so we are inherently weakening the bores and in some engines as I've kind of mentioned, we may not have enough thickness in the sleeves or liners to actually allows us to bore the engine.
Good one that I am aware of because I've just gone through a build is the GM LS1, which runs a 3.9 inch bore. We can't bore and hone that so basically we end up with a piston where we're relying on the cylinder wall, the cylinders coming out in good condition with a 3.903 inch finished bore diameter. So a little bit dicey, particularly if you've got an engine that wasn't in great condition in the first place. Kind of get back on track with your specific question here, really what you need to do is before you order pistons, get your engine stripped down and actually check the bore condition. You should be able to have an engine machinist give you a fairly good assessment of this and JE will give you a finished bore size.
As I mentioned in that spec sheet, there will be a finished bore size that will suit those pistons. So straight away your machinist will be able to tell you if honing the bore is gonna give you a good enough finish and the right bore size and then it becomes a bit of a balancing act as to if you need to clean up the bores a little bit more, whether you want to go slightly larger on the piston to cylinder wall clearance to get perfectly round bores and a nice fresh hone pattern, whether you want to compromise that slightly and keep the piston to cylinder wall clearance closer to where it should be. I went through exactly that situation as well on our Toyota 2JZ build that we did as part of our practical engine building course. This was supplied with a set of stock 86 millimetre bore CP pistons. They target around about 3.5 thou and even though we were dealing with a brand new block straight from Toyota, we found that we were already slightly over 3.5 thou.
Given the horsepower application for that motor, I was quite comfortable actually going to four thou with that particular engine. But this is the sort of situation you can find yourself in. So long winded answer, I'm sorry there, hopefully Robin that's given you a little bit of information around that question though. Chris Pratt has asked, if in all motor piston to wall clearances around the 0.004 or four thou of an inch, how much more piston to wall clearance do we need for a forced induction engine? Do you give more clearance by horsepower, how do you know what specs to use? OK so this is a tricky question here and the answer really comes down to the piston manufacturer. In your particular example there if you are targeting around about four thou piston to cylinder wall clearance for a naturally aspirated engine, of course depending on the amount of boost pressure you're aiming to run, you may want to be somewhere in the range of maybe 4.5 thou piston to cylinder wall clearance.
May not necessarily be that much. If on the other hand you were running a very high boost engine, maybe it's a drag engine running on a gasoline based fuel, you may need to be up at five thou or larger. Just to show you that there are some complexities around this and nothing's ever cut and dry, with some of our drag engines, because we were running methanol fuel, while we were making a lot of power, the methanol fuel runs a lot colder than a gasoline based fuel. So even with our 1200 horsepower Mitsubishi 4G63s, we were still somewhere in the range of about five thou, maybe 5.5 thou piston to cylinder wall clearance. So to answer the second part of your question, yes the horsepower, or more specifically the specific horsepower level, so what I'm talking about that is howerpower per cylinder because of course making 1200 horsepower on a six litre V8 versus 1200 horsepower on a four cylinder two litre engine, very very different things.
The last part of your question though, and this is really what I wanted to get to here, this is a common one obviously. How do you know what specs to use? And it's a valid question. And this is where really it's a great idea if you are in doubt, if you're doing something that really veers away from what the OE, the piston manufacturer would really intend, so a run of the mil standard use for that piston, if you're doing something really out of the ordinary, ring the piston manufacturer, they all have tech support helplines. And they would rather you ring and get the right information from one of their tech guys, or girls, rather than just make some assumptions, put the engine together and have the piston fail because then they're gonna look bad, you're not gonna have good things to say about their product, so it doesn't really help anyone out. In the past I've done this several times with some of our extreme builds and in our case we generally deal with JE, I've found them exceptional to deal with and they've been very helpful so that's how I'd deal with that.
Low Dynamics has asked, are there benefits or downsides to asymmetrical pistons? In terms of the benefits, really what we're doing is two fold, we're reducing the amount of material on the piston, so that has the advantage of reducing the weight of the piston, obviously the weigh of the piston, significant aspect of our rotating assembly mass so the less weight we can have here the better. I should actually back up for those who aren't aware of what I'm talking about. I don't have one here to show you but an asymmetrical piston, the skirt dimension or size is different from one side of the piston to the other, depending on whether we're talking about the thrust face or the non loaded face. So basically more material, a wider surface is placed on the thrust side of the piston because that's where it's going to be forced against the cylinder wall so we want that support here. The other advantage with an asymmetric piston design is that it reduces the frictional losses inside the engine because we have physically less piston contacting the cylinder wall.
In terms of downsides, probably not the ideal solution for a very high horsepower engine. You've got less piston skirt supporting the piston there so stength, probably not as good as a full round or a forged side relief piston at the very high end power level. Alright that's brought us to the end of our questions there. As usual if our members have got any further questions, please ask those in the forum and I'll be happy to answer them there. Thanks for joining us and I hope to see you in our next webinar.