123 | Measuring Bearing Clearances Using Plastigauge
While using a micrometer and bore gauge is undeniably the most accurate way of measuring engine bearing clearances, plastigauge is still a technique relied on by many. In this webinar we’ll discuss how to correctly use plastigauge to get the best possible results during engine assembly.
It's Andre from the High Performance Academy and welcome to the webinar where we're going to be looking at the use of Plastiguage during the engine assembly processs, now, Plastigauge is a really really cheap and simple way of checking bearing clearances and it's also probably for the most part fairly heavily frowned upon by those people out there who are professionally assembling building engines for a living and while it's frowned upon, I still find that Plastigauge is a technique that is quite heavily relied on by a lot of home enthusiasts who are assembly just perhaps their own engine in their workshop and aren't really interested in investing heavily in the sort of tools necessary to perform the bearing clearance checks in a more professional way. Now, before we move on with the Plastigauge just for those of you who aren't aware, I will cover off the alternative option here, I'll just talk about it briefly and we will cover this in more detail in a future webinar. So what we normally do, what we prefer to do is if we have our journals on our crankshafts and we use a micrometer or mic to measure the bearing, the journal diameter so this micrometer would then be set at the diameter of the journal on the crankshaft. What we can then do is we can grab our bore gauge and the bore gauge uses a dial gauge and what it does, we zero this inside our micrometer here and once we've zeroed our dial gauge inside our micrometer, this essentially means that at it's tightest point, the dial gauge here is reading zero. Once we've zeroed our dial gauge, we can get our connecting rod in this case or we can also check this if we are using the main bearing journals, if we're checking the main bearing journals we can fit this straight into the engine block and we can insert our bore gauge into the conrod and then again we can just rock this through until we can see our clearance and in this case, I'm not quite sure, I can't see if you can actually see the value here, but this is a metric bore gauge and that was reading 500ths of a millimeter so that's the preferred technique.
This is the technique you'll find most professional engine builders performing. If you wanna learn more about this, it is covered in detail in our engine building fundamentals course. If you are considering building a few engines, what we will find these days is you actually don't even need to spend a huge amount of money on this sort of equipment. It's come down massively in price over the last few decades and we can now buy what is still quite good quality equipment for a relatively low sum of money, so, certainly, if you are considering building a few engines, it's still worthwhile investing in quality equipment. It's going to make it much easier to do this task thoroughly and be much more accurate with it.
However, moving back to Plastigauge, Plastigauge is as I've said, still relied on by a number of people and even when I do have the correct equipment, and as I do here, it's also quite a nice sanity check for some instances just to go through and check your bearing clearances using Plastigauge. One of the advantages, I find, with using Plastigauge which is probably often overlooked is because we are laying the Plastigauge strip over the full width of the journal, or that's my preference, at least, what allows us to do is very quickly visually see if there is any taper in that journal and that's to say our bearing clearances change from the front of the journal to the rear of the journal. So that's, as I say, an often overlooked advantage we can see at a glance. The downside of Plastigauge is that we don't have that same accuracy depending on the grade of Plastigauge we're using. We might be able to see, for example, that we have either two thou or three thou of clearance but it's very very difficult to say exactly where in between our clearance may lie.
And of course, as with any tool, it does require some understanding of the product and how to use it in order to get the best results from it. So, what exactly is Plastigauge, we've been talking about it for a little while and we still don't really know exactly what it is if you haven't seen it before. I've got a couple of packets of it here, so Plastigauge is essentially a thin wax strip that we can lay inside on the bearing journal on the crankshaft and we can torque the cap down and that's going to then squeeze the Plastigauge tight and it's going to expand out or crush as we torque down the bearing cap and then by removing the bearing cap again and inspecting the crushed Plastigauge strip, we'll be able to see what our bearing clearance is. So I've got this particular pack here which is a green pack and this is designed to measure between one and three thousandths of an inch. If you're more used to working in millimeters, you'll be happy to see that on the opposite side we have the metric scale.
For today's webinar, I will be working now in imperial thousandths of an inch. Very very simple to convert between, though, as long as we remember that there's 25.4 millimeters in one inch. I've got another pack here which is the red type and this one is to measure between two and six thousandths of an inch, there's also a blue pack available, so here in New Zealand, we're buying this under the ACL brand, it may be available under different brands depending where exactly you're based in the world and you should be expected to pay just a few dollar for one of these packs. It's incredibly cheap. So each of these packs includes a length of this wax and what I'll try and do here is just zoom in with our separate camera and give you a look at what exactly that looks like.
So, hopefully that will focus there, so you can see it's just a thin string of wax, yeah it's not really going to focus for me particularly well, it's just a thin string of wax that we want to break off and lay across the journal on the crankshaft. Okay, so what we're going to do is go through a demonstration of how we can use Plastigauge and it's important to go back a step from where we actually are here with our Toyota 2JZ demo engine and what we want to do is go through and basically complete a dummy assembly of the components that we're looking to check. So we're going to start here by talking about the main bearing clearances. See, for those who aren't aware of exactly what I'm talking about here when I use the term bearing clearance, I'll go back one step as well, when I'm talking about the bearing clearance, what I'm talking about is the clearance between the outside diameter of the crankshaft journal and the inside diameter of the journal that the crankshaft runs in the engine block or the conrod for that matter. And what this clearance dictates is the oil clearance in that particular journal and it's important to make sure that that oil clearance is exactly what we want it to be, it's going to have a large impact on the reliability of the engines and how well our bearings are going to last, so, it's a very, very critical measurement and we want to be very sure that that measurement is correct.
Okay, so what we want to do is start by ensuring everything is 100% clean and we're assembling the engine exactly as if we're going to do a final assembly. Now, when I'm putting the crankshaft in to the engine block, however, what I'm going to do while I'm checking the clearances using Plastigauge, is I want to assemble the crankshaft in to the journals dry and by that I mean I'm not going to use any assembly lube or any engine oil on the bearing surfaces. What we want to do here is ensure that the oil film isn't going to effect our measured clearance so we want to make sure that everything is assembled clean and dry. So I've done that, you can see that I've got all of our main caps barring the center main cap installed already. And what I'm going to do is check the clearance on our center main here.
So what I want to do is start by grabbing a length of our Plastigauge and you can either cut this, or, I find that it's reasonably easy to just break with your fingers or your fingernail and to get the best results in order to be able to see how the clearance across the journal works, we want to make sure, as I've touched on before, that we are laying the Plastigauge across the entire journal, so, I'll just install this now and then we'll have a slightly closer look at that. Okay, so I've just laid the Plastigauge there across the journal, let's go to our other camera and we can see the Plastigauge lying there that she offset it very slowly, so let me just move it and centralize it. Now it's stuck to my finger which isn't ideal and now we have lost it. Let's do that again. Alright, with our second piece of Plastigauge.
Now, if you actually are having trouble with your Plastigauge staying put like I just had, what you can actually do here is you can just put a little drop of either oil or grease just on the Plastigauge and that will hold it firm. Let me just fish this piece of Plastigauge back out so I can try and install it correctly here. This will just save me from having to cut another piece. Nope, it's not gonna play the game for me, alright, so we'll just cut another piece here and we'll go through that process again. Normally, if you're not filming this, it'll tend to go much smoother than what you're seeing here.
So only when you try and demonstrate it to someone that you actually have problems with it, normally. Alright, so I've got another piece there and we'll just lay that across the journal and this one, unfortunately, is still a little bit short, but it's going to demonstrate the technique anyway. Alright, let's have another look here, we'll just zoom in so we can see our Plastigauge there is lying across the surface of the crankshaft and we do wanna be a little bit careful. Obviously you can see we've got our oil hole in the crankshaft journal, so we want to obviously make sure that we don't lay that Plastigauge directly across our oil hole, either. Okay, so now we've got everything set up there.
We can now install our main bearing cap. Again, we want to install that with everything dry so we're not going to be adding any additional lubrication. What I'll do is I'll just place that gently down over the studs and we'll just go through and put a little bit of lubricant on the washers, this is an ARP stud kit which is relatively common these days on just about any sort of high performance build. So we'll just put our washers and our nuts on and just a little bit of ARP moly lube on each of those and then what we'll be able to do is just torque down our main bearing cap. For this particular stud kit, the recommended torque is 60 foot pound using the ARP moly lube and what I like to do is do this in three stages.
At the moment, you can see I'm not using a torque wrench, all I'm doing here at the moment is just pulling the billet main bearing cap down into the register in the block, so I'm not going to actually perform all of my work here with the strong arm. We've got that pulled down into the register in the block now, so, I'll just swap across to my torque wrench, we'll just turn that on, I'm just using a Snap-On digital torque wrench here but any torque wrench will obviously do the job and what we'll do is we'll do this in three steps, we'll start with our first step at 25 foot pound and pulling this down in three steps like this just has the effect of making sure that it is torqued down evenly, we're not going to end up with an uneven amount of torque applied. So we'll go to 40 foot pound now and finally the rating of 60 foot pound. Okay, so once we've got that torqued down, the next step is just simply to remove it again. Once we've removed the bearing cap, we'll be able to have a look and see what's actually happened to our Plastigauge and we'll see exactly how crushed that is.
Now, when you're performing this sort of work, these particular billet main caps may have life very easy because they have a little machine recess here so it's very easy to get in there with a screwdriver or pry bar and actually remove the main bearing cap. It's quite difficult in some instances to remove the cap easily and what we want to make sure we don't do is rock the cap backwards and forwards. We really wanna make sure that the cap is pulled directly up from the block because if we're moving it backwards and forwards, this can have the effect of crushing or effecting the crush on our Plastigauge. So in this case, all I'll do is just use the up pry bar here and I just can pry the bearing cap directly up. Let's just get our block back level.
Remove the bearing cap and our Plastigauge has remained there on the journal and what we're going to do is now have a quick look at that Plastigauge. Let's jump across to our other camera. So we can see one of the first things I do tend to look at when I'm doing this is does the thickness or width of that Plastigauge look consistent and in this case you'd have to say that yes it does and once we've done this, we'll just jump back and have a look at how we actually check our clearance and as we saw on our pack of Plastigauge, we do have a scale on the Plastigauge pack. So all we want to do is cut off a small section of this, physically hold it over the Plastigauge and see where that aligns to see what our clearance is. So let's try and do that now, see how well this works out with our other camera, let's switch across to our other camera now and we can see is if we compare it to the two thou scale, we're a little bit thinner than our two thou scale, but if we move across, we're obviously much wider than our three thou scale.
So this is where the Plastigauge has it's limitations because all we can tell from that is the clearance is larger than two thousandths of an inch but clearly much smaller than three thousandths of an inch. If I had to take a guess there, I'd say we are probably looking at somewhere around about 2.1 or 2.2 thou, but it's obviously impossible to be very, very accurate with that. And it is to say it's a good way of getting a sanity check on our bearing clearances and getting a good visual indication of our bearing clearances as well as whether or not we have any taper there. Now, there's actually a step that I did miss out, which I'll just touch on. It didn't actually catch me or trip me up there, but it does on occasion, present a few issues.
What you'll often see is that the Plastigauge ends up staying stuck to the bearing shell rather than staying on the journal of the crankshaft. Now this makes it much more difficult because we can't easily get in to the curved nature of the bearing shell using the scale on the Plastigauge pack to check our clearance, so, a good trick for that is to give the bearing shell a light coat of a silicone spray, so in this case, I'm using a product that's made by CRC and all it is is a lubricating silicone spray. It just needs a very, very fine spray on that bearing shell and then what that will do is it will ensure that our Plastigauge stays on the crankshaft and it doesn't transfer across to the bearing shell, so that's a nice little tip for you there. Okay, so at this point, we've checked our clearance, we're happy with that, or, we at least know what we've got. We do still have this piece of Plastigauge stuck to the journal on the crankshaft and it is important to make sure that we thoroughly remove that Plastigauge before we go on.
The way I tend to do this is just by using a clean rag and a brake clean product, so we just want to spray the brake cleaner onto the rag and really any cleaning product will do. And we'll see if we can show this using our second camera here and all we want to do is just simply clean away the surface of the journal. Clean the Plastigauge off it and you can see even after that cleaning, there is actually still a little residual Plastigauge stuck here, it is actually quite difficult to remove and it does require a little bit of attention, so, whenever you're using Plastigauge, make sure that the journal is thoroughly cleaned both before and after you've done your measurement with Plastigauge. Okay, so we've looked now at using the Plastigauge on a main bearing, however, we also obviously have our big end or connecting rod bearings and we want to obviously check and make sure that those clearances are accurate and correct as well. They're just as important as our main bearings and that provides a slight difficulty when we're using Plastigauge.
And what I've done is I've just assembled a connecting rod onto our engine here, and I'll just show you why that can be problematic. So, typically, or, the easy way of doing our test is what we've just seen here. We would have placed the Plastigauge down on the top surface of our journal. That makes it very easy for us to place the Plastigauge and even then, you also saw that I managed to drop my first test piece, so, the top of the journal is where we really want to place that Plastigauge to be convenient. However, this become a problem when we go to remove the conrod bearing cap after we've torqued it down so that we can check what's happened to our Plastigauge.
And the reason for that, I'll just show you, is when we go to remove our bearing cap, we need to free it from the connecting rod body and I'll just undo these two conrod bolts and what we can see is as I've undone these two connecting rod bolts and I've wound them out of the connecting rod, the connecting rod cap stays firmly fixed to the body of the connecting rod and that's because it's dowed into location, so, there's no easy way of freeing that conrod cap from the body and if we go back to our wide angle here, the easy way of doing this is to remove the bolt just a few turns as you've seen me do here and then we can just tap on our extension there and you can see that what that's done is it's freed our cap now from the body of the conrod. Now, once it's free like that then we can easily remove the connecting rod cap and then remove the conrod assembly. The problem when we're using Plastigauge is that hammering effect that I just performed on the conrod bolts, what that tends to do is it also crushes out our Plastigauge and it gives us a completely unrealistic value. It makes it look like our bearing clearance is much tighter than it really is. So, this isn't going to help us, it's going to be completely misguiding so my own technique when I am checking the conrod bearing clearance with Plastigauge is I'll actually use the Plastigauge or fit the Plastigauge onto the upper half of the bearing shell.
This presents a bit of a problem because we either have the difficulty of fitting the Plastigauge in to the conrod body, let's just have a look at that by what I'm talking about. We want to fit the Plastigauge across the upper bearing shell which ends up in the conrod body as opposed to the cap, so two options equally awkward there to install the Plastigauge on to the bearing shell in the conrod while once the conrad's been installed into the engine block, or, alternatively try and precariously locate the Plastigauge into the conrod then install the piston and the connecting rod into the engine block. So it all gets a little bit messy and the trick here is if we just use a small drop of oil or a very, very small dab of grease, this will allow us to locate the Plastigauge quite nicely where exactly we want it, it's going to mean that it's not going to go missing and it's not gonna jump out of the way or get knocked out of the way while we're installing the connecting rod into the engine and while we're torquing that cap in place. Then, if it's installed on the upper side, what that's going to mean is that when we remove the cap, as I've just demonstrated, that's not going to end up crushing the Plastigauge out, so, just a little tip there if you are going to be using Plastigauge for doing your big end oil clearances. Now we're going to move into some questions and answers now so if you do have anything that you'd like me to explain, or anything that I've done during this demonstration that you would like me to go into more detail on, please ask those in the chat now and Ben or Colin will transfer those through to me.
I will just have a quick look and see if we've got any questions. We do have one from TDEChamp Already who's asked "Is it normal to be "using larger clearances than stock "with performance engine builds? "I ask because two thou seems large, "but I may be just used to Subarus”. Yeah, okay, really good question there Tyler and the answer is yes it is actually quite normal but your comparison there with the Subaru is probably not particularly fair to a Toyota 2JZ and the reason for that is what we tend to find with the aluminum blocks in the Subaru engines is everything grows and moves around quite dramatically at a normal operating temperature and this can have quite a large impact on our actual oil clearances under normal operating conditions, however, if I remember off the top of my head correctly, the recommended or O.E. oil specification for the Toyota 2JZ is in the range of about 1.6 thou on the main bearings. So we have actually purposely selected to go a little bit looser on the oil clearance here with our 2JZ build and I will just go into a little bit of detail about exactly why we do this 'cause I think it's an important addition to this Webinar.
What we need to understand is that the oil clearance is critical, that oil film is critical to the reliability of our engine and contrary to what many seem to think, the bearings, the actual engine bearings, should never contact the crankshaft journals. They are always supported by a film of oil and as soon as we get any metal to metal contact, we can end up with almost instantaneous carnage, instantaneous catastrophic failure, so we need to make sure that that never occurs. Now, the other aspect, even with a cast iron engine block like our Toyota 2JZ. When you're talking about an engine that might be revving to eight or nine thousand RPM or even further and potentially an engine that's running very high boost pressures and making upwards of a thousand rear wheel horsepower, which is what we're expecting from this particular example. What happens is the engine block and the crankshaft, everything is actually moving around and flexing quite significantly and that flex can be very, very dangerous because as the crankshaft flexes, this can result in bearing contact between the journal and the bearing.
If the flex basically exceeds our oil clearance. So, it is quite common on a high performance build to just go slightly looser on our clearances and that tends to be the safer option. Now, I'll counter that with there is a more recent that the idea of a looser build for a high powered, high performance engine is fairly old and tried and tested technique, it's certainly nothing that I've come up with by myself. There is a more modern take on that where we're seeing a high performance engine build sort of go completely the opposite way and go for thinner or narrower oil clearance and one of the reasons they're doing this is the narrow oil clearance allows a thinner grade of oil to be used and it allows potential for a lower friction loss inside the engine. The downside of doing this, is in order to get away with it, you need to make sure, be very sure, that the rigidity of your crankshaft, your engine block, all of your components, is very high because if we get almost any flex or any movement in those components when we reduce our oil clearance, this can end up very easily resulting in metal to metal contact.
Dave Co Motorsports has said "I notice you spun the crank dry, "is that an issue, I've always "been extremely worried about doing this". Certainly what I'm doing here, unfortunately, is trying to demonstrate two aspects to you here in a webinar form and certainly no, I wouldn't recommend that you assemble the crankshaft dry and then move it. Particularly when we are doing this example here, this task here, where we are checking the clearances with Plastigauge, we're going to do this in two separate ways. We're going to check all of the main bearing clearances and we can do this with the crankshaft sitting there in one single location, we don't need to, and we definitely don't want to rotate the crankshaft and particularly when we are checking our clearances using Plastigauge, that's one of the big no-no's, I should have mentioned that already, we do not want to move the crankshaft, it can smear and basically, smear the Plastigauge and effectively render any of our results completely inaccurate. In this case, I did want to demonstrate to you the task of checking the main bearings and the big end clearances and I just wanted to show you this one conrod that I've installed and it required me to rotate the crankshaft.
What I've done, is I've lightly lubricated the bearings that we weren't looking at, we weren't interested in there, but your point is valid, I definitely wouldn't recommend rotating a crankshaft if you have assembled it dry. Terry's asked "Does Plastigauge have a shelf life, "and if so, about how long approximately?" You know, that's a really good question and it's one that I've never actually gone into any consideration on, however, it is a wax product, so, I can't specifically see a good reason for it to have a shelf life and I have never seen a best before or a use before date on a packet of Plastigauge, so, I'd need to possibly do a little bit more investigation into that myself. Barry G has asked, "How important "is a harmonic balancer to prevent the flexing?" Well the task of the harmonic balancer is to remove harmonics or vibrations that are occurring naturally inside the crankshaft, this in its own right, I don't believe goes too far to actually preventing flex in the crankshaft and I've certainly run our own drag engines with solid front pulleys with no harmonic dampener and I've noticed no ill effect from doing so in terms of the crankshaft moving around and what I'm talking about here is my own 4G63 drag engines and these were a classic example of where you can really clearly see the effect of the crankshaft flexing when you're running a crankshaft like that in a 4G63 to 10 and a half or 11 thousand RPM and over 1100 wheel horsepower. What you tend to do is when you disassemble the engine, you can see shiny areas on the sides of the bearing shells where this being wear starting to occur and this is really indicative that the crankshaft is essentially moving around like a large noodle under those sorts of conditions, it's no longer the solid piece of cast material that you started out with, a forged material that you started out with. So, yeah, I don't believe that the harmonic dampener is going to remove or eliminate the flex, that's really more of an aspect of the material that your crankshaft is made out of as well as the forces that you are putting into it.
Alright, guys, that looks like we have taken through to the end of our questions, and as normal, if you do have any more questions that crop up after this webinar has aired, please ask those in the webinar section of the forum and I'll be happy to answer them there. Thanks for joining us, I look forward to seeing you all next week.