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Practical Engine Building: Ring Gaps

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Ring Gaps


00:00 - When we're dealing with aftermarket pistons, one of the critical steps during the assembly process is to correctly set our piston ring end gaps.
00:08 With aftermarket pistons, the ring packs that are supplied are typically what's referred to as a file fit ring, and what this means is that the rings are supplied in an oversize form and it's the engine builder's task to file and correctly set the piston ring end gaps.
00:25 Now if those ring end gaps are too tight, what this means is that the rings can end up butting together as they heat up and expand in operation, and this can result in catastrophic engine failure.
00:37 On the other hand if our ring gaps are too large, this can result in excessive blow by, oil consumption, and a possible reduction in engine power.
00:47 For this demonstration we're going to have a look at gapping a piston ring for a Subaru FA20 engine.
00:54 I have the block sitting here in front of me, and we're going to have a look at gapping the rings using two separate techniques.
01:02 First of all I'll be using a electric ring file, and then I'll also be repeating the process using a manual ring file.
01:10 Now if you are expecting to build several engines, it's definitely worth considering paying a little bit more for an electric ring file.
01:19 This is going to make your task much easier.
01:22 It's faster to file fit the rings, and you're also going to find it's easier to maintain consistency across all of the ring end gaps than what you can achieve with a manual ring file.
01:33 That's not to say that the manual ring file is incapable of doing a good job, but it does require a lot more attention to detail while we're performing the task.
01:44 Before we start gapping the rings, we want to find out what the existing ring gaps are as the rings are supplied from the manufacturer.
01:53 So what I'm going to do here is take the ring for piston number three and I'm going to fit it into the bore of our FA20 block.
02:01 And when we are measuring the ring end gaps, what we want to do is make sure that the ring is square in the bore and we want to locate it slightly down from the top of the bore.
02:11 Somewhere around about 15 to 20 millimetres is normally adequate.
02:16 This is because there's a slight taper on the very top of the bore that's designed to help lead the rings into the bore when we're installing the piston.
02:24 We don't want that to distort the actual reading we're getting.
02:29 So we want to make the measurement in an area of the bore where the rings are actually going to operate.
02:35 Now also it's important to point out that if you have used a torque plate during the honing process, then it is recommended to perform your ring gapping with that torque plate installed.
02:48 Now I'm not going to be doing that for this demonstration because it does make demonstrating the techniques a little more awkward because we need to work down through the thickness of the torque plate.
02:59 But definitely if you have torque plate honed your block, it is recommended to also gap your rings with that torque plate fitted.
03:07 Before we start, you can see that I've laid out all of the rings, and I've laid them out in order.
03:12 Now once we've gone through this process of gapping our rings we want to make sure after that that the rings remain with the correct piston.
03:21 We may find that there are slight differences, very minor differences in bore diameters, and what we're doing here is gapping the rings to a particular cylinder.
03:31 So once we've done this we don't want to end up mixing and matching the rings to other cylinders.
03:36 Let's grab the top ring now from number three cylinder, and we're going to locate that into the bore.
03:43 Now what I'm going to do for a start is just manually compress the ring and I'm going to slip it right into the top of the bore.
03:50 So now that it's located there, we can press the ring further down into the bore.
03:54 I've got a couple of options here.
03:57 There's a very cheap tool which I've got that allows the ring to be located in the bore.
04:03 And what we can do is just compress this to the correct bore diameter, place it over the top of the ring, allow it to expand out, and then we just want to gently press down.
04:13 And when we do this, what happens is that these little raised sections here on the ring locator, will locate on the outside of the bore, and this just means that the ring is installed down from the top of the bore as I was explaining, and we also ensure that ring is square.
04:31 Once we've done that we can make our measurement.
04:34 Now the other way we can do this if you don't have access to one of these ring locating tools, is to simply fit a ring onto a piston.
04:43 So in this case I've got one of the pistons that's going to be going into this engine, and in this case I've just fitted the second compression ring, we can also do this with the oil control ring.
04:54 And now if we just turn the piston upside down, we make sure that the ring is relatively central on the piston, and we just press down.
05:02 What we'll find is that the ring will support the piston, and this ensures again that the ring is located a fixed amount down from the top of the bore.
05:12 Now what we're going to do is grab our feeler blades, and we're going to use these to measure the ring end gaps.
05:19 So what we're looking for here is just a light amount of friction as we press the feeler blade through the ring end gap.
05:28 At the same time while we're making our initial measurement, it's really important to take special note of the existing ring gap.
05:36 What we're looking for here is whether that ring end gap is parallel.
05:40 It's very common to find that when we first install the piston rings that the end gaps aren't parallel.
05:46 And we're going to need to correct this during our gapping process.
05:51 Right so I'm just going to grab our feeler blades here, and I've got our 10 thou feeler blade, and I'm just going to press that through the ring end gap.
05:59 And what I find is as I do that, initially from the inside diameter of the ring, I can move that 10 thou feeler blade into the ring end gap, but as I move through the ring gap, it becomes tighter.
06:12 So this means two things.
06:13 The first is that our ring end gap is not parallel.
06:18 So as we move towards the outside of the cylinder, our ring gap actually narrows down slightly.
06:23 And it also shows us that we're approximately 10 thou at its largest at the moment.
06:29 So we're now going to correct that, and set our ring end gap.
06:34 Now in this particular engine, we're running a 3.4 inch bore, and if we were to set our ring end gap with a four thou per inch end gap, we're going to be aiming for somewhere around about 13.5, 14 thousandths of an inch.
06:52 So in this instance what I'm going to do is just show you the process of removing a couple of thousandths of an inch of material from that ring.
07:01 What we're going to do now is take our ring out of the engine block and we're going to fit it to our ring file.
07:06 We'll start with the electric ring file.
07:08 We want to make sure that the ring is installed square in the ring file.
07:12 We can use this also to help correct any error if our ring end gap wasn't parallel.
07:18 We can also adjust this particular file for different bore diameters and this is going to ensure repeatability when we're fitting subsequent rings into the file.
07:28 So what I'm going to do now is just allow a little bit of protrusion from the ring file and we'll lock that down.
07:34 Now what we can do is just wind the ring file until we just make contact with the grinding wheel.
07:41 And once we've done that we can zero the dial indicator.
07:46 Now the advantage here of the dial indicator is that we know exactly how much material we're removing.
07:53 So once we've got our dial indicator set, we can begin removing some material.
07:57 And there's a couple of ways we can do this, what we can do is just run the ring down against the grinding surface and then while the ring is against the grinding surface, we can turn the little dial on the end of our file and that's going to move the ring against the grinding wheel.
08:14 Again we're going to be taking note of the dial indicator.
08:17 Alternatively what we can do is make our adjustments to the ring file while the ring is off the grinding wheel here.
08:24 We can wind the ring file in and then move it against the grinding wheel.
08:29 If we're going to do that we want to make very small adjustments of perhaps a thousandth of an inch or less at a time.
08:35 Let's have a look at the process now.
08:37 So I'm starting at zero.
08:57 Ok so we've made a couple of small adjustments there.
09:00 I've made a cut there that's approximately two thousandths of an inch.
09:04 I've also gone some way hopefully there to remedying the fact that the ring end gaps weren't parallel.
09:10 Now even while I've got a dial indicator here, I'm always a big advocate of making small adjustments and frequently checking your work.
09:18 It's very very difficult to add material back onto the ring once you find that you've filed the ring end gap too large.
09:26 So it's always better to make small adjustments and frequently check your work and creep up on the correct ring end gap.
09:33 So let's install the ring back into our bore and we'll check our new measurement.
09:50 So once the ring's installed in the block, the first thing I'm going to be looking at is again whether or not my end gap is parallel, and straight away visually it looks a lot better than it was.
10:01 I'm going to take my feeler blades, again I'm starting with my 10 thou feeler blade, and this time we see that the feeler blade slips nicely between the ring end gap with absolutely no resistance.
10:12 So this means that we can step up.
10:15 Let's go up to our 11 thou feeler blade.
10:23 OK and our 11 thou feeler blade now is a nice snug fit there between the ring end gaps.
10:30 So what we've done though, that's the important part here, is we've corrected initially that lack of a parallel end gap.
10:37 We've now got exactly the same tension as we move through the ring end gap.
10:42 So the process from here is simply to continue.
10:45 We'll fit our ring back into our ring file, and continue removing small amounts of material until our ring end gap measures exactly what we want.
10:56 Let's move on and we'll have a look at the process being applied now with our manual ring file.
11:02 When it comes to manual ring files there are a variety of designs but essentially they all work on the same principal where we have a manual hand crank that we turn in order to move an abrasive wheel against our ring surface and remove material.
11:16 One of the key things we need to keep in mind when we are using a manual ring file, is the direction that we're turning the ring file.
11:23 It's critical to make sure that we always grind from the outside of the ring surface towards the inside.
11:30 A lot of rings will have a surface inlay on the outside of the ring and if we grind the wrong way, the grinding process can actually pull or remove this inlay from the ring and damage it.
11:43 The other mistake a lot of people make when they are using a manual ring file, is to squeeze the ring against both surfaces of the grinding material.
11:54 So instead of grinding one side of the ring, they'll tend to push the ring together and grind both surfaces together.
12:02 What this does is almost guarantees that we aren't going to end up with a parallel ring end gap.
12:09 So we need to make sure that when we're removing material with a manual ring file, we are only removing material from one side of the ring.
12:17 So the process begins the same by checking our initial ring end gap, making sure that our ring end gaps are parallel, and then we can account for any error we find while we're adjusting our ring gaps.
12:30 So let's have a look at that process now, we'll remove a little bit of material from the number one piston ring.
12:36 So what I'm going to do, normally this would be fixed to our workbench to ensure it isn't going to move around, but for our demonstration I've just got it sitting here.
12:45 What I'm going to do is use my right hand on the crank for the ring file.
12:50 And again we're going to turn this in an anticlockwise direction so that we are grinding from the outside of the ring to the inside.
12:58 We do have some little locating tabs here on the ring file to locate the ring.
13:03 And that's where we're going to apply some pressure.
13:05 I'm going to use my left hand here.
13:07 And my index finger just to locate the ring, and I'm applying a little bit of pressure down on the bed of the ring file, as well as pushing the ring into the stop.
13:18 As I'm doing this I can then move the ring in against the grinding wheel as I'm turing it, so let's have a look at that process now.
13:34 Again just like with our electric ring file, it's important to make small adjustments and then check our work in the cylinder block to make sure that we are remaining parallel.
13:45 So I've only made a very small adjustment there, I've only removed a very small amount of material.
13:50 So let's just try this in our number one bore.
13:55 And we'll again drop our ring down to the correct height, and check our ring end gap.
14:18 So again with a very small adjustment that I've made I'm expecting to still be very close to my starting point of 10 thou so we'll start with our 10 thou feeler blade.
14:29 What we find is that the 10 thou feeler blade slides in very easily to start with, so we're a little over 10 thou at the inside diameter of the ring.
14:38 As we move towards the outside of the ring, the end gap is still tighter.
14:43 So what this means is that our end gap isn't parallel.
14:46 So in subsequent adjustments that we're going to make with our manual ring file, I'm going to need to focus on removing material from the outside of that ring in order to get that ring end gap parallel.
14:58 Again just like with our electric ring file, the process is a case of making very small adjustments and measuring our work multiple times to make sure that we don't end up with ring gaps that are too large.
15:12 Once we've completed our ring gap filing, the other process that we need to go through is deburring the ring.
15:20 Now when we go through a grinding process, what this is going to result in is small burrs on the ring end gap and we want to remove these before we install them on the piston.
15:29 There's a couple of ways we can do this.
15:32 The electric ring file that we've just demonstrated actually has a deburring wheel on the end of it, however the other, probably more common way, is to simply use a small needle file.
15:44 So what we want is a very fine needle file.
15:47 And the process of deburring the ring is simply a case of taking the end of the ring that we have filed and we want to just run our file, our needle file, on a 45 degree angle to just break down the surface of any burrs that we can feel on the ring.
16:06 It's important here as well to make sure that we don't excessively remove any material, we want to remove just the burrs that are present on the ring end gap.
16:17 From here we're going to simply repeat the process on the remaining piston rings until all of our end gaps are correctly set.
16:25 Most of our work here is going to be focussed on the top and the second compression rings.
16:30 We'll find that usually the piston ring manufacturer will simply specify a minimum end gap for the oil control ring rails, and quite often straight out of the box, this end gap will already be larger than that minimum.