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Practical Engine Building: Piston Balancing

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Piston Balancing

18.35

00:00 - When it comes to balancing the piston assembly for our engine, this is a task that's often left to the engine machinist, however it can be easily performed in the home workshop without requiring thousands of dollars worth of equipment.
00:13 What we are going to require if we want to balance our own pistons is a set of precision scales, and some way of removing material from the pistons.
00:22 There's a variety of techniques we can use to remove material from the pistons, depending on exactly how much we need to remove.
00:30 Here I've got a couple of die grinder options that we can use for removing material from the pistons.
00:36 And we've also got a power file or linisher which can be used for removing material quite quickly.
00:42 Now when we are balancing the piston assembly, what we're trying to do is achieve balance to within a 10th of a gram.
00:50 It's not strictly necessary to try and achieve any finer balance than that, and if you are trying to balance the piston assembly to within perhaps 100th of a gram, you're going to end up spending a lot of time chasing gains that really aren't going to make a huge difference to the performance of your engine.
01:08 If we can achieve balance to within a 10th of a gram, we're going to end up with a very smooth operating engine.
01:14 Now when we're considering that, this will play into the type of scale we need.
01:20 And what we need is a set of scales that can measure down to a 10th of a gram.
01:25 And it'll be useful if they can cope with around about two kilograms maximum weight.
01:31 The set of scales I've got here actually do measure down to 100th of a gram and we'll be just rounding this to a 10th for our demonstration here.
01:42 So I've laid out the piston assembly for a Subaru FA20 engine.
01:47 These are JE forged pistons and the entire piston assembly consists obviously of the piston, then we have the wrist pin, we have the wrist pin locks, and we also have the ring pack.
02:01 Now what we'll generally find is that the pin locks as well as the ring packs are normally quite well matched straight out of the box, and the main work we're going to need to do is on the piston assembly and also potentially on the wrist pins as we'll see.
02:19 Let's just turn on our scales and we'll allow them to zero, and we'll just have a quick look at the weight of our wrist pin locks.
02:27 So let's take two of those now and we'll place them on the scales.
02:30 We can see that they measure 1.91 grams.
02:33 I always like to make two measurements just to ensure that we are getting some repeatability, so 1.92 grams, and we'll try another pair of those locks.
02:44 Place them on the scales.
02:46 So we can see that essentially there is no difference with our wrist pin locks.
02:52 We're balanced there across those two sets, to within a couple of hundredths of a gram so we definitely don't need to worry about those.
03:00 Let's have a look at a couple of our ring packs.
03:05 So our first ring pack measures at 21.32 grams, I'll try again.
03:12 And 21.31 grams, we'll try another ring pack there from another piston.
03:18 And we can see 21.33 grams.
03:24 21.33 grams.
03:26 So what we can see there is both the ring pack as well as our pin locks are very very well matched, where both of those components are within a couple of hundredths of grams, so we really don't need to be too fussy about these.
03:40 Of course I'd still recommend measuring them and if you are seeing significant differences across those components then we can factor that into our balancing.
03:49 The important thing to understand here is what we're trying to achieve.
03:54 And what we're trying to achieve is a matched balance across all of those components, so we can factor in the components that we need in order to achieve that balance.
04:03 Obviously if all of our ring packs and all of our pin locks are measuring exactly the same, we don't need to take those into consideration, we can focus just on the piston and the wrist pins.
04:15 Now the point to note here is if we are measuring a difference in our ring pack weights, it's important to make sure that our balancing work is performed after the ring end gaps have been set.
04:29 And the reason for this is during the ring gapping process we are going to be filing or removing material from the rings, and this understandably will have a very minor impact on their mass.
04:41 So if you are factoring those into your equations, make sure that you have gapped your rings first.
04:47 So in this example we're going to focus solely on our pistons as well as our wrist pins.
04:52 So right now I'll just remove all of the other components out of the equation.
05:02 And now what we can do is go through and make a first weighing on our pistons.
05:06 I've just arbitrarily numbered the pistons as well as the wrist pins here before I've started using a vivid or sharpie marker.
05:14 This just means that while we're going through the weighing process or the balancing process, I'm going to know which piston is which, and which wrist pin is which.
05:24 So let's go through now and weigh our components, we'll just turn our scales back on.
05:29 Allow them to zero, we'll weigh our first piston.
05:45 So we can see there we measured 317.86 grams, and then 317.87 grams.
05:53 Remember here we're only balancing to a 10th of a gram so I'm going to round that and that becomes 317.9 Let's measure our second piston now.
06:22 So our second piston weighs exactly the same at 317.9 grams.
06:29 We'll try our third piston.
06:44 So our third piston weighs 317.4 And now our fourth and final piston.
07:06 And our fourth piston weighs 317.7 So now with our pistons weighed we can see that they're actually relatively close.
07:14 We've only got a difference there of 0.5 of a gram from our heaviest to our lightest piston.
07:21 Now what we're going to do is go through and perform the same process with our wrist pins, so let's have a look at that now.
07:39 So our first wrist pin weighs 132.8 grams.
07:58 Our second is 132.6 Our third weighs 132.7 And our final pin weighs 132.5 OK so at this point we have a couple of options.
08:44 What we can do here is choose to balance each of the components individually.
08:48 By that I mean we can balance all of the wrist pins to match the lightest wrist pin, which in this case weighs 132.5 grams.
08:57 So we can simply remove some material from the heavier wrist pins.
09:01 Now if we choose to do this, what it means is that we can mix and match the wrist pins between different pistons when we assemble the engine, and if we are going to remove material, what we're probably best to do here is use a carbide bit on our die grinder, and we want to remove material from the inside diameter of the chamfer on the wrist pin.
09:22 Now the reason we're going to remove material from here is because this is a low stress area of the wrist pin so it's not going to affect the strength of the wrist pin.
09:32 And simply what we want to do is run our die grinder gently around the inside diameter of the chamfer in order to smoothly remove material.
09:41 You always want to make sure when you're doing this process that we don't leave any sharp edges that can form a stress razor, and we also want to balance the amount of material we're removing from one side and the other, so we don't want to perform all of our balancing by removing material from one side of the wrist pin.
09:59 Now the other option we can perform here, and really our decision is going to be based on the weights of the pistons and the pins, is we can balance the piston and wrist pin assembly as a pair.
10:13 Generally once we've got the wrist pins fitted to a piston they're going to remain with that piston.
10:19 If there's a subsequent teardown of the engine, we're going to fit the same wrist pin back into the same piston, so they're going to sit as a matched pair anyway.
10:28 And what we can do here is help balance our components by matching the heaviest wrist pins with the lightest pistons.
10:37 In this case there's not a huge amount of work to do because as we've already found our piston weights are very evenly matched anyway.
10:44 We've already got two at 317.9 grams, and we've only got half a gram difference from our heaviest to our lightest.
10:53 This is my preferred technique though, I like to match the pistons to a wrist pin and then make the balancing adjustments on the actual piston itself rather than the wrist pin.
11:05 So let's look at how we can do that.
11:07 If we note here our heaviest wrist pin was number one, 132.8 grams.
11:12 What I'm going to do is match that with piston three.
11:17 And piston three remember was our lightest piston at 317.4 grams.
11:23 Our next heaviest wrist pin is 132.7 grams, pin three, and I'm going to match that to our piston number four which is our next lightest piston at 317.7 grams.
11:43 From here we've got wrist pin two and four left, and doesn't really matter which piston we put these with because both of the pistons weigh the same, so we'll just match those to our pistons.
11:55 Now that we've done that what we can do is go through and perform an overall weighing of our piston and our pins so let's go and do that now, we'll start with piston and pin number one.
12:19 So we get 450.4 grams there.
12:26 Piston number two.
12:40 Piston number two comes in at 450.3 Try piston number three now.
13:02 Which weighs in at 450.1 And finally piston number four.
13:13 So piston number four comes in at 450.5, I'll just double check that.
13:22 450.4, I'll just make a third measurement.
13:35 So if we look at our total weights now, we've got two pistons that measure in heaviest at 450.4 grams.
13:42 Our lightest piston measures at 450.1 grams.
13:47 So our process here is simply to remove material from the heavier pistons until we match our piston number three assembly of 450.1 grams.
13:57 So what we want to do is remove some material.
14:00 Let's have a look at doing that on one of our pistons, and we'll start with piston number four here.
14:06 Piston number four we need to remove 0.3 grams.
14:12 So it's important to understand where we should be removing the material.
14:16 Obviously when we are removing material to balance the pistons, we want to be very careful that we aren't affecting the strength of the piston.
14:24 We don't want to do anything that could adversely affect the piston in operation.
14:29 So what this means is that there's no ability for us to remove material from anywhere on the crown of the piston, nor on the skirt, that's going to affect its operation.
14:39 And what we want to do is look at the underside of the piston and inspect where we can remove material.
14:46 Now this will depend to a degree on the piston you're dealing with as well.
14:50 On these JE forged pistons what I'm going to be able to do is simply add a chamfer to the underside of the wrist pin boss, and that's going to be the predominant place I can remove material without affecting strength.
15:05 I want to be very careful because the actual underside of the wrist pin boss will affect the strength, so all we want to do is add a slight chamfer there on the edges, take off those sharp edges.
15:17 On a factory piston on the other hand, quite often what we will find is that there are specific balancing pads on the underside of the piston skirt that are designed to be removed for the purposes of balancing.
15:30 OK so remember we wanted to remove in this case 3/10ths of a gram.
15:35 What I'm going to do is just use my die grinder with a carbide cutting bit and we'll do some light chamfering and see what our results are.
15:43 Obviously when we're doing this process, it's always a good idea to make small adjustments to the piston weight and measure constantly.
15:52 There's no point in removing a large amount of material then finding that you've gone too far.
15:57 So it's all about making a small adjustment, seeing how much affect that's had on the piston weight, and that's going to also guide you as to how much more material you need to remove.
16:08 Let's just have a look at that piston weight again and piston number four weighed in at 317.7 grams.
16:16 OK so our target here is 317.4 grams.
16:21 Let's remove some material and see what we can do.
16:59 So we've removed some material from our piston now and what we want to do is weigh it again and just check exactly how we're getting on, how much more material we need to remove.
17:09 Remember we started at 317.7 grams, we're trying to remove 3/10ths of a gram of material in total in order to balance this to our lightest piston.
17:20 Let's put it back on our scales and see how we got on.
17:25 So we can see at this stage we've removed just under a 10th of a gram.
17:29 So we've got approximately 2/10ths of a gram left to go.
17:33 So the process here is simply to repeat what we've done, make small adjustments to the piston weight, and continually check on our work until we've matched the set of pistons.
17:44 Now in the particular technique I've used here where we have matched the wrist pins to the piston, it is obviously essential for balance that that wrist pin remains with that particular piston, so after this process is complete, it's a good idea to engrave the piston location on the end of each of the wrist pins so that there's no chance of getting these confused.
18:09 Another tip here as well just to avoid any stress raises, once we have completed our balancing work on the piston it's a good idea to just go back over with a light sanding roll and smooth the surface finish.