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Practical Engine Building: Step 5: Balancing Parts

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Step 5: Balancing Parts


00:00 - At this point we're ready to balance our engine components, this consists of our connecting rods and our pistons.
00:07 Because of the way the SR20 inline four cylinder engine works, we can balance these separately to the crankshaft.
00:13 So we're gonna start here with our JE forged pistons.
00:17 And I've got all of the components laid out on the bench.
00:19 Now obviously what we're trying to do here is achieve an overall balance with all of these components, the piston, the wrist pin, the two wire locks, as well as the ring set.
00:31 Typically in most instances the ring sets and the wire locks will all weigh pretty well exactly the same so we don't need to be too worried about those.
00:38 Of course we'll go through the process here and just prove that that is the case.
00:43 Now for this worked example, we are using a set of digital scales that will measure down to two decimal places or 100th of a gram.
00:50 We don't need to be anywhere near that fussy though.
00:53 All we're going to be doing here is rounding our results and we're going to be trying to achieve a balance of plus or minus a 10th of a gram.
01:01 Of course you can balance these components to whatever specification you're personally happy with.
01:06 So let's start by checking our ring sets.
01:09 We'll just try two ring sets here on our digital scales.
01:13 We can see that our first ring set there weighs 21.34 grams.
01:18 We'll try another one just to confirm that they are the same.
01:24 And we can see there 21.36 so we're within 200ths of a gram there.
01:28 Certainly not going to be worried about that.
01:31 Likewise we can check our wire locks, so we'll put a pair of wire locks on our scales.
01:35 And we've got a result there of 1.93 grams.
01:39 Try another pair of wire locks.
01:43 And our results there are within 100th of a gram so again we're not going to be worried about our wire locks either.
01:49 So let's get those out of the equation there, we'll remove our wire locks and we'll remove our ring sets.
01:58 Now we can get down to weighing our pistons and our wrist pins.
02:01 We've numbered our pistons, at this point we haven't numbered our wrist pins because if we do have a large discrepancy in the weight on the wrist pins we can mix and match these around our piston set to try and offset some of the imbalance that we have.
02:14 We'll start though by weighing our pistons on their own.
02:29 So with our four pistons weighed we can see that our heaviest piston is 313.1 grams and our lightest piston, a full gram lighter at 312.1 grams.
02:38 So we've definitely got a little bit of work to do there.
02:41 Let's go through and see what our wrist pins weigh up like.
02:56 So all of our wrist pins in this instance weigh exactly the same at 98.3 grams.
03:01 So we don't need to do anything with our wrist pins, they're not actually going to help us with our imbalance, so we can disregard those just as we've done with our ring set and our wire locks.
03:13 Alright so now the process comes down to removing weight from the pistons that are heaviest.
03:18 So in this case our lightest pistons remember is piston number two, 312.1 grams so we're not going to be doing anything with piston number one.
03:25 We're going to go through and remove weight from the other pistons.
03:30 So let's start with number one piston, remember this was 312.5 grams, so we're going to be removing 0.4 of a gram, 4/10th of a gram of material from that piston.
03:40 It is important to inspect our piston and decide where we're going to be removing this material from.
03:45 So we want to remove the material from a place that's not going to have any affect on our piston strength.
03:50 Generally this is going to be the inside of the piston skirt and the inside of the pin boss.
03:56 And we want to make sure that the material that we remove is as evenly as we can from these areas so we're not focusing our die grinder on one area for too long.
04:04 So let's go ahead and remove some material now.
04:33 So we've gone through there and we've removed enough material from number one piston to balance it so let's put it back on our scales now, we'll just confirm that, allow our scales just to zero, put the piston back on, and we can see that that comes in at 312.07 grams so we round that of course to 312.1 grams.
04:51 So our piston now weighs the same as our lightest piston, number two, 312.1 grams.
04:57 So the process is to remove material slowly, you wanna get a feel for just how much material we need to remove in order to make a change of perhaps a 10th of a gram.
05:07 And this will guide us, there's no use taking out too much material and finding out then we need to remove material from what was our lightest piston.
05:14 And we also want to make sure that we remove that material smoothly and consistently around the underside of the piston skirt.
05:21 We don't wanna focus our efforts in one place for too long.
05:24 So at this point we're gonna go through and we will balance the remaining two pistons.
06:49 So there we've got all of our pistons balanced to within a 10th of a gram.
06:53 We're now going to have a look at balancing our connecting rods.
06:57 In order to balance our connecting rods we've set up our conrod balancing fixture.
07:01 And the process we're going to go through here is to measure the big end weights of all of the rods, and we're going to find the lightest rod and we're going to be matching the weights or reducing the weights of the heavier rods.
07:12 And when we're going this, it is a little bit trickier with weighing conrods to get the sort of accuracy and repeatability that we get with our pistons.
07:20 So we tend to use a slightly broader range of tolerance.
07:24 In this case I'm going to be aiming for a half a gram imbalance maximum between the heaviest and the lightest rod.
07:31 So let's get started and we're going to begin by measuring the big end weights of all of the rods and I'm going to take five measurements for each connecting rod and we're going to discard the highest and lowest reading and we'll average the remaining three, this will give us the best chance of getting repeatability.
08:10 We've gone through there and we've weighed all four of our connecting rod big ends and we have got a reasonable variation here.
08:16 You can see that our two rods here that are heaviest, 385.4 grams each.
08:21 Our lightest rod 383.1 grams and then we've got one in between at 383.8 grams.
08:28 So we've got a little over two grams variation between our heaviest and our lightest so we've got a little bit of work to do here.
08:35 Now we do need to be a little bit mindful of how much material we remove here and sometimes we do need to balance the effects of how much material we will remove and any potential for weakening the rods, versus our desire to get our balance absolutely perfect across our set of rods.
08:53 We'll have a quick look here and we'll see what we can do to remove material from our rods to help with our balance.
08:59 What we're going to be doing here is removing material by putting a gentle chamfer on the sides of the rod here alongside the big end connecting rod bolts.
09:09 And what we want to do is make sure that the material is removed evenly from all four of these faces.
09:14 We want to remove as little material as we can to ensure that the strength is retained.
09:20 So what we'll do is go through a demonstration here.
09:23 We're going to start with number four rod, 383.8 grams, we know that our lightest rod is 383.1 grams.
09:31 I'm not going to be trying to match this to within a 10th of a gram as I've said.
09:35 I'm gonna be happy if we're within about half a gram here.
09:38 So let's go through that process now and we'll see how it's done.
09:49 So we've gone through there, we've made one round of modifications to our number four connecting rod and we've actually managed to get that weight exactly the same as our number three rod at 383.1 grams.
10:01 It wasn't our intention there remember for this particular set of rods, I'm trying to get that weight within half a gram of our lightest rod.
10:09 And when we're doing this as well, when we are checking our work, it is always a good idea to check frequently and make small adjustments to our rod weight, we don't wanna go too far and we also want to make sure again that when we're making those adjustments when we're removing material, we're doing it as evenly as we can across those four surfaces we've got to work with.
10:27 From here it's going to be simply a rinse and repeat of the process we've looked at, removing material from our other two heavy rods until we're all matched within half a gram.
10:37 Let's jump ahead and we'll take a look at what we're going to do once we've got all of the big ends matched and weighed.
10:43 When it comes to balancing the small end of the connecting rod, we don't need to use our conrod balancing fixture.
10:48 If we know that all of the big ends of the connecting rods weigh the same, then all we need to do is weigh the overall rod and any imbalance now will be located at the small end, we can use this to guide us with how much material will need to be removed.
11:02 So our first step here is to go through and weigh all four of our connecting rods.
11:15 So with our four rods weighed we can see our maximum imbalance is one gram, our lightest rod is number two at 528.2, our heaviest number four, 529.2 Number one's not far behind though at 529.0 grams.
11:29 So remembering here we're targeting an imbalance of no more than half a gram so we're going to need to remove some material from number one and also number four.
11:38 Now in order to do this, what we're going to be doing is remove material from around the outside of the pin boss.
11:44 We want to do this nice and evenly, we don't want to focus our efforts in one place, we want to remove as little material as we can, evenly around the whole outside of that pin boss, to ensure that we retain maximum strength.
11:57 So let's go ahead and do that now.
12:07 Alright so we've removed some material there.
12:09 As usual we want to make small adjustments and check our work frequently so let's take another measurement and see how we've got on.
12:17 So we can see there we've removed 6/10ths of a gram, we've gone from 529.0 down to 528.4 Now that brings us to within our half gram tolerance there so we're happy with that rod.
12:29 We can go on and complete the rest of the process.
12:31 We'd repeat this on number four rod until all of our rods are weighed in within that half gram tolerance.
12:38 Once this is complete, we can move on with the next step of our process.

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