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Practical Engine Building: Step 5: Balancing Parts

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Step 5: Balancing Parts

12.44

00:00 - In this step we're going to be looking at balancing our piston and conrod assemblies.
00:04 Now this step has actually been completed a little bit out of sequence.
00:08 Because we're dealing with a V8 engine here, the weight of the piston and conrod assemblies needs to be taken into account by our engine machinist when they're balancing the crankshaft.
00:19 This goes into a bob weight calculation and this is a mass that is physically attached to the crankshaft journal during the balancing process.
00:28 Alright we're going to start with the balancing of the pistons and this really includes the pistons, the wrist pins, the spiralocks that hold the wrist pins into the piston and of course the ring set.
00:39 In this case, as in most instances, the weight of the ring sets and the spiralocks are irrelevant, they're all essentially identical.
00:49 We'll have a quick look at a couple of our wrist pins just to see how they vary so let's turn on our scales here.
00:55 We'll just look at two of these at the moment and just see how close they are.
00:59 We've got 105.8 grams for our first one.
01:04 We'll just use another one here and have a look at that.
01:07 And 105.7 grams so really when we're talking about our wrist pins here being within a 10th of a gram, I'm not concerned about that at all, that's close enough and what we're actually going to be trying to do here with our balance of our pistons is we're going to be looking to balance them to within half a gram.
01:24 Now this is a little bit of a broader range or broader tolerance than what you would have seen in the practical skills section of the course and the reason for this is we're now dealing with a piston that weighs around about 450 grams.
01:36 So we're looking at a percentage of that and there's no need to be balancing these down to a 10th of a gram, we can be a little bit coarser.
01:44 Of course you can choose to use whatever tolerance you like.
01:48 So let's go through here and we'll take an initial weighing of all of our pistons.
02:08 Alright we've gone through and weighed all of our pistons there and they're actually really close straight out of the box from Wiseco.
02:13 In this case our lightest piston number three is 441.0 grams, we've got number six matches that exactly as well and our heaviest piston, number eight there is 442.0 grams.
02:26 So we've only got one gram variation across that set.
02:29 So what we're going to do here is highlight the pistons that we are going to remove a little bit of material from and in this case piston number two, piston number five, piston number seven and piston number eight, we're going to remove a little bit of material and see if we can get those down to within half a gram of our lightest piston.
02:46 So let's go ahead and do that now.
03:14 Alright so we've gone through there with our number two piston, we've removed some material and we can see that our weight now comes in at 441.3 grams, so we're within that half a gram tolerance that we're looking for.
03:25 Now in removing material from the underside of this piston, we're always looking for areas where we can remove that material without adversely affecting strength.
03:33 And we've only needed to remove a small amount of material, around half a gram from this so you can see there isn't a lot of material I have removed but I've done so evenly, not focusing it all in one area just to make sure we're retaining as much strength as we can.
03:48 So with our first piston balanced, the remaining balancing process is just a repeat of what we've looked at, so we're going to jump ahead now and we'll see how we can finish off once we've used our die grinder to remove material from our heavy pistons.
04:02 At this point we've gone through and we've got all of our pistons balanced.
04:05 As we can see here, we've now got a lightest piston still 441.0 grams, our heaviest piston, number eight, 441.5 grams.
04:13 Everything's falling within that half gram tolerance that we're shooting for.
04:17 There is one last task that we want to complete here though.
04:20 What we can see is that once we have used our die grinder on the underside of our piston, we end up with a slightly rough surface finish here.
04:28 Now that rough surface finish can end up becoming a stress raiser that could end up promoting crack formation.
04:36 So we want to smooth that surface off.
04:38 What I'm going to do in order to smooth that surface on the piston is just use a fine sanding roll on a Dremel die grinder.
04:46 So this isn't going to remove much, if any material.
04:50 Of course we can still check and confirm that our balance hasn't been affected.
04:53 All we're trying to do here is smooth that surface finish and remove any of those rough marks that could promote cracking.
05:00 Let's go through and do that now.
05:12 Alright so with that task completed, we can see how much smoother the underside of the piston is where we've removed that material.
05:19 We'll just check our weight here and make sure that that hasn't changed appreciably.
05:24 You can see that we are measuring exactly the same at 441.3 grams so that hasn't affected our balance or our weight at all.
05:32 So we'd then of course go through and we'd complete that same task on any of the remaining pistons that we have balanced making sure that they're all smooth and there's no areas of stress raises.
05:42 With our pistons all balanced we'll move on and we'll have a look at our connecting rods.
05:47 We've got our conrod balancing fixture set up here and we're going to go through and initially just take a weighing of each of the big ends of our connecting rods.
05:54 I've also marked each of the connecting rods, both the body of the rod as well as the caps so that we can keep track of which rod is which.
06:02 Now in this case, we're going to be trying to match the big end weights to within half a gram.
06:06 So let's get started, we're going to take five measurements on each rod.
06:10 We'll discard the highest and lowest and we'll average the other three.
07:25 So we've gone through, we've taken our measurements of each of the connecting rod big ends, we've averaged our results and what we can see is that our heaviest connecting rod is number seven here at 464.5 grams.
07:36 Our lightest is number eight at 463.2 grams.
07:40 So we've got a variation from heaviest to lightest there of 1.3 grams.
07:45 Now in itself that's actually already pretty good but we're going to see if we can get that just a little bit tighter.
07:51 So what we're going to do is remove some material here from the big end of number seven connecting rod and we're going to see if we can match that down a little bit closer to our number eight rod.
08:00 Again remembering here we're aiming for a tolerance of half a gram.
08:05 It's important to have an idea where we are removing the material from here.
08:09 And we've got a couple of options on the rod.
08:11 First of all we're going to be adding a chamfer to the side of the rod where the connecting rod big end bolt goes and if we can't remove enough material from that position there we can also remove some material from these little pads on the connecting rod as well.
08:28 Everything we're doing here we're trying to remove material in a way so that it's not going to affect the strength of the rod, obviously that's paramount.
08:36 So let's get going with our linisher now and we'll take a small amount of material off the rod.
08:55 Any time we're balancing our connecting rods or anything for that matter, we want to make small adjustments, we want to remove a small amount of material and check our work frequently so that we don't overshoot our target.
09:05 So we've made a small adjustment here to all four edges of the connecting rod and we can go ahead and take another measurement and see how close we are.
09:20 Alright so with that adjustment there, we've got our conrod back on our scale so we can see that we're at 463.5 grams.
09:27 So we're straight away right within our tolerance there of half a gram.
09:32 So the process we're going to continue now with is just a repeat of what we've looked at there.
09:37 All of the rods that fall outside of our half gram tolerance, we're simply going to be removing material as we've just seen until they're within that tolerance.
09:46 We're going to jump ahead now and we'll have a look at how to deal with balancing the small end once all of our big ends are within tolerance.
09:54 So we've gone through at this point and we've balanced the big ends of all of the connecting rods and we can see we've got our final weights now.
10:01 So 463.3, 463.6, 463.5, 463.5, 463.4, 463.6, 463.5 and our lightest rod, 463.2, so all eight of our connecting rods are right within that tolerance of a half a gram that we wanted.
10:21 The next step is to balance the small ends.
10:24 Now we don't need our conrod balancing fixture in order to do this.
10:27 If all of the big ends weigh the same, then we can simply weigh the rod overall and any imbalance now will be on the small end of the rod which is where we're going to remove material if we've got rods that are too heavy.
10:40 So let's go through now and we'll weigh each of our connecting rods.
10:59 Alright so we've gone through there and we've weighed our rods and we can see that the results are actually pretty good.
11:04 Our lightest rod there is 649.5 grams, we've got that on rod four and on rod five, our heaviest weight there, our heaviest rod there is number six at 650.6.
11:17 So actually with the exception of our number six rod, all of the other rods already are within our half gram tolerance.
11:24 So we don't actually have a lot of work to do here which is great.
11:27 What we're going to do now is remove some material from the small end of number six so that we can get that back down to within our tolerance.
11:35 Now when we're doing this we do need to obviously be very careful where we're removing that material from so we're going to be using our linisher to just smoothly remove material from the outside of the pin boss.
11:46 We want to remove that material as smoothly as we can right around the circumference, we don't want to focus our linisher on one place as this can end up impacting the strength of the rod.
11:58 So let's get our balancing underway.
12:18 So at this point we've got our number six rod down to an overall weight of 650.0 grams.
12:24 So this brings us within our tolerance for our full set of rods.
12:28 Our job's done, all of our components are balanced and we'd now be able to ship these off to our machinist so that the crankshaft can be balanced with accurate weights for the pistons and the conrod assembly.