Practical Engine Building: Step 5: Balancing Parts
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Step 5: Balancing Parts
26.43
| 00:00 | - The next step of our process is to balance our engine components and while we have left the balancing of the crankshaft itself to our engine machinist, as we've discussed during the body of the course, this isn't something we can do in the home workshop. |
| 00:13 | We are going to undertake the balancing of our connecting rods and our pistons, as these tasks are relatively straightforward. |
| 00:21 | Now we're going to start here with our pistons and we've got our set of scales set up here. |
| 00:28 | And we've got all of our piston components laid out because of course it isn't just the pistons we're worried about, it's actually the overall mass and that includes the pistons, the wrist pin, the ring set and the wire locks. |
| 00:40 | Typically i find little to no variation across the ring sets and the wire locks, sometimes we can see a variation with our wrist pins so it's always a good idea just to get an idea of these components as to whether or not we can discount them and just focus on the piston itself. |
| 00:57 | So let's just start here with our ring pack. |
| 00:59 | So we've got our ring pack for our number one piston and we'll just lay this onto our scales and see what we've got here. |
| 01:06 | You'll notice here I am using a set of scales that actually measure down to 1/100th of a gram. |
| 01:10 | That's much more precise than we need it to be but this will work nicely for our purposes and gives you a better sense of exactly what the component weight is. |
| 01:20 | So 20.8g there, 20.80 to be precise and we'll put on our next set of rings. |
| 01:27 | And 20.76 so while we're measuring down to 1/100th, I'm really rounding here to a 10th so those two ring sets, exactly the same and if we'll continue, that's what we'll find. |
| 01:37 | So what this means from our ring set perspective, we don't need to worry about that. |
| 01:42 | Let's have a look at some wire locks as well and we'll see if the same holds, so we'll put two wire locks here for our number one piston, 1.92g, obviously 1.9 when we round, and we'll take another set of wire locks here, and 1.91g so we're within 0.001g there so as expected, no need to worry about those components. |
| 02:03 | We do however see occasionally some variation in our wrist pins which we can then use in conjunction with the differences in our piston weights to help our overall balance so we'll see how these all go so we'll start with number one here. |
| 02:17 | 132.4g. |
| 02:24 | Again 132.4. |
| 02:29 | 132.45 so we would round that to 132.5g. |
| 02:38 | And 132.4 so we've got a variation of just under a 10th of a gram essentially across our set of wrist pins so again very very minor. |
| 02:48 | If we want to get fussy or depending how extreme we want to get with our tolerances there we could use these to our favour, when we're basically within a 10th of a gram there, I'm not particularly concerned about that variation so for simplicity we're going to disregard those so let's get into our pistons themselves. |
| 03:06 | Now what I have done here you'll notice I have just marked with a sharpie on the underside of the piston, the piston number and this just allows us to keep consistency as we go through the build process. |
| 03:17 | This is a personal preference, I usually use a sharpie here, you can also use a dremil to actually engrave the piston number, obviously a little bit more permanent. |
| 03:26 | But once we fit it to the rod that's really not a big concern so let's go through, we'll weigh the pistons and mark down our weights. |
| 03:42 | Alright we've initially measured our pistons as they've come out of the box and we can see number 2 piston's our lightest there, 309.5g and they're all relatively tight, we've got our number 1 piston there is our heaviest at 310.5g so all 4 are within 1g in weight so that's a pretty good starting place and in a lot of applications we could run these exactly as they are. |
| 04:06 | However we like to increase or improve that tolerance and get the balance across that piston set a little bit tighter if we can and this leads us to the next question, what sort of tolerance should we be shooting for? This really comes down to the individual, the application that you're using the engine for, the RPM limit among other things and generally I like to be within about half a gram total in balance so with our lightest piston obviously being 309.5g, we want our heaviest to be no more than 310.0, that's the sort of range, 0.5g total, so not 0.5g plus or minus for a 1 gram total, I'm looking at a 0.5 g total imbalance. |
| 04:48 | So that's what we're going to be shooting for. |
| 04:50 | One of the points that is quite often made when we release information about balancing pistons and rods is what's the point in trying to be too precise with our balance when the components will be affected by the weight of oil clinging to them in operation. |
| 05:04 | And we obviously can't control that and there's likely to be some difference from one piston and rod to the next. |
| 05:10 | And while yes that's absolutely true, my argument with this is always that we want to try and control the things that are within our control, we can control really accurately the base weight of our pistons and connecting rods so if that's within our control we may as well get it as close as we can, it doesn't really take much more effort on our part to improve the tolerance when we are balancing those components so that's my personal take on it but again it's up to the individual, the process however, irrespective of what tolerance you're prepared to accept is exactly the same. |
| 05:43 | Next we need to look at our pistons and decide where we can safely remove material because obviously we don't want to adversely affect the strength so let's get the piston under our overhead here and we'll have a quick inspection. |
| 05:55 | And with most aftermarket pistons particularly those from JE which we use a lot here, we generally see there's quite a generous amount of material left on the inside of the pin bosses here on these 4 corners that I've just pointed out. |
| 06:08 | And if we tip the piston over, the important point to note here is that this gives us the ability to remove material without adversely affecting the thickness there down to the wrist pin or the wall thickness which then denotes the strength. |
| 06:21 | So those are the places I'm going to be focusing on and we know that we only really need to remove about 0.5g maximum from our heaviest piston to it's not a lot of material that we're going to need to remove and we should be able to do that reasonably easily. |
| 06:37 | We're now going to go through that process. |
| 06:39 | In order to do that, I'm using an air die grinder with a carbide burr on the end of it, I've selected one that has a rounded nose profile so that any material I will be removing will be removed nice and smoothly. |
| 06:50 | So let's go ahead and do that now. |
| 07:14 | Alright we've got our first piston balanced now, piston number 1 has come down from 310.5 to 309.8 so we're now within 3/10ths of a gram of our lightest piston number 2. |
| 07:26 | Now the process of doing this as well, particularly when you're just getting started, you're not going to necessarily have a good guide on how much material you need to remove so obviously it should go without saying here, we want to start by removing a small amount of material, take multiple measurements as you go, work slowly, check your work, it's always easy enough to remove more material if you find that your piston is still heavy. |
| 07:51 | If you shoot past your target weight and end up with a piston lighter, that can end up with you chasing your tail removing material from what was previously the lightest piston so it's just a case of having a little patience and working slowly. |
| 08:03 | In particular with our number 1 piston, you can see that I did end up actually removing material slightly down the strut that connects our wrist pin boss out to the skirt of the piston as well and again just doing that equally all around the piston and only removing a small amount of material, that gets our balance where we need it to be without adversely affecting our strength. |
| 08:24 | From here we still need to remove a little bit of material from our number 3 piston, number 4 is within my tolerance there at 309.9g anyway, we're 4/10ths heavier than number 2. |
| 08:37 | So we need to remove essentially 3/10ths of a gram at least from number 3, let's get that done now. |
| 08:54 | Alright we've got all 4 of our pistons now balanced to within our tolerance of 0.5 g, our heaviest is number 4 untouched at 309.9 and of course number 2 our lightest at 309.5, number 1 and number 3 the ones that we did remove material from, both come in at 309.8g so relatively easy process to complete. |
| 09:15 | Now depending on the quality of the burr or the coarseness of the burr that you are using the remove that material, this can end up leaving some sharp edges and that's not desirable on a stressed part like a piston so a nice way of just cleaning up those marks when you've finished is to use a die grinder with a sanding roll on it and that won't really remove enough material to impact on the balance of the pistons but it is going to allow you just to smooth over any rough surfaces. |
| 09:42 | Let's get our pistons out of the way now and we can move on with our conrods. |
| 09:46 | When it comes to balancing the conrods, our task is a little bit more complex because as we've already learned in the body of the course, it's not just a case of balancing the overall weight of the rod, we do need to separate out the big end weight with the overall weight so that's where our conrod balancing fixture is coming into play. |
| 10:03 | Now with the rods themselves, we actually end up getting some information from Boostline on the individual weights of all of the components of the rod, the big end, the small end and the overall weight, along with some dimensional measurements as well. |
| 10:18 | And Boostline actually provide a serial number on each of their rods so we can see that they're marked on both the cap and the body of the rod so we can align that with their relevant information on that particular card so we'll be able to actually check and see how closely that matches up with what our measurements are. |
| 10:37 | Just in terms of marking those rods, while, as I've just mentioned, the Boostline rods do have a serial number, that's not overly relevant or useful for us when it comes to assembling an engine, in this case the rod's numbered 248. |
| 10:50 | So we do need a number on our rod that indicates where it's going to go in the engine. |
| 10:54 | If we turn the rod over we can actually see in this case, our machinist has done this for us, they've used an engraver to mark the body and the cap of the rod and this just ensures that there's no chance of us mixing and matching between different caps and bodies. |
| 11:09 | These rods when they are manufactured, they are machined and then honed as one piece so what this means is we cannot exchange the rod cap and body between different rods, we're going to end up with dimensional inaccuracies, particularly with our bearing clearances so that's a big no no. |
| 11:26 | When it comes to how we mark those though it's just worth discussing briefly. |
| 11:29 | The engraving technique is probably my preferable, it gives us a permanent mark and it doesn't negatively affect the strength or reliability of the rod. |
| 11:39 | One technique to stay away from is using metal stamps, those can negatively impact on the rod. |
| 11:45 | The other technique which I do use from time to time is simply to use a sharpie marker. |
| 11:50 | Now that'll give us a mark that we can use during the assembly process. |
| 11:55 | Obviously it's not permanent though and particularly we can end up easily washing that off using brake clean so comes down to a bit of common sense there and a bit of personal preference. |
| 12:05 | Now again as we've looked at during the body of the course, when it comes to using a conrod balancing fixture like this, they are a little bit fussy to get good repeatability and obviously we want that repeatability in order to really be assured of what our masses are. |
| 12:19 | Now the way I'm going to use this is no different to what we've seen in the body of the course, we're going to take 5 measurements of each big end weight, we'll be discarding the highest and lowest measurement and we'll be then averaging the remaining 3. |
| 12:32 | It is important to mention though, even when we're doing this, if you're seeing a massive variation from one measurement to another then that's a bit of an alarm bell that something's not right. |
| 12:42 | Generally we should probably expect to see, if we're using this correctly and we've got good quality scales, we should see no more than a variation of maybe 3/10th of a gram or thereabouts, something like that so if you're seeing a full gram or 8/10ths of a gram difference from one measurement to another, stop and reassess what you're doing. |
| 12:59 | So what we'll do here, we'll get our scales zeroed and I've already gone ahead and roughly set up our fixture. |
| 13:06 | What we want to do is make sure that when the rod is hanging, it is horizontal. |
| 13:10 | I want to make sure that the little hanging arm on our fixture also hangs perfectly vertical so we're not basically making that move where we've located the rod. |
| 13:20 | We also want to make sure that the rod is located laterally as well so it's not skewed over, that's going to again give us the best chance of getting good results. |
| 13:30 | So let's go ahead and we'll get our rod set up on our fixture here and we can take our first measurement. |
| 13:43 | Alright we can see our first measurement there, it's just moving around a little bit, 458.2 once it's stabilised, 458.1 I should say once it's stabilised so we'll write that down. |
| 13:58 | We're now going to go ahead, remove the rod from the fixture, reinstall it and take 4 further measurements. |
| 14:15 | Alright we've got our 5 measurements taken there and we can see that our lowest there, we've got 2 at 457.8 excusing my dyslexia there on our third reading which should in fact read 457.8. |
| 14:27 | And our highest, 458.2 so not too bad there, we've got 4/10ths of a gram variation between them so what we're going to do is eliminate here our lowest, 457.8 or one of our lowest and we'll eliminate our highest here, 458.2. |
| 14:45 | So what we want to do there is take out our calculator and let's go ahead and average those remaining 3. |
| 14:56 | Alright as we can see there, averaging our 3 measurements, we've got 457.96, 457.97, we are working to one decimal place there, when we round that we actually end up at 458.0 so it's a little tricky there with the number of decimal places and the accuracy that we can work to but in our case, 458.0, that's the average for our first conrod big end weight. |
| 15:26 | Let's go through now, we'll repeat this process and speed this up and we'll check our remaining rods. |
| 16:16 | Alright we've got all of our big ends weighed there and we can see we've actually got a bit of a disturbing trend here. |
| 16:23 | We've got number 1, number 2 and number 4 rods all within, in this case 3/10th of a gram which is pretty good. |
| 16:32 | Then we've got this outlier here of number 3 which is sitting at 456.5 so couple of grams there lighter essentially than our other 2. |
| 16:42 | Let's continue now, we'll now remove the conrod balancing fixture from our scales and we can measure the overall weight of each of those rods so we've got a better picture of what's going, we can see where that discrepancy on our number 3 rod is coming from. |
| 17:06 | Alright so we've got our overall weights all measured there and we've got a pretty good match between number 1, number 2 and number 4. |
| 17:14 | 666.3g for our heaviest 665.2 so we're within about a gram there which is about what we'd expect for a production aftermarket conrod. |
| 17:26 | This number 3 conrod though, still a little bit worrying there, 663.7g so significantly lighter than the remaining 3 and what is interesting as well is if we actually compare this to the card that came from Boostline, that's not the story that the card tells us. |
| 17:45 | First of all before we dive too deep into this, we need to just understand the specifics of the numbers, and we can see if we look at the total mass on the card from Boostline, the numbers that they're measuring are actually quite different to our 662 down to 661.5 so they've got everything within 0.5g. |
| 18:06 | We're measuring obviously significantly heavier, a few grams heavier on all of rods and that much big a variation particularly with number 3 so what I want to talk about there is the actual numbers and our scales here are almost certainly going to be a cheaper alternative to what I'd have liked to think Boostline are using at their laboratory so the actual number, probably is not that accurate from our perspective, I would trust the Boostline numbers normally over and above what we're getting from our much cheaper scales. |
| 18:39 | The important thing from our perspecitve here, because we're not calculating a bob weight, the actual weight of the rod from our perspective is not that critical. |
| 18:48 | What we want to be able to do is make sure that every time we put the rod on our scales we get the same number. |
| 18:53 | The repeatability, particularly when we're looking at overall weight, that does work out really nicely with our scales so I just want to clear that up, the fact that for example on number 1 rod, 248, we're measuring 665.2 which in this case on the Boostline rod sheet for the same number, 661.5 so quite a significant variation, I'm not too worried about that variation. |
| 19:17 | What I am worried about though is this number 3 rod, 077 serial number which is measuring a good significant amount lighter than our other rods. |
| 19:27 | And most of that imbalance, most of that difference there as we can see is made up from the error in the big end weight. |
| 19:35 | If we look at number 77 on our conrod sheet though, everything lines up pretty much perfectly there, 454.5g on the rotating mass and 661.5. |
| 19:47 | That's simply not the case. |
| 19:49 | And this is one of those awkward situations where trust but verify really comes into play. |
| 19:54 | 99.9% of engine builders, particularly at the hobbyist level, would take Boostline on their word, believe the numbers on that card, never check any further and you're going to assemble an engine with an imbalance in the rod of a couple of grams, actually a little bit more than that. |
| 20:10 | In our case though, we've found that there is an imbalance so we're going to have to fix that. |
| 20:16 | Now this comes to the next point, how much material can we feasibly remove from a rod in order to balance it. |
| 20:23 | We're in this awkward situation here where we've got one outlier that is significantly lighter than the remaining 3 and in this case about 2.5 odd grams difference between the heaviest and our lightest. |
| 20:35 | Normally I don't like to try removing much more than about a gram of material. |
| 20:39 | A lot of this does come down to the design of the rods though and with some rods with a lighter construction, without as much excess material, we could find that trying to remove 2.5g of material to balance it, could end up negatively affecting the strength of the rod. |
| 20:54 | Obviously that's not what we want there. |
| 20:57 | So a little bit of compromise and a little bit of understanding is really important there and sometimes we may need to compromise our overall balance of our rods in order to make sure that we are maintaining strength. |
| 21:09 | In our case, these Boostline rods however are actually quite a chunky rod and there is quite a bit of material on them. |
| 21:17 | And I have actually discussed this issue with Boostline. |
| 21:20 | Typically what I would do in this case is go back to the manufacturer and say hey look, we've got a set of rods here that don't match what your card says, the imbalance is greater than I'm comfortable with, most often, that's going to result in them sending out a new set of rods. |
| 21:36 | Interestingly in chasing this up a little bit further and going through the issue with Boostline what we found was that we have 3 rods from one batch of manufacturing and our 3rd rod, our 4th rod I should say, that number 77 which is light, comes from a separate manufacturing batch and I guess that goes a long way to explain the scenario we've got. |
| 21:55 | It shouldn't have happened, I'm not particularly happy about the fact that the card also says everything's just fine. |
| 22:01 | But that's life and that is where we are at. |
| 22:04 | So as I've mentioned, these rods are quite chunky and we'll just get this under our overhead shot here and we'll just have a look at some of our options in terms of where we can remove material. |
| 22:14 | So first of all we've got these areas on each side of the rod. |
| 22:18 | Basically along the parting line here and we can easily remove quite a bit of material from these and obviously that's on both sides. |
| 22:25 | We also have the ability to remove some material from these two points here and removing material there is not going to adversely impact on the strength of the rod. |
| 22:34 | Lastly, and this is not my favourite place to remove material, the two ridges here on the back of the cap. |
| 22:42 | These, after talking to Boostline, have purposefully been over engineered to provide a position to remove material for the purposes of balancing. |
| 22:51 | So this is one of those areas where we do need to be mindful, I see a lot of people removing material from these ridges without understanding that they are actually engineered into the design to improve the rigidity of the cap so if we remove too much material from these areas, this is going to adversely affect our strength. |
| 23:09 | In this case, some material is designed to be removed from here so that's another area where talking to your rod manufacturer if you're in a situation like this, can give you some insight into what they recommend. |
| 23:22 | So what we're going to do now is go ahead and remove material from our heavier rods and this is an iterative process. |
| 23:29 | What we're going to do is use our linisher and we're going to grind material from those locations, just like you've seen in the body of the course and we're going to be checking our work frequently, creeping up on our target weight. |
| 23:42 | And in terms of target weight here, what I am targeting is an imbalance of no more than 0.5g so let's go ahead and do that now, we'll jump forward to see our end results. |
| 23:53 | We've now completed balancing the big ends of the rods and just like we saw in the body of the course, this is an iterative process, we want to make small adjustments, checking our progress frequently so we don't end up overshooting. |
| 24:05 | The other tip when we are removing material using a linisher from the rods is always to make sure that we are removing that material in line with the beam of the rod, we never want to linish across the beam, that can introduce stress raises which can result in cracking. |
| 24:19 | So with our 3 rods now balanced to within 0.5g we can see our big end weights, 456.3 for number 1, 456.9 for number 2, of course number 3 hasn't changed and number 4, 456.8g. |
| 24:36 | The process from here is simply to now weigh the overall rod and of course what we're going to be doing is finding the heavier rods. |
| 24:45 | Now any imbalance will be at the small end of the rod so let's go ahead and weigh the 3 rods that we've just balanced. |
| 24:58 | Alright as we can see there, we're actually already really close. |
| 25:01 | Again our overall imbalance I'm looking for all 4 rods to be within 0.5g. |
| 25:06 | We can see our lightest there, number 1, 663.1g. |
| 25:10 | Heaviest, number 3, the rod that we hadn't adjusted so of course that weight just gets transferred down here, I didn't need to go and reweigh it, 663.7g. |
| 25:19 | And we've got two that are 663.5g. |
| 25:22 | Now with our 0.5g imbalance tolerance there, we can see that we've actually got 6/10ths of a gram between our lightest at number 1 and our heaviest at number 3 so what we're going to do is remove a small amount of material from the small end on number 3 rod. |
| 25:37 | And when we're doing this, again we're going to be using our linisher, again we want to make sure that we are linishing along the beam of the rod, not across it and obviously with the small end of the rod we want to be very mindful of removing the smallest amount of material possible. |
| 25:51 | Equally, around the circumference of that pin boss, this is just to ensure that we maintain the maximum thickness in the wall that we can and that we aren't going to adversely affect our strength. |
| 26:03 | The other thing we want to be mindful of when we are doing this as well is just not concentrating the linisher in one place for an extended period of time which can end up overheating the material. |
| 26:12 | Alright we've finished linIshing the small end of our number 3 rod so we'll just grab that and get that back onto our scales and we can see, 663.2g for that rod which I've already noted down on our sheet here, so we can see that we are now within our tolerance of 0.5g there for both the big end and the overall weight. |
| 26:35 | So at this point we've balanced all of the components, we're ready to move on with the next step of our process. |
