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Practical Engine Building: Conrod Balancing

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Conrod Balancing


00:00 - Another one of the engine components that we can balance ourselves in the home workshop is the connecting rod.
00:06 Now if we are going to be doing this work ourselves though, we do need to understand what's involved as balancing connecting rods is quite a lot more involved than a simple component such as the piston.
00:17 When it comes to the connecting rods, we're not just interested in matching the overall weights of the connecting rods across the set but we actually need to separate out the big end and the small end weights of the connecting rods and then make sure that all of the big end weights across our set of conrods match as well as obviously our small end weights.
00:36 Now the reason we need to do this is because of the way the conrods operate in the engine.
00:42 The big end is rotating along with the crankshaft while the small end is attached to the piston by the wrist pin and this is moving up and down in the cylinders.
00:51 So in order to balance our connecting rods, we're going to need a couple of pieces of equipment which I've got here in front of me.
00:58 First of all, we're going to need a set of precision digital scales and the scales will need to be able to read down to a 10th of a gram.
01:07 It's also useful if they can measure up to approximately 2 kg because we will at the end of this process be weighing the entire conrod so it needs to be able to cope with the entire mass of a connecting rod.
01:19 What we'll find is that a lot of the precision scales on the market will only read up to perhaps 500g and that's not going to be enough for a lot of the conrods we'll be dealing with.
01:30 Next we've got our conrod balancing fixture and this is an essential part of balancing the connecting rods.
01:37 This allows us to support just the big end of the connecting rod on our scales while we go about making our measurements.
01:43 The connecting rod balancing fixture provides a low friction way of supporting the conrods and this is a really essential part if we want to get repeatable results from our measurement.
01:54 Lastly I've got a linisher here which we're going to use to remove some material from the connecting rods in order to balance the set.
02:02 Now it is possible to use a power file to do the same task but a linisher certainly is an easier way of getting a good surface finish.
02:10 OK let's get started with our conrod balancing and what I'm going to do first of all is go through and measure the big end weights of all of the connecting rods.
02:22 Now even with the conrod weighing fixture that I've got here in front of me, it is very very difficult to get absolutely repeatable results.
02:32 So in order to overcome this, what I've adopted is the technique where for each of the connecting rods, I'll take 5 separate measurements of the big end weight.
02:42 What I'm going to do then is eliminate my highest and lowest value and I'm going to average the other 3 and this is just a good precaution to make sure that you're not going to be influenced by the way that you fit the conrod onto the fixture.
02:58 What I'm going to do now is just turn our scales on and zero our scales and I'll fit one of our connecting rods to the fixture, we'll just talk about the important aspects of that fixture that we need to keep in mind.
03:15 OK so we've got our connecting rod fitted there and I've already previously set up this fixture for these particular rods, these are a Brian Crower conrod for a Subaru FA20 engine.
03:27 The important point to note is that when we have the fixture set up, we want the conrod sitting horizontal on our scales.
03:34 We also want to make sure that the support arm for the small end of the connecting rod is hanging very close to vertical so it's just going to improve our chances of getting repeatable results.
03:46 So as you can see, our scales at the moment are showing 388.7 grams so that's our first weighing of the big end of number 1 conrod.
03:54 I'm going to write that down and then I'm going to take 4 further readings.
04:02 So we've taken 5 measurements there with our first connecting rod, we can see we have our masses ranging from 388.7 grams at the lightest to 388.9 grams at the heaviest.
04:14 Now this is a point that's really important to understand because it is incredibly difficult with these fixtures to get completely repeatable results.
04:24 You can see there our variation though is only 2/10ths of a gram so I'm happy with that sort of variation, if I was putting the connecting rod onto the fixture and I was seeing a variation of 0.5 grams every time that I placed the conrod back on the scales and that's going to indicate we've got a problem with our equipment, that's not sufficient repeatability.
04:47 The other aspects that are important to note when we are placing the connecting rod on the fixture is to make sure that we use consistent alignment between the small end and the big end.
04:57 In other words we don't want to be placing the conrod on the fixture at an angle, we want to be very consistent in how we do that.
05:03 I also want to watch our scales every time we remove the conrod from the fixture and just ensure that they do come back to a true zero.
05:11 All of these things can affect the consistency of your results.
05:15 So with that being said, let's have a look at our results now, we'll eliminate the highest and lowest readings and then we'll average the remaining three.
05:49 So again just working to one decimal place or a 10th of a gram there we have an overall weight for our big end on number one connecting rod of 388.8 grams.
06:00 I'm going to go ahead now and I'm going to weigh the remaining three connecting rods.
06:10 OK we've completed weighing the big ends on our 4 connecting rods and we can see that our heaviest connecting rod was 388.8 grams and we have two that weigh at 388.3.
06:24 So in this case we've got about a 0.5 gram variance from our heaviest to our lightest connecting rod.
06:30 The next step will be to remove some material from those heaviest connecting rod big ends in order to match them down to our lightest connecting rod.
06:38 So in this case we're going to take our number one rod and we want to remove 0.5 grams.
06:44 So it's important to understand how we're going to go about doing this, where we can safely remove the material from.
06:51 Obviously while we're removing this material, we don't want to do anything that could adversely affect the strength of the connecting rod.
06:58 So particularly when we're looking at an aftermarket connecting rod like this, it's quite common practice for people to remove material from the ribs on the cap of the big end.
07:10 That's actually not an area we want to remove material, those ribs are quite important for the strength of the connecting rod.
07:17 Instead what we're going to be doing is just applying a very small chamfer to the edge of the connecting rod, down where the connecting rod bolt sits.
07:26 Now we want to take a minimal amount of material off and what we also want to do is average out the amount we're taking, so in other words I want to remove a little bit of material from each of the chamfers so I'm not focusing my energy too much on one single point.
07:42 The other aspect here is when we're using the linisher we want to be very careful not to overheat the connecting rod.
07:49 We don't want to apply pressure on the linisher for too long so we end up with a blue colour on the connecting rod material and this can end up damaging the heat treatment of the connecting rod.
08:03 So let's remove a little bit of material from our number one connecting rod now.
08:51 The key when we're balancing any engine components is to make very small adjustments to the component weight and check our work.
08:59 It's always easier to remove more weight, whereas if we go too far then we constantly end up removing material from the remaining parts of the set.
09:08 So let's put the connecting rod back on our scales and we'll see what the affect of that first very small change was.
09:15 Remember our average was 388.8 grams and we were trying to remove 0.5 grams.
09:29 So we can see there with our first weighing of the connecting rod we're at 388.5 grams so that small amount of material that we removed equated to 3/10ths of a gram.
09:40 So obviously we would just continue the process until the weights of our conrods match or the weights of our big ends of our conrods match.
09:49 Once we've match the masses of our connecting rod big ends, and here I'm trying to get the weights within a 10th of a gram, what we can then do is remove the fixture from our scales and what we're going to do is zero our scales again and now what we can do is weigh the entire connecting rod.
10:08 In this case, if all of the big ends of the connecting rod weigh the same, now any discrepancy in weight we will have is a result of the small end of the conrod and we can make adjustments to the small end in order to correct that, so let's go through our set now and we'll weigh each conrod.
10:40 With all our 4 conrods weighed now, their overall mass, our heaviest conrod is 537.4 grams and the lightest is 536.9 so the process here as I mentioned is to remove material from the small end of the connecting rod in order to match the overall weights.
10:59 So what we're doing here is we're just going to lightly linish the outside surface of the small end of the connecting rod and again we want to remove a minimal amount of material, we want to do this smoothly as well, we don't want to focus our linisher on one area and remove excessive material, we want to smoothly remove our material from the entire outside diameter.
11:22 And again we want to be careful that we don't focus our linisher on the small end of the connecting rod for too long and overheat that material.
11:30 So the process there is simply to go through our set, make those adjustments to the small end of the connecting rod until our overall weight is within a 10th of a gram and at this point our connecting rod set is balanced.

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