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Practical Engine Building: Measuring Bearing Clearances

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Measuring Bearing Clearances


00:00 - Measuring and confirming your engine bearing clearances is one of the most important tasks during the assembly process of your engine.
00:08 Even if your machinist has set or checked the clearances for you, it's still your responsibility to make a final measurement and assessment of the clearances and ensure that you're happy with them before final assembly of the engine.
00:22 For this particular demonstration, we're going to have a look at how to correctly measure the big end oil clearances on a Subaru FA20 engine.
00:31 While we will be using the Subaru FA20 for our example, the same principals can be applied regardless what engine you're checking bearing clearances on, and regardless whether you're looking at the connecting rod, big end oil clearances, or the main bearing oil clearances.
00:47 So for our example we're going to be measuring the clearance on the number one big end journal.
00:52 And before we've started I've grabbed one of our Brian Crower forged h-beam conrods, and I've torqued the bearing cap on with our ACL race series bearings fitted to the conrod.
01:03 Now this is going to be an important aspect because we're going to be using a bore gauge here to actually check the clearances.
01:11 What we need to do before we get started is make sure that our big end bearing journal is clean.
01:16 So what I'm going to do here is just grab a rag, and I'm just going to grab some brake clean, we're just going to remove any of the oil or protective film from the big end journal, just to make sure that there's no debris or lubricant that's going to affect our measurements.
01:34 At the same time I've got my micrometer here, and I'm also just going to clean the contacting surfaces of the micrometer, again just to improve the accuracy of my measurement.
01:45 So what I'm going to do now is I'm just going to drop my micrometer down over the big end journal, and I'm just going to tighten the micrometer, and while I'm doing this I'm just moving the micrometer backwards and forwards, to make sure that I have got it at the widest point of the big end journal.
02:04 Once we're happy with that measurement, we can then lock up the micrometer, and remove it.
02:09 Now this is obviously an ideal opportunity to also check that the big end journal is on the correct size.
02:16 For today's demonstration what we're going to simply be looking at is how to use that measurement that we've just taken, in conjunction with a bore gauge to check our actual oil clearance.
02:27 So once we've locked up our micrometer here, what we're going to do now is take our bore gauge, and we need to zero the bore gauge.
02:35 So the bore gauge itself is a measurement tool that gives is a relative measurement.
02:40 So what we're looking at there is the measurement in the big end bearings in the conrod, compared to the size of the journal.
02:48 So what we're going to do before we start is we're going to zero our bore gauge with our micrometer.
02:54 So this does get a little bit fiddly.
02:55 What we want to do is locate the bore gauge into the micrometer and we just want to again move the bore gauge backwards and forwards, we're finding the widest point here in the micrometer.
03:11 What we want to do is just adjust the dial gauge if we need to, until the needle is just swinging to zero at the widest point.
03:20 So we can see that's what's happening now.
03:22 As I move the bore gauge and I open up the gap there, we can see that the needle moves away from zero, and as I come back through the zero point, we can see right at the narrowest point, we're sitting right on that zero mark on our dial gauge.
03:39 So this means that our dial gauge is correctly adjusted.
03:43 And at this point we're ready to start taking a measurement in our conrod.
03:48 So what I'm going to do now is I'm going to grab our connecting rod.
03:53 And what we want to do is just locate the bore gauge into the big end journal of the connecting rod.
03:59 Again what I'm going to do here is just swing the bore gauge past zero, and I'm looking for the measurement at the narrowest point.
04:08 What we can see there is we're just a little bit under 500ths of a millimeter.
04:14 So this is how we're going to measure our clearance.
04:18 Now it's also important to make sure that we make our measurement perpendicular to the parting line of the connecting rod.
04:26 The actual bearing shells are slightly acentric in their shape so as we get towards the parting line, the oil clearance will change, and what we're looking for is the clearance perpendicular to the parting line.
04:38 So that's the process of measuring our bearing clearances using a bore gauge and a micrometer.

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