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914World.com _ 914World Garage _ Main bearing clearances?
Posted by: stugray Jul 28 2012, 08:56 PM
I am getting ready to put my short block together and I have what appears to be a fairly new crank.
Main bearing journals: 2.361.
The bearings appear to be new unused STD-STD.
When I put on the main bearing that goes directly beneath the cam gear dry and wiggle it I get ~4 thousandths "wiggle" when measured with a dial indicator stuck to the flywheel.
Does that sound correct?
Would that be considered two thousandths bearing clearance (2 on each side = 4)?
Stu
Posted by: mrbubblehead Jul 28 2012, 09:14 PM
I am getting ready to put my short block together and I have what appears to be a fairly new crank.
Main bearing journals: 2.361.
The bearings appear to be new unused STD-STD.
When I put on the main bearing that goes directly beneath the cam gear dry and wiggle it I get ~4 thousandths "wiggle" when measured with a dial indicator stuck to the flywheel.
Does that sound correct?
Would that be considered two thousandths bearing clearance (2 on each side = 4)?
Stu
stu, have you ever heard of plastigauge? i have never used it, but from what i have read about it, it would be more accurate measurement. then you could check the manual and see if your within tolerance..... beyond that i cant help anymore. im sure someone with more experience will chime in...
Posted by: stugray Jul 28 2012, 09:58 PM
Yes I have heard of plastigauge, but you cannot use it in this application because the two bearings I am trying to measure are the full circumference bearings. There is no way to get it in there and then clamp.
That would work for the rod bearings or the center bearing.
Thanks though.
Stu
Posted by: mrbubblehead Jul 28 2012, 09:59 PM
Yes I have heard of plastigauge, but you cannot use it in this application because the two bearings I am trying to measure are the full circumference bearings. There is no way to get it in there and then clamp.
That would work for the rod bearings or the center bearing.
Thanks though.
Stu
gotcha
Posted by: stugray Jul 29 2012, 10:08 AM
Bump.
Bolting halves together today.
Anyone else ever measured bearing clearances this way?
I dont have an inside Dia. micrometer.
Stu
Posted by: McMark Jul 29 2012, 10:11 AM
I've never measured the rock but it's definitely there. Your journals are on the lower limit of STD, so there will be more rock. You may also have to use a larger oil pump.
Posted by: Racer Chris Jul 29 2012, 10:32 AM
...
When I put on the main bearing that goes directly beneath the cam gear dry and wiggle it I get ~4 thousandths "wiggle" when measured with a dial indicator stuck to the flywheel.
Does that sound correct?
Would that be considered two thousandths bearing clearance (2 on each side = 4)?
Stu
Thats not a good indicator of installed bearing clearance.
The only way with a full circumference bearing is to bolt it into the case and measure with a dial bore gauge. Then subtract the crank journal diameter to obtain bearing clearance dimension.
Bearing clearance is spec'ed as total, not per side.
Posted by: nathansnathan Jul 29 2012, 10:45 AM
The crank journal should be between 2.3610 and 2.3618. I've got some red and blue plastiguage here that measure 2 to 6 and 4 to 9 thousandths, respectively. Green is 1 to 3 thousandths. Is there a smaller color? - because the whole range of tolerance is +-4 ten thousandths, or 'four tens' as they say.
The other thing about measuring crank tolerances on the crank or the case is how true it runs, if it's whiplashed. You need a jig to check runout, a machine to check the concentricity of the crank bores in the case, if it needs align boring or not.
Posted by: stugray Jul 29 2012, 11:03 AM
The crank is bolted to the flywheel sitting on the bench vertical.
I put the magnetic base dial indicator on the flywheel and measure the total movement of the main bearing (opposite end of the crank) with the bearing dry.
I dont have an inside micrometer to measure the bearing so this was just a sanity check.
It seems a little more loose than normal, but it is very hard to judge.
Of course once you put lube on the bearing and put it on it doesnt seem to have a lot of slop.
Stu
Posted by: nathansnathan Jul 29 2012, 12:18 PM
I've always taken my crankcase, crank, and flywheel to a machine shop for this stage of building an engine. There's no sense in putting all that cash and work into it without knowing what they can tell you. To check the thrust that may be pounded, etc. It almost sounds like you are talking about endplay, with your odd measurement procedure there. 
Posted by: stugray Jul 29 2012, 01:26 PM
No I am measuring how much the bearing moves side to side (perpendicular to crank centerline) with a dry bearing.
.004 seems to be OK, but it seems that not many people do this "odd measurement" as a final sanity check before putting the crank in the case.
So I dont have anyone's measurements to compare to.
Stu
Posted by: sixnotfour Jul 29 2012, 02:50 PM
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Posted by: ldsgeek Jul 29 2012, 07:07 PM
The only way to check the clearance on those bearings is before they are assembled on the crank, and you need either an inside mic or snap gauges. When the case halves are torqued they compress the bearing shell slightly, meaning the clearance you are seeing is not what it will be when assembled. At this point you can either say "close enough for gov't work" and assemble it or disassemble the crank to check it properly. It all depends on how confident you feel.
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