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Old 04-08-2008, 12:52 PM
GMR GMR is offline
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main bearing clearance

I've read that the main bearing bore size (without bearings) for a chevy 350 is 2.6410. I've measured a std main bearing at .097 thickness. Subtracting .194 from 2.6410 equals 2.447. The crank main journal is 2.45. How does this work? I'm guessing the crush applied to the bearing somehow. Would like an expert to straighten me out please.

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Old 04-08-2008, 01:21 PM
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When I first started assembling engines I Mic'ed them this same way, and double checked the clearances with plasti-gauge. It always came up with the clearances I Mic'ed at. Yours will be .002 by the numbers you listed. A crush if any would be caused by the housing bores if they Mic up to the smaller size in the housing bore specifications.
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Old 04-08-2008, 04:39 PM
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How did you arrive at .002? The crank is 2.450 and the bore, based on the calculations I mentioned, is smaller at 2.447. That's my confusion.

I guess I understand that the eventual clearance may well be within spec. I don't understand, given the numbers I mention, how it turns out that way. Are my numbers correct? Is 2.6410 a typical 350 main bearing bore size without bearings. Is .097 a typical std main bearing thickness? Is the arithmetic to arrive at 2.447 legitimate? If yes, then how does a crank main journal of 2.450 fit in a main bearing bore with std bearings which measures 2.447?
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Old 04-08-2008, 04:59 PM
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Pardon Me! .003 clearance which is still well within spec's.
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Old 04-08-2008, 05:34 PM
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The crank should Mic out @ under 2.450 by spec's with 2.4493 @ the max for #1 journal, (minimum 2.4484) Journals 2,3,4 2.449 (minimum 2.4481) and Journal #5 2.4488 (minimum 2.4479). I don't have a standard bearing to check it's thickness with a ball Mic. to see if the measurement you gave is accurate for it's stock thickness. The housing Bore you give If I remember right is pretty accurate, but keep in mind the housing, bearing and crankshaft are usually measured in 10ths of a thousand tolerances.

I wish I had a good block to measure the one I have needs align bored. It's measuring 2.6431 and egg shaped.
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Old 04-08-2008, 06:00 PM
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Ha just found these clearance specs for the main bearings. #1 .0008-.0020 #'s 2,3,4 .0011-.0023 and #5 .0017-.0033
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Old 04-08-2008, 08:21 PM
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Quote:
Originally Posted by GMR
I've read that the main bearing bore size (without bearings) for a chevy 350 is 2.6410. I've measured a std main bearing at .097 thickness. Subtracting .194 from 2.6410 equals 2.447. The crank main journal is 2.45. How does this work? I'm guessing the crush applied to the bearing somehow. Would like an expert to straighten me out please.
couple questions, did you use a ball mic to measure the bearing shell? and are you sure the crank measures 2.450 ? 2.450 is .0007 above the high limit, using a ball mic your bearings should measure very close to .0955.

i always torque the bearings to be used in the bore housing, i measure the crank and set the dial bore to this size, then check the bearings with the bore gauge and this will tell you your bearing clearance.

with the specs you posted your crank is going lock up as soon as you torque the caps.

sam-missle
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Old 04-09-2008, 08:03 AM
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carsavvycook, thanks for your replies and taking the time to dig up the information. It is helpful.

sam-missle, I used dial calipers, not the best way I know. So thanks. I then used a bore gauge to measure the main bore without bearings and then with bearings and came out with a .194 difference, so it seemed to jive. I'm using inexpensive ACL bearings so maybe they're slightly thicker or, more likely, my measuring needs improvement.

Anyway, my question was more a theoretical one and I wasn't sure how to word it but your replies were very helpful.

I have two other questions. Something carsavvycook mentioned about his main bore sizes and egg-shaped. Dumb question but what the heck - if I measure the main bore at different points (perpendicular to the parting line and then diagonally) the measurements should be the same - yes? Mine are not.

Second question - my main bores without bearings measure out to: #1 2.6470, #2 2.6551, #3 2.6549, #4 2.6555, #5 2.6487.

Can align boring make this block useable?
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Old 04-09-2008, 08:43 AM
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Yes it can, possibly align-honed. Take it to a local engine builder you trust and get his opinion.
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Old 05-30-2008, 08:22 AM
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Forgot to let you guys know how it turned out.

Found a good machinist and he fixed the problem with some old caps he had laying around and align honing. Thanks again for your responses.
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Old 05-31-2008, 09:16 AM
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I think you measured your bearings wrong, they should be about .0955", if they are .097" thick they are not standard!
Also how were you using your dial bore gage without a micrometer?
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