main bearing plasti gauge clearance. What do you think? - Pontiac GTO Forum
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post #1 of 4 (permalink) Old 08-12-2018, 11:39 PM Thread Starter
 
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main bearing plasti gauge clearance. What do you think?

Ok guys got my crank block etc back from the machinist, got the cam sitting in the block and decided to check the main bearing clearance with plasti gauge before final assembly. Here are the results .1).001 or just slightly larger than .0015 2) .0015 3).0015 4) between.002-.0015 5).0015 all are close to the same except the thrust bearing. do you think I am good to go? I guess I expected them all to be the same. I guess they could be with the proper tools.
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Last edited by RMTZ67; 08-13-2018 at 07:10 AM. Reason: more info
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post #2 of 4 (permalink) Old 08-13-2018, 08:53 PM
 
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Originally Posted by RMTZ67 View Post
Ok guys got my crank block etc back from the machinist, got the cam sitting in the block and decided to check the main bearing clearance with plasti gauge before final assembly. Here are the results .1).001 or just slightly larger than .0015 2) .0015 3).0015 4) between.002-.0015 5).0015 all are close to the same except the thrust bearing. do you think I am good to go? I guess I expected them all to be the same. I guess they could be with the proper tools.
Factory '68 manual for the 400 says mains - .001 - .0020". Rebuild books say .0025".

First, temperature can make a difference. Heat expands things so reading can be off. My machinist has a room just for engine assembly and I believe he keeps it a 70 degrees. So if you just dropped the crank in the block and then put on the mains to check the clearances, I would perhaps let the engine set overnight so block/crank/caps all attain the same temps first, then do a check.

Next, you can also swap bearings around if one seems to tight or another too loose - with exception of the thrust bearing. Sometimes playing musical chairs with the bearing shells can change readings.

I think you know how to use the plastigauge - dry and not put any oil/lube on the crank pins as this will throw things off. Make sure the bearing shells are firmly seated and nothing gets under them like a spec of dirt or fiber from a rag.

I can't tell you if the numbers are good or bad, but I would definitely consult with your machinist on the numbers to get his opinion. Different manufacturers sometimes have different clearances from one another even though you order for your needs. You can also order bearings under sized/oversized just enough to get closer to where you want to be.

If the bearing clearances are too tight, you are more apt to spin a bearing. I don't think you plan on hammering on the engine, so I think (my opinion here) I would go with a minimum of .0015" but no more than .0020".

Did some checking on the web just for fun. Bearing clearances are all over the place for a 400CI. Some say use .0007 or .00075 and multiply by journal diameter. Others use .001 and multiply. I also get .0022-.0028", .0030", and everything in between.

Go figure.

Last edited by PontiacJim; 08-14-2018 at 09:16 PM. Reason: Corrected my clearances - not enough zero's
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post #3 of 4 (permalink) Old 08-14-2018, 07:08 AM
 
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RMTZ67 - My opinion is that you should be peachy-fine with those clearances, assuming this engine is for street use. FWIW, that half-thousandth variance in your results is easily within the practical accuracy of Plastigage measurements, IMO.

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post #4 of 4 (permalink) Old 08-14-2018, 03:12 PM
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Nice tight clearances, almost too tight. I like about .001" per inch of crank pin diameter max. About 7 years and 15,000 miles ago, helped a friend build a 389 on the cheap, and his main crank clearances were all .003". Pretty loose, but the engine runs great, uses no oil, and has been very reliable and quiet. In your case, I would run an oil NO THICKER than 10/30.
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, Lemans & GTO Complete Engine Compartment Discussion

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