# New mains same clearance



## mjc707 (May 28, 2015)

I have my 6 389 apart so I thought I would replace mains to be on safe side. The old ones plastigage between .0015 and .002. I ordered same clevites that were in there MS483G. I did plastigage on new bearings and came up with same reading. Is this too loose for new bearings? 65 service manual says .0005 to .002 for tolerance. I dont really want to have crank ground to .010 since this is still in spec. Also old bearings have oil channel all the way around and new are just tops. Cap sides are solid. I finally have everything and ready to assemble just concerned about being close to tolerance. Oil pressure was good before teardown. Thanks


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## PontiacJim (Dec 29, 2012)

mjc707 said:


> I have my 6 389 apart so I thought I would replace mains to be on safe side. The old ones plastigage between .0015 and .002. I ordered same clevites that were in there MS483G. I did plastigage on new bearings and came up with same reading. Is this too loose for new bearings? 65 service manual says .0005 to .002 for tolerance. I dont really want to have crank ground to .010 since this is still in spec. Also old bearings have oil channel all the way around and new are just tops. Cap sides are solid. I finally have everything and ready to assemble just concerned about being close to tolerance. Oil pressure was good before teardown. Thanks



OK, here is what I found in my research - keep in mind I am not a machinist nor expert engine builder, so what you do is up to you. You did not mention rod bearings.

My info says that Pontiac 400CI engines came with fully grooved main bearings but does not say anything about the 389CI. Fully grooved bearings give the crankshaft oil through the entire 360 degree of rotation versus 180 degrees of rotation. Technically, the fully grooved bearing does not offer the same load surface as the upper grooved/solid bottom bearing *BUT*, the fully grooved main bearings offer better oiling to the connecting rod bearings - which is a must for a Pontiac. So, my recommendations are to use a fully grooved main bearing set for any hi-performance engine. A 3/4 grooved bearing would be my choice, but it seems that only a competition type bearing material is offered in 3/4 groove for the Pontiac and is not for a softer cast street crank. Here is a guide you might want to use in bearing selection: http://butlerperformance.com/i-2445...496p-10.html?ref=category:1234737;brand:99933

Next, using plastigage to check bearing clearances is not the best way to get an accurate reading. In my opinion, your clearances are OK as the factory spec is .0002" to .0020". I would rather have my bearings on the high side (loose) for an engine that will be spinning higher RPM's. I pulled my '68 Pontiac Service Manual and it actually says to place a .0002" brass shim between the crankshaft journal and the lower bearing shell on each main bearing cap next to the one that is being checked with the plastigage and torque these to the proper torque. This causes the crankshaft to be forced against the upper bearing and insures an accurate measurement of the total clearance. Then plastigage the main bearing you want to determine the clearance on.

That said, my opinion is that you want to get the fully grooved bearings and that your clearances are within spec. Here is the Clevite listing of bearing types : More Information for MAHLE/CLEVITE MS483G You can either buy a new set of mains having the full grooves or another set of what you have to make up a full set of fully grooved mains. Best to email any manufacturer/supplier as to their recommendations as to which bearings to use for your application. :thumbsup:


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## mjc707 (May 28, 2015)

I talked to Clevite and they said full groove and half grooved comes down to preference and that there is data proving each way. he also said it would have originally came with fully grooved. I already have the kit with half groove. Is there a difference that I need to know about before I assemble this weekend. Thanks


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## geeteeohguy (Feb 2, 2008)

IMO, you have a crankshaft that has virtually no wear, and the original bearings are still in spec. You can replace them if you want to, or run the old ones. No reference to mileage on your engine, but the bearings in my '67 400 were all still in spec when I tore it down for an overhaul at 173,000 miles.


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