Oversize vs. Undersize bearings - Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum

Chilly · #1 08-14-2009, 04:56 PM

Is it correct that oversize bearings for the crankshaft and/or rods would decrease the clearance (oil space) between the two surfaces?

And the opposite would be undersize bearings that would increase the clearance between the bearing and the rod/crank?

I'm just trying to figure out why most replacement bearings are listed as undersize when bearing/crankshaft wear should actually increase the clearance between the two surfaces.

Chilly

the roadie · #2 08-14-2009, 05:17 PM

The term "undersize" refers to the I.D. of the bearing after installation, which would mate closer to a worn crank.

You're right, there is actually more metal in an undersized bearing.

DJDarknez · #3 08-14-2009, 05:27 PM

Unless they come in increments of .001", you shouldn't install oversized bearings in an engine without grinding the crank. Most bearing manufacturers run oversizes in .010, .020, and in some cases .030.

For the 4.2, they're available in standard (2.7549"), .010", and .020" over.

Chilly · #4 08-14-2009, 07:06 PM

Thanks for the replies!

So, my understanding of this now is that if you grind the crank you would need to put in undersize bearings with more metal that will mate closer to the crankshaft bearing surface. (Of course the crankshaft journals would have to be properly measured to determine the size of the undersize bearings.)

Undersize bearings therefore have a smaller inside diameter than standard, and oversize bearings would have a larger inside diameter than standard!

Amazing how these terms get interchanged once in awhile!

DJDarknez · #5 08-14-2009, 07:24 PM

Yea, just kinda depends on who you talk to. Some people say over, some say under.

For instance, when a crank gets .010" ground off it's journals, you'd say "well, it's a 10 under crank". Then you would go look for .010" under bearings, since the crank is .010" under.

There is a specific range of oil clearance that engineers design into bearings (usually measured in tenths of an inch) which is why you'll only find certain size bearings, and the crank should be ground accordingly.

Does that make sense? It's kinda weird how I write stuff sometimes.

DJDarknez · #6 08-14-2009, 07:29 PM

Also, if anyone needs it, here are the ranges according to ACL:

Rods:
2.2319" - 2.2326" (journal size on crank)

Rod Housing Bore:
2.3749" - 2.3755" (ID of big end of rod)

Mains:
2.7546" - 2.7552" (journal size on crank)

Main Housing Bore:
3.0736" - 3.0743" (ID of block and main girdle)

Chilly · #7 08-15-2009, 11:53 AM

Definitely understand it now. Almost feel like an expert!