F Engine crank/bearings

knucklebuster · Post by **knucklebuster** » Tue Nov 16, 2010 11:24 am

04/55 build on an M38A1 - has the F engine showing a rebuild tag of .020 oversize bearings and .040 on the bore.

The #1 rod bearing had spun and affter removing the cap and looking at this journel it appears it will need another .010 shaven down with bearings to make it right.

My question is: How far is oversize not trustworthy on these engines - speaking of longevity as well. Or is it better at this point more reliable to drop a new crank in it and go back to STD bearings ?

Post by **wesk** » Tue Nov 16, 2010 2:54 pm

That would be 0.020 undersize. Was that for rod and main journals both? Max undersize is 0.030 which would work just fine. You should mic everything before planning a course of action. That depot overhaul plate from 1955 may not be the last time machine work was done on your engine.

The 0.040 oversize bore is OK and .060 is about the max safe limit although you can find 0.080 rings and pistons.

These are long stroke low RPM engines and they are not normally hard on bearings unless you have oil pressure and flow issues.

The #1 rod wear issue is common when pressure drops at the front end of the oil gallery.