Chevy 235 Main Bearings, are they OK?

I decided to check the bearings on my 235 before putting it in my 59 Apache. Plastigauge says the clearances are .002-.003 but the rear bearing has some kind of shallow pitting on it. The crank is nice and smooth, and the oil holes are clean.

Since main bearings cost a fortune now (along with everything else) I dont want to replace them if I don't have to. I figure I'll drive this truck rarely, and in the future Ill rebuild it proper once I find some new rocker shafts and run the engine to find any problems.

I think this is damage caused by cavitation, but how does one fix the cause so it stops or doesn't happen to new bearings? There was a TON of sludge in the oilpan when I pulled this thing apart. Maybe the jello consistency of the oil causes flow problems which leads to cavitation?

Also, do the markings on the back of the bearing say they're .030 under?

 

since you have it apart i'd replace them...and why not do that proper rebuild now too? i know you don't want to spend the money right now , but you do have the engine out and apart . why cause extra work by doing it later?

as for being 0.030 under....a micrometer will tell you for sure

 

Yep .030 under. That looks like corrosion damage to me ,the over plate flaking away. Never heard of bearings cavitating in my 50 years + of screwing with old cars. That said I would run them if this is just a normal street motor.

 

Thanks for the quick replies. Acid was my first guess too. The oil cavitation I was reading about was once I started looking on the internet from a diesel website and on this page

http://www.bobistheoilguy.com/bearingwear/bearingwear****ysis.htm

so it wouldn't surprise me that it doesn't apply to normal duty gas engines.

What would cause acid in the oil? Leaving it in there too long, carb running rich? This was a junkyard motor, so its history is anybody's guess. There was some lichen growing on the truck, so I reckon the donor truck was sitting for a couple years before getting s****ped.

I don't want to do a real rebuild now 'cause it ran fine on the engine stand after a carb rebuild; plus I just spent a lot of time repainting it and I don't want the machine shop to undo all my recent hard work

 

Not acid,, Moisture/water. Thats what caused the pitting. Somehow excessive moisture was in the oil and the engine was stopped for a while with the oil/water mixture trapped against the bearing.. Short cold trips for beer/smokes will do it..

 

caution regrinding crank and new bearings may cause lock up un less you have the original shims for the main caps

 

By 1959, 235s no longer used main bearing shims.

 

That pitting is caused by normal acids that build up in oil. That's why oil changes are popular....I have run engines with worse than that, it doesn't seem to hurt a Stovebolt. If the engine sat for years, that explains the pitting, the crud settled in there and had a long time to eat away.

 

i see O'Reilly has Sealed Power main bearings for about $100........and they are available STD .010 .020 .030 .040 .050 .060 undersize

 

Moisture combined with the end results of combustion that get in the oil create the "acid" that etches the bearings in that manner. Pretty normal for an engine that got parked with dirty oil and sat for a long time.