Hi, New here and I've got a question.

I've been building a 1954 Chevy 235 truck engine, and I read about shimming the main bearings.

Shop Manual says "place three .002" shims on each side of each bearing and install bearing caps."

Does anyone know WHY the precision main bearings need to be shimmed in this year engine?

Chevy used shims on all six cylinder engines up to and including 1955.In 1956 they changed the type of bearings used and quit using the shims.You need to check the bearing clearance to make sure they are not to loose or too tight.
EvilDr235

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Two types of people drive old cars.Rich people because they want to and poor people because they have to.
Remember EvilDr235 has a cure for Your sixness.

The shims are essentially "wear shims". That is they could be removed one by one as the crank journals/bearings wore thereby allowing you to adjust rotational clearances without removing the crank. This was done using the amp load system. That should be in your shop manual. I completed the rebuild of my 235 recently and because I have a recut (.010/.010) crank with new undersize bearings I had to shim .002 on just one cap to get the proper clearance measuring with Plastigage.
I think if I had the block line bored I probably would'nt have needed that shim.
In short, check your clearances. You may not need any shims until your next rebuild.


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Drew
'55 Chevy

EVILDR235 & CatDog,
Thanks!

I had been wondering if the block had been align bored with the shims under the caps: or align bored and then shimmed with the final eccentric bearings.

I found an old 1950's bearing manual this weekend. It does'nt go into the Chevrolet bearings much, maybe 1 or 2 detailed pages.
Let me read it a few times and I'll get back about what it says.