VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1937-1945 1938 Main Bearing Shims

Ran into a snag rebuilding my engine. None of usual places like Chevs/40's or Egge can get main bearing shims. It's a December 1937 216 casting and i'm putting in fresh .010 bearings with a freshly ground crank.

Anyone know how my machine shop can work around this? I can dig up a variety of old used shims but don't want to create a mess. It's a semi-retired old-timer working on it so I'm sure he can get creative but was curious if anyone had been down this road.

Anyone know who might have some of these or an alternative shim material or solution?

Thanks!

Tim,
Check my post in "Parts Wanted" concerning bearing shims.
I have also found a package of used shims in a package with the front main bearing pt#.

Send me a personal email: don3955chevy@sbcglobal.net if interested

Tim,
I would like to ask to what dimensions you ground your crank for the .010 undersize? [mains and rods] I think there are errors in the dimensions listed on drawings and maybe even in the 1937 and 1938 Chevy Shop Manuals by Chevrolet. I have not been able to verify the correct dimensions as some folks sware they are the same as the 1939 Chevy Shop Manual by Chevrolet and the years that follow. I am inclined to think these numbers (1939-) are correct by have been unable to understand how the other multiple errors have arrison. The errors are of 2 types: 1. In 37 and 38 Shop Manual there is .002 inches addes to each of the 1939 numbers for all four main bearings and nothing added to the rod journal. 2. In other places the drawings follow the 1939- pattern but have .003 added to just the front intermediate main crank diameter (thus the same as the ID of bearins yielding zero clearance, an obvious mistake).
Mike (rebuilding a 1938 216)

Thanks for the ideas everyone. I'm still sorting this out. I'm getting my .010 main bearings from Chevs/40's and they are insisting (via email but I'm waiting for a phone call) that the mains are a true .010 oversize and no shims are necessary.

My machinist who does nothing but old engines and has for 45 years, is saying that the main bearing sets are sized to .010 but this is WITH the standard shim pack installed (I recall he said 60 thousands was the standard shim pack size). He may be used to dealing with old stock bearings or a particluar brand as he even mentioned line boring them to make sure they were bang on.

So I'm not sure if the current main bearings available have "evolved" so that the shims are not necessary (or because they are just not available) or whether chevs/40's is just not understanding.

From my understanding the crank is a true .010/.010 after machining. I'm hunting through my stash of rods as the new set I provided the machine shop were 0.030 so would need a few hours to machine and would be a "waste" if one of my other used sets was closer to 0.010.

At this point I think I have found some shim sets so I will buy them and we'll know the whole story once the machinist has the bearings in his hands...

Mike, PM me with your rebuild details or start a new posting so everyone can watch and learn (or help us undo a mistake!)

Tim,
In 1938 and the immediately following years (up until about 1948 or so) the main bearings were babbilt cast (I think by spin casting ie spinning the holder during metal pouring), and placed in the engine, and then the ID was line bored with the bell housing in place and properly aligned. After 1948 precission bearings became available to replace the original style main bearing and these did not need to be line bored as they were accurately made to fit without this final "adjstment step".

It is my understanding that there are two .002 and two .001 shims on each side of the main bearing cap when the engine was shipped from the factory (all engines came with them). Thus these shims that totaled .006 (6 thousands , not 60 thousands) were necessary to make the bearing fit properly when new and when the crank wore these shims could be removed in increments of one .001 shim (one on one side but not the other) or two .001 shims(one from each side) or one .002 and one .001 (one from each side) and so forth to come up with a way of adjusting the bearing ID to keep the clearance in the .0015 to .0035 range. Therefore, when new main bearings are installed they would need the entire set of .006 shims on each side of the the bearing cap to be correct. Thus as the new bearings wore and the crank wears the same "adjustment" proceedure is available.
I noticed that my crank journal diameters are wore about .005 inches undersize or .007 undersize (from new)depending on whether you assume the origianl diameter is the following first set of numbers or is the second set of numbers below:

front bearing crank journal = 2.6835 to 2.6845 inches
front intermediate bearing = 2.7145 t0 2.7155
rear intermediate bearing = 2.7455 to 2.7465
rear main bearing journal = 2.7765 to 2.7775
or

front bearing crank journal = 2.6855 to 2.6865 inches
front intermediate bearing = 2.7165 t0 2.7175
rear intermediate bearing = 2.7475 to 2.7485
rear main bearing journal = 2.7785 to 2.7795

These second set of figures are the ones listed in the 1937 Chevy Shop Manual. They are also listed that way again in the 1938 Chevy Shop Manual. Both of these were printed by Chevrolet Corp.
If one subtracts .010 from these figures, he should arrive at the correct dimensions for the machinists to grind to. But which set is correct? If you have your crank ground, you could measure it and tell me?
If I am wrong on any of this posting, please let me know by those of you out there who can set me straight. Just for the record, Gene (ChevyNut) has allready posted to me that the first set of figures should be correct and they are what are listed in his 1939 Chevy Shop Manual. They are also the ones listed in my 1942 Chevy Shop manual. The only safe thing to do is to purchase mains first and measure the ID with the bearing clamped and with an internal micrometer (careful not damage bearing) and then decide on the crank journal grind dimensions. That said, I am still driven crazy trying to understand where this dual yearly mistake occurred in printing these figures. They are listed in my "Motors Manual 1935-1952 Book" the same as the second set of figures (.002 larger)???????????
Mike

My 1938 Canadian Chevrolet/Pontiac manual has the 2nd set of numbers listed for the crank.

I have main bearings and shims on the way so once I have them all in the machinist's hands I'll let you know what/how he was measuring and whether I actually need the shims with the main bearings I bought from Chev's of the 40's...

Tim