VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1937-1945 Hot Debate on Crank Size and Main Bearing Sizes

Now that I have stirred us all up about the crank size for 37-38 versus 39 (see "Piston Ring Types for 216", "Forty 216 Main Bearings" and "Will someone with a 37 Shop...Dia Crank") under this section 37-42 of VCCA, let me throw some more gasoline on the fire: If one looks at these 216 engines (37-39) they will see two different forging numbers 838281 and 838877 for the same part number of 838345. (???) Ok now if they go to the following web site: http://chevy.tocmp.com/1929_54chevyparts/00/098.HTM
they will see main bearings listed for .010 .020 and .030 etc undersizes. But what you will also see is a very curious listing there too for .002 undersize! I sugggest that this might have been for the 1939 crank which is .002 undersize compared to the 1937 and 1938 cranks. I have no proof of it but I wonder also if the 838281 was a 1937-38 crank and the 838877 was the 1939 crank? http://chevy.tocmp.com/1929_54chevyparts/00/098.HTM

I could be wrong but I wonder why the "1937 Shop Manual", the "1938 Shop Manual" and "Motors Manual 1935-1952" all lists the .002 differnce between the 37-38 and the 39 crankshaft. Further, since there was no plastigage during the 1937 to 1939 time period, all the main bearing adjustment was done by removing shims that were installed new and also required when installing replacement mains,this .002 was generally not a problem since a 1938 crankshaft (.002 fatter) could be used in a 1939 engine by adding a .002 shim to each side of the cap. Someone has got to remember this from long long ago?
Motor on, Push as hard as Posssible, Jump in, select second, pop clutch and ... away Mike

Most all precision bearings were available in standard and .002" undersize.Some even in .001" under.The undersize was to compensate for wear.Same for the rod bearings in the later years.
There can be more than one casting number for a single part number and there can be several part numbers for one casting number.
The 1937-47 connecting rods all interchange but there were a few different forging and part numbers thru the years.
The original 1937-47 main bearings came in a "rough" size and no under size.They were installed in the block and line bored to size.
In 1948 the new engines recieved precision type main bearings.They did not require line boring and if necessary one bearing could be replaced -or the complete set.
After about 1949 Chevrolet discontinued the "rough" bearings for the 1947 and back (to 1929) and sold precision bearings for replacement.With these, after they were installed, one or all the bearings could be replaced and line boring was not necessary.At any rate .002" difference would make no difference as the shims would compensate for that either way,
The parts book has always listed the same main bearings for 1937-39 ,both in the early "rough" type and the later precision type.
The front three bearings fit up to 1947.
The only thing that ever changed in the 1937-39 crank was the clutch piolet bearing opening.The 1939 used a roller bearing and the 1937-38 a ball bearing.The hole diameter changed.Possible caused change in casting and casting number.