The Stovebolt Site Forums The Shop Area Engine Shop Main bearing cap shims ????

ALL 216/235/261 factory made Chevrolets have Babbitted lower connecting rods and crankshaft main bearings .

The shims used on the precision interchangeable main bearings were of great benefit on the Chevrolet factory motor assembly lines. The fresh "green" engine blocks were mated with freshly ground crankshafts at mass produced speeds. The dimensional shapes of the main bearing shells (they were often eccentric at the crowns) were made to fit with a "shimpack" installed, and when torqued , would "crush" to give a proper average oil clearance. {ex 1954 235-261 .0010/.0030} IF the fresh crankcase had any warpage, or the crankshaft was unlikely imperfect, then shim leafs could be removed to make for slight adjustment at fast factory assembly line speeds. THEN latter, after the engine was used and worn a lot by the owner, a shop mechanic could remove some more shim leafs to reduce the worn oil clearance to get them closer to spec. This was never perfect as the vertical oil clearance was what was primarily effected by using "shims". The saddle bores when shims are used are NOT round.
The shimming was labor intensive, it worked great. It is a lost art.

Soggys further opinion (Soggy is old and confused after watching Matlock on TV)--- 42465967 On your rebuild : crankshaft should have been turned to the proper (under)size, and then IF the mains' precision bearing shells included matching shim-packs, those particular main bearings would have had their shape designed to go with (match) the shim-packs .... {note: shims should not extend onto the bearing shells @ parting line)
If the shims were in place w/new shells, yes the re-ground crankshaft should initially have had that .0010/.0030 ,~.002 (feel or drag or measurement on each main bearing). The aftermarket bearing boxes used to come with a small paper note inside that told jobbers what to do with the shims and what clearances to use, but that may have disappeared 30 years ago! 42465967, you can possibility look on the underside of the bearing shell for a "manufacturers" number to tell you what bearings the rebuild shop used. If the bearing shells were made to go with a matched set of shimpacks, and the shimpacks were not used, I would logically think they would have been very "tight" on break-in .
Yes Plastigage could have been used to spot-check average oil clearance after the shell bearing fittings , IF used correctly !!!: Or a long brass metal shim could have been used as HRL mentioned. The bearing tightness does have a definite "feel" to the engine re-builder.

Regarding the lower connecting rod bearings , well the Full Pressure lubricated (slang say hi-pressure) 235/261 did use replaceable shell babbitted bearings instead of the prior Combined Pressure lubricated (slang say lo-pressure) direct babbited (centrifugally spun) lower rod bearings. My personal opinion is that the shims were not worth Chevy using on the Full Pressure replaceable shell lower rod bearings as the shells were cheap to install & replace. The direct babbited earlier 216/235 connecting rods were great and could be maintained with shims, but they were becoming OBS because the replaceable shells in the Full PSI engines were so cheap and faster/easy to build and rebuild.

Finally, Soggy thinks that in late 1954-55 big changes came to Chevrolet engines with their (Cadillac inspired) V-8 . One of the biggest hold-ups for introducing that Chevy V-8 was the engine blocks were very intricate sand castings and they could not be reliably produced fast enough in mass quantity until Chevrolet used a new "greensand" casting process. This is just my opinion, but it may mean that the I6 OHV engines in late 1954-55 were also getting better engine blocks that didn't need shimming on the main bearing shells from the factory ??? Just my opinion........ Soggy is done, dog eating his slippers and kids need chasing off the lawn.