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

I have seen this on the 216 and now on a 1954 261. Why do/did they use shims on the main bearing caps with replaceable bearings?

If the crank is turned to a standard undersize with matching bearings, may shims still be necessary?

I do not find shims on the 261 rod caps.

I believe that it is just a holdover from the way GM always did mains in the past.
The same question can be asked as to why they used main shims after they stopped using babbitt bearings for the mains on engines much earlier that 1954.
It could be due to imprecise casting/machining capabilities of the time back then.
Carl

Then on setup, if they did not have plastigage back then --maybe they did --- how did they determine where, how much to shim each main bearing cap. On this 261, it looks like they just replaced the main and rod bearings standard size and placed the cap shims on the mains where they were prior. As I understand this engine stopped running after 6 miles. Engine is not seized but rod bearings are solidly into the copper and the mains are very grooved along twith the crank.. I do not know yet if .020 will clean up the crank or not yet. I believe the mains clearances is what eventually stopped the engine. I have an old engine rebuilder close I will get to check the crank.

The early 1954 235 called for the shims. When the 235 was updated mid-year the shims were no longer specified.

From observation of the rods, they appear to be dowel upper and tangs. The bearings used had no dowel and I believe they all had spun. The crank main bearings appear to be dowel only. Let me know if this is possible.

I have seen main shims up through 57. From 58-62 no shims. A good machinist doesn't use Plastigage for initial set up. The crank journal is measured and then the bore in the torqued down cap is measured. The correct shims are then used to give the desired clearance.

Before Plastigauge, a small piece of .002" shim stock was placed between the lower main bearing shell and the crankshaft, the cap was torqued, and the flywheel was given a firm tug. If the crankshaft turned freely, a shim was removed. Once the crank turned with a moderate drag, the clearance was assumed to be .002" and the procedure was repeated for the rest of the main bearings. Babbit rods got the same procedure. 216's, early 235's and 261's had main bearings with dowels top and bottom. Midyear 1954, the upper bearing shells got anti-rotate tabs instead of dowels to make main bearing changes with the crank in the block simpler. I've never seen rod bearings with dowels instead of tabs.
Jerry

The rod itself has a hole in the top of the area where the bearing sits. The hole exits further up the rod and provides an oiling function.

The hole in the rod is the "spit hole" that lubes the cylinder wall every time it aligns with the drilled passageway in the crankshaft. Chevy insert bearing rods, either 235 or 261, simply do not have anti-rotation dowels. There are tabs at the parting line of the rod and cap, for both top and bottom bearing shells. A properly-reconditioned rod doesn't need a tab to prevent the bearing shell from rotating- - - -there's enough "crush" to keep the bearing from moving.
Jerry

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.

Soggy is mistaken- - - -NO 235/261 engines had Babbited rods- - - - -that stopped with the 216's. 1941-53 235's had Babbit rods, bit there were NEVER any Babbit-rod 261's! 1954-up 235/261 engines have replaceable rod bearings, not Babbit rods.
Jerry

What about the "early" low-pressure 235 engines, Jerry - 1941-1953?

http://chevy.oldcarmanualproject.com/chevyresto/53145.htm

YES , Soggy is correct.
The lower rod bearings on the 235/261 have Babbitted replaceable shell bearings.
Babbitt babbit babbitt babbbitt ............

3-layers- - - -steel/copper/Babbit.

Yet another semantic smokescreen!
Jerry

Cool info, I did look up the specs on a 1954 235/261 crankshaft and it amazed me that every journal was different on the mains -- front 2.6835-26845, #2 2.7145-2.7155, #3 2.7455-2.7465, #4 2.7765-2.7775. Why?????

Rod journals were all the same at 2.311-2.312.

??? babbitted rods or babbitted rod bearings ???

"NO 235/261 engines had Babbited rods"
"3-layers- - - -steel/copper/Babbit."

Babbitted rods vs babbitted insert rod bearings - semantics?

Jim Carter sells reconditioned babbitted rods to take precision insert bearings.

It should also be noted that 1954 235 (and probably 1954 261s) rods had insert rod bearings and shims.

I think that all later rods/bearings were "precision" and did not have shims?

Almost all the older GM inline engines had progressively larger main bearings going toward the flywheel. That would include Buick, Olds, Pontiac, etc. The prevailing idea back then was that the flywheel/clutch/transmission needed more support from a bigger bearing due to increased leverage and weight loading. In about 5-4-3-2-1 seconds, some "engineer' with no hands-on engine rebuilding experience will tell you I'm full of BS!
Jerry

Read what you wrote up above, again, Jerry.
The statement below is all that I questioned.

"NO 235/261 engines had Babbited rods"

No 235 engines had Babbitted rods?
All 1953 and earlier 235s (1937-1953) had babbitted rods.
These 235 engines had babbitted rods?

(I am not sure about the 1953-55 Corvette 235s)

Your original statement has nothing to do with the material used on 235/261 insert bearings.

Read the edit I made a few minutes later. Pre-1954 235's had Babbit-lined rods. There was never a 261 with that style of rod, since the first ones were made in 1954. Getting picky about what's on the top layer of an insert bearing just confuses anyone without a lifetime of hands-on engine building experience. No engines that I know of adjust insert rod bearing clearance with shims, which is how this fire hydrant watering discussion got started.
Jerry