VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc NOS GM Main Bearing sizes

I have a question on the sizing of NOS GM main bearings listed in the 1941 and 1942 Chevy manuals. These are the bearings which I believe are the ones that were made before they came out with precision bearing sizes.

I sold a customer a set of NOS GM main bearings for a 1937 Chevy 216 engine. These were 603943 front main, 603944 intermediate main, 603946 intermediate rear main and 603945 rear main all in the original boxes. My customer went to install the crank (standard size mains) once these bearings were installed and the crank will not go down into the bearings. He is tell me that these are undersized and figured around .030 or so. Is this common for these bearings? In the parts catalogs, all it list is "BEARING, Crankshaft Front" etc. with the different year applications but doesn't say anything about undersized so I assumed these were standard size. Guess I was wrong. Are all the original GM replacement main bearings from 1929-48 undersized if there is nothing in the parts catalog specifying whether they are standard or not. I was under the impression that all these main bearings especially the ones with 6 digit number were all standard size from the factory before they came out with the precision size 7 digit number bearings shown in the 1954 parts catalog.

Can someone shed some light on this?
Bob@marxparts

Original NOS bearings for those years were not finished to size and needed to be installed in the block and line bored to fit the crank. For that reason they will appear to be way undersize out of the box. They will have the advantage of you being able to size to whatever your crank is, but the disadvantage of requiring finishing.

After about 1948 Chevrolet supplied the precision bearings for all earlier engines that originally came with the line bored bearings. They came as standard, .002 under for a "slight polish", and .010, .020 etc under for reground crankshafts.
So the bearings you supplied are right for a 37 216 but will need to be bored to fit the customers crank. The later precision bearings will also fit, but you need to know the crank size.

The 1948 engine was the first to use precision main bearings. This eleminated the line boreing of the main bearings and just one beaing could be replaced with nothing but direct installation.
At this time the "bearings in the rough" used from1929-1947 wee no long sold.They had extra thick babbitt and could only be sold in complete sets and required line boring and just one could not be replaced.
The precision inserts then could be installed but all were reqired and the part numbers changed.As an example the 1937-1947 bearings werre part number 3847064 for a standard size front, 3847065 front intermeadite, 066 rea int. med and 067 for the rear. They came in Std., 002", 1010-020 and 030 under size. Any 1948 and up parts book will have all the numbers for the earlier models.
The other advantage to these bearings is they had thinner babbitt and the babbitt did not compress as much in use and the bearings did not "loosen up" as much.
Also the major aftermarkket bearing makes offered them shortly after.

For 1948 and up the rods also had thinner babbitt for longer life.

Chevrolet sold only standard size rods but after market sold under size rebabbitted. The under size usually lossened up faster due to babbitt compressing.

That is an advantage of rod inserts as the insert backing is made thicker so the babbitt stays thin as the original standard size.

As note if a main bearing was chipping out babbitt on a earliermodel it was know that just the lower insert from a precision bearing could be installed and adjusted properly and it worked just fine. The upper half of the insert never carried any load so always remained OK.

Mr Schneider
My apologies for sort of hijacking the thread, but are you saying that if I pull the main caps on my 46 and don't find enough shims left to get a proper adjustment... I could put new standard or .002 under precision shells in all of the caps only, and shim as required and that will work?
I hope that's what you are saying as it would save a lot of work! :-)

That would save pulling the drive shaft, transmission, radiator, damper and pulley, and timing cover. dropping the crank 3/8" etc.
Was that really done?
If it was, and it worked, I might try that!

While we're on bearings what is your feeling on filing rod caps when the babbitt is still fine but the shims are all gone?
I know it destroys the ability to rebabbitt or machine for inserts, but if I'm not concerned about that, is it a "passable" practice?

Thanks for your thoughts and expertise.

First Mr. Schneider was my father, Just call me Gene.

I am not sure if replacing all the lower inserts would work OK. AllI remember that in 1950 I graduated from high school aand was working at a large Chevrolet dealer. I bought a 1941 Cevrolet with 75.000 miles on it and was never overhauled. Had one of the mechanics overhaul the engine and he found the babbitt begining to chip out on the rear intermeadiate main. He replaced the lower insert only with a precision bearing and adjusted it correcly. I drove the car for over a year and 20,000 hard miles with no problem. When I worked in the parts Dept. it was not unusual to give out a main bearing and just the lower was used.
In your case I do not know if replacing just the lower would reduce the clearence but it probably would as the lower wears and the top untouched. If it were mine I would consider doing it.
Another thing that was done is place feathered shim stock under the bottom insert. I have some that was made by Perfect Circle.

Thanks very much!
Nothing beats hearing about how things were actually done and what worked in reputable shops when these vehicles were current and state of the art. I may give this a try.
A 1941 was your first car then? Always thought 40 and 41 were some of the best looking cars Chevrolet made. I like the look of the 40 a little better, especially the dash etc, but the 41 has the advantage of several improvements to the old 216.
Thanks again.

Actually my first car was a worn out 1939 4 passenger coupe. The 1941 club coupes were very popular in 1950- with the young guys, I seldom kept a car very long in those days. The 1941 was replaced by a 1949 in 1951 and the 1949 replaced by a1950 Belaiir in 1952 and that replaced by a new 1953 and next was a new 1955 and 1957 and a wife in 1958.

Hi Gene and Ole
Thank you both for your replies and insight.

This is what I kind of figured after the fact. I purchase a pop can skid of NOS 1929-48 main bearings (mostly 37-48) in the original boxes about 25 yrs or more ago at a swap meet in Northern Wisconsin at a great price. This was the first set I ever sold out of the lot as no one ever asks for standard size bearings. I was always under the impression that they were standard size. I do know that the after market did sell the precision sets in different undersizes as did Chevrolet which is listed in the 1954 parts catalog. Time to do some measuring to see exactly how much undersize these bearing that I have are. This way I can sell a set of bearings to someone that is actually grinding the crank to undersize and line bore the bearings to fit.

As for just replacing the lower main bearing if you have any normal wear. This is not a problem if the upper bearing shells are ok. As Gene said, the lower bearng shells are the ones that wear because of the weight of the crankshaft and gravity. The upper bearing shells will still be located the same distance away from the center line of the crankshaft which will have the same oil clearance as before once the lower bearing shell is taken up for wear by adjusting the shims, placing shim stock between the bearing shell and bearing cap or replacing the lower main shell. As long as the bearing shells are stable and can not move around under load and maintain the correct running clearance, the fix will work.

It should be noted that which ever main bearing controls crankshaft thrust will only have 1/2 the surface to limit the movement. There may be a tendency to scrape oil from the shaft and accelerate thrust wear. Just posted so people will be aware of the potential. Likely not a real problem for our low yearly mileage babies.

This is an interesting thread. Thank you all for sharing your expertise but I think I'll just take the old 216 to the machine shop and let them have at it instead of adding a cap here, boring a bearing there, here a shim there a shim, etc. Pretty soon one would be humming "Ol MacDonald had a farm." Mercy!

Stovbit You said:

"I like the look of the 40 a little better, especially the dash etc, but the 41 has the advantage of several improvements to the old 216."

We need to talk. :nana

Best,

Charlie