Fitting main bearings on '41 201, use of tapered shims

[12905]

Carla:

Don't us the taper shims! I'd rather see the m/b reground to 0.020 under - or metallised and reground standard. Many, many years ago I worked in a rebuild shop where we rebuilt virtually every component from shock absorbers to engines. At one point the shop investigated the practicality of using tapered shims on freshly-ground cranks with new bearings and the result of their research was a resounding 'don't do it!'

My understanding is that those shims were designed to be used with worn shells that would normally have worn so the bore with the shells in place would be an ellipse with the major diameter in line with the cylinder bore. In that case, inserting the shim would turn the bore in the rod or main into an ellipse with the major diameter at right angles to the cylinder bore. This elliptical situation would force the elliptical bore of the worn shells back into an approximate round and somewhat extend the life of the bearings.

If you put shims under new shells, you are just distorting the shells and defeating the purpose of "precision inserts" as they were known when they first appeared. It's a really bad idea. Personally, I wouldn't let the darn things inside a shop door.

[David Pollock]

I have used tapered shims with good results, and have had no bearing failures. With a new bearing, such as the rear main you describe, a straight shim would keep everything concentric but the tapered one would avoid what is mentioned in the next sentence.

I have mixed feelings about dressing the ends of the shell so that the shimmed shell does not interfere with tightening the cap. It would be bad news if the shell itself could move, however, too much "crush" could also be harmful.

A rear main which is at the tight end of specs is a lot less likely to leak at the seal than a loose one

[Carla]

Hello, Plymouth people,

You may remember the story of Bob S.'s engine, from a few months ago, the one which was allegedly rebuilt by a shop in Campbell, at enormous expence, then had a bearing failure when it first ran. Well, the saga of the '41 engine progresses, after seemingly endless waits for some parts and farmed-out machine work......

I got started on the re-assembly the other day, but found the main bearing clearance to be just a little bit more than I'd really like. Appearently, when the crank was ground, the grinder went .001 under the nominal for the .010 grind, so the mains checked out at .0025 to .003 clearance, instead of the works manual spec of .0005 to .0015.

I was able, after quite a bit of calling around, to find a set of the old Perfect Circle tapered shims, in .002, and had planned to use them to tighten up the bearing clearances to spec.

Now, years ago, when I was in the engine business, we commonly used tapered shims to snug up bearings which were just slightly too loose, it was a standard shop practice. The tapered shim, in this case a .002 shim, is a rectangle of steel shim stock of the dimensions of the bearing insert,  .002 thick in the centre, tapering out to nil at the ends, which is fitted under the insert shell, between the shell and the bearing cap.

Its seldom that they're needed, but when they are, I've used them on several engines, one time and another, with good results.

The reason I'm bringing up this item here is that Bob got into conversation with a well known Plymouth restorer, who told him quite emphatically that using these inserts was a really bad idea, so now Bob is quite concerned.

So, I thought I'd ask.......has anyone here had bad experience with taper shims when fitting bearings, and, if so, what happened?

(and, yes I know that the generality of mechanics, today, would simply assemble the engine with that .0025-.003 clearance on the mains, and say there's nothing wrong with that much clearance.......and I'm willing to do that if the engine's owner insists on my doing so......but I'd really rather bring the clearance down to factory spec.)

What think you, gentlemen?

cheers

Carla