Main cap sizes verses block main size.

T

Tom Elliott

Senior Member
After looking at several old main bearing caps it appears the are of a different diameter (or radius) than the block main bearing seats. I realize that a lot of caps were filed down but even taking that into consideration the diameter seems quite a bit smaller.
Does this make any sense to anyone or am I missing something here? The block main seat is half of the radius and the cap with shims should be the other half. (Hope I'm making sense to you all) If the original shims were .004 and the caps have been filed it makes sense to add extra shims to bring the cap to its original position. But the amount of shims on some caps to do this would be overkill. What are your thoughts? Thanks, Tom
 
Tom Elliott said:
After looking at several old main bearing caps it appears the are of a different diameter (or radius) than the block main bearing seats. I realize that a lot of caps were filed down but even taking that into consideration the diameter seems quite a bit smaller.
Does this make any sense to anyone or am I missing something here? The block main seat is half of the radius and the cap with shims should be the other half. (Hope I'm making sense to you all) If the original shims were .004 and the caps have been filed it makes sense to add extra shims to bring the cap to its original position. But the amount of shims on some caps to do this would be overkill. What are your thoughts? Thanks, Tom
Click to expand...

I also see this all the time as I have not seen a unmolested engine for a long time hard to say if that is std or they all have been filed but most seam not to have been filed judging by the finish
 
Just to clarify the block main bearing tunnels seam to be bored larger than the caps making the Babbitt thicker in the block then the caps
 
I am somewhat confused also. Are you measuring across the parting line (diameter), or are you measuring the depth of each (radius)? Measuring the depth of cut in the cap can more accurately tell you how much was filed away from the cap.

Do you have some numbers to give us?
 
From my observation the finished babbitt thickness in chemically bonded caps is thinner than in mechanically bonded caps ---the earlier forged rear caps have thinner babbitt than the later cast caps.
I have a set of NOS caps,some finished, some rough,some Ford, and a few "GYP" and unmolested original caps/babbitt that still had 2 shims per ---ruined by honing and ridgecutting without proper cleaning, I'll take some pic and measurements

I can take pictures tomorrow but won't be able to post till late
 
The thickness in the forged caps are about half the thickness as in the cast caps.

As Kurt said, when tinned, it bonds the babbitt. You don't need the thickness.

But on a peened cap, and block, you need the thickness of the babbitt for strength.

When I first got on the fourm here, what was pasted around for the truth, was you can't grind a crank over .020, as thet makes the babbitt to thick, and it will break up and fall out, and this was from guys that poured some.

Translation, "My babbitt falls out, and I know it couldn't be me, so I think it is the thick of babbitt".

You have to get the hole round, at least for the slinger area, so as to fit the crank spacing, of the rear thrust O.D., and that takes more shims then Original.

Most bearings, insert type in other cars with babbitt, will have from .015 to .050 thousandths babbitt for the wall thickness.

After a bearing has worn .006 thousandths or more, it is junk.

Herm.
 
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Thanks Herm, that explains why the forged caps are a smaller diameter. I found a round tube that fit perfect in the block main bearing saddle and the rear main cap also fit that round tube perfect, but the front and center caps wouldn't start to fit that tube. They were too small a diameter. ( I'm talking without Babbitt of course) That fits with the description of how the thickness of the Babbitt is thinner in the forged front and center caps verses the cast rear cap and block.
 
Tom, no, B caps would be bigger. We have several front and center A caps and they all are a smaller diameter than the rear cap and block bearing saddle. I am thinking because front and center caps are forged and rear cap and block are cast is why the difference.
 
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