Inliners International Forums General Discussion Engines 261 engine - cracked front crank end cap

Yes exactly. When the engine is installed the crank is riding on the shells in the bearing caps and a film of oil - with the upper bearing shells providing the clearance were the oil pressure is delivered. The distance above the parting lines is an engineering spec - with upper and lower bearing shells being matched.

I checked the '54 in my garage - its front bearing cap looks identical yours but it does have a casting number (3701245) along with that same raised dot casting mark between the oil drain holes:


So this older design cap may well be of use to you (with the newer design bearing shells). But there are some details to look after when you are measuring the clearances.

The 1954 and First Series 1955 engines still used shims to establish a precise fit on a particular block and crank pairing:

This meant that during assembly all four caps were measured and selectively fit with shims of differing thickness'.

The second series 1955-62 engines used more precisely ground caps and bearings which allowed an engine to simply be assembled without shims:


As you can see the specifications on the cranks and bearings are pretty much identical (GM engineers also gave up the fractions in favor decimals). The newer design calls for tighter clearances on the front two caps vs the rear two.

If it were in my garage I would proceed as follows. Take the old (broken) cap and the old bearing shell and measure the clearance with Plastigauge - crush-able plastic that is placed between bearing and bearing surface: torqued to spec and then remove the cap and bearing shell and read the clearance. Clean off the crushed plastic and replace it with new. Now repeat the process using the old bearing shell and the new Cap. I am curious to see if the clearances are different and if so by how much.

But before you do anything - I have an important question. Are you trying to avoid taking the crank out of the block?