To say that I'm now totally confused is a massive understatement. Here is a photo of the old cracked cap and its bearing and the newly found one and its bearing. What is the difference?
Indeed. They "look" identical to me too - save the relief on the front face (left side in the photos). I have a '54 in pieces in the garage - I need to refresh my memory to see what that is for.
I just looked at the change to tang bearings - it affects just the upper shells that go into the block. Apparently they did this to allow the top bearing to replaced without taking the engine out of the car (or dropping the crank for that matter). The old bearing could be rolled out and the new one rolled back in - the opposite direction - with the tang acting as a stop. Not sure if there are any other differences in the way the bearings fit with respect the parting lines between cap and block.
The change in the part numbers may be as simple as that relief on the front face. But it is something that would certainly need to be noticed early on - because the timing cover fitment is late in the assembly - making for a lot of lost time (if the wrong cap were used).
