2 BBC rear main seal questions

[Ron Gusack]

I have a very small leak that appears to be coming from the rear main. I pulled the engine out to weld in a couple bungs and think I should put a new seal in at the same time. Should I be concerned about the end play changing when I put the cap back on? Should I offset the 2 halves of the seal? I've read both yes and no on that.

This is a drag race only MK IV with a 2 piece seal. I use manifold vacuum to get pan vacuum and it currently pulls about 4 inches with the seal in the normal way. Thanks for any advice.

[modok]

if the sizes are in range, and the alignment decent, shaft surface is good, and installed correctly......seals are SUPER reliable.
Although you don't have much control over the housing and alignment it at this point, if you do all else right and it still leaks then....that's why.

I like to do the overlap thing maybe 1/4=3/8 inch, but, I don't think it is very important, works both ways for sure.
I do think it is important to very lightly chamfer...just "dull" the sharp edges of the seal bore at the parting with a small chamfer, maybe 30 degrees.
Safer have a tiny gap there which sealant CAN fill rather than risk shaving off a piece of the seal.
And I just do a thin LINE of sealant from the seal out to the gasket, maybe a little heavier at the step of the register because the cap has a chamfer on the edge there and that needs to be sealed also.
don't over-do it with the sealant.

[vortecpro]

It will depends on how flat the edges of the cap to block mating surfaces are. When I block prep a BBC I'am very careful not round these surfaces when deburring. At one time I had a employee prepping the blocks, we had a rash of leaks, I always put the seal in flush, I got rid of the employee and the problems at the same time. Bottom line: if the mating surface has flaws, you dam well better stagger the seal. Another thing to look for if the seal is distorted when the cap is torqued up, in that case we cut the seal down.

[Ron Gusack]

It will depends on how flat the edges of the cap to block mating surfaces are. When I block prep a BBC I'am very careful not round these surfaces when deburring. At one time I had a employee prepping the blocks, we had a rash of leaks, I always put the seal in flush, I got rid of the employee and the problems at the same time. Bottom line: if the mating surface has flaws, you dam well better stagger the seal. Another thing to look for if the seal is distorted when the cap is torqued up, in that case we cut the seal down.

Are you saying: If the distorts when the cap is torqued it means the halves are too long and one side of one half needs to be trimmed to reduce the butting of the halves?? If that's what you mean by cutting the seal down, how do you cut it to still be sure it stays square to the other half?

[leahymtsps]

It will depends on how flat the edges of the cap to block mating surfaces are. When I block prep a BBC I'am very careful not round these surfaces when deburring. At one time I had a employee prepping the blocks, we had a rash of leaks, I always put the seal in flush, I got rid of the employee and the problems at the same time. Bottom line: if the mating surface has flaws, you dam well better stagger the seal. Another thing to look for if the seal is distorted when the cap is torqued up, in that case we cut the seal down.

Are you saying: If the distorts when the cap is torqued it means the halves are too long and one side of one half needs to be trimmed to reduce the butting of the halves?? If that's what you mean by cutting the seal down, how do you cut it to still be sure it stays square to the other half?

Morning Ron,
It usually means the cap has been cut for align boring - honing and the ridge that holds the seal is now kind of egg shaped. Best
thing to do is bore the ridge back to it's original diameter and you won't have to modify the seal, also it can reduce drag on the
crank if the cap has been cut very much.

Tom

[Ron Gusack]

Thanks Tom, It has been align bored and after that I had fretting issues so maybe they took a little off to get rid of marks left by the fretting. This leak is so minor that I'm tempted to let be. I just hate leaky stuff. If the problem is the seal not being round, I should be able to see that, right?