Stiil need some advice on this 68 390 rear main seal leak, not a drip. I did a two piece seal, and didnt offset the joints with the mating surfaces. Will that definitley cause a flowing leak? or is it just a good thing to do? There does not appear to be a score on the crank where the seal rides. new pcv and has vacuum. NOT the oil pan, and at this point I have not pulled the fly wheel for further investigation. People told me loose bearing clearences, and the seal wont be able to do its job. Or, too much blow by increasing the presure and the seal cant hold it back. But my main question is the two piece seal, does it NEED to be offset?? It is just a old work truck that is not worth too much more time, but I would hate to dump it if it is just an easy fix. Any thoughts

 

I didn't offset mine either but I used black RTV bewteen the main cap and where the seals meet together. No leaks so far!

 

this is a tough one. maybe do what ratsmoker said? im stumped too.

 

Could it be possible that the rear-main seal was put in backwards? That would cause a pretty good leak I would think.

 

If this is a two piece rope seal, then you do not have to offset the pieces. From the factory, they are flat even with the main cap and block surfaces. You can cause a leak by leaving little bits of rope between the cap and block. They, in effect, will increase the bearing clearance, and thus allow more oil to blow past the bearing, sometimes overcoming the seals effectiveness. If the side seal pieces are not pushed in flush with the meeting surface, you can get leaks there, too. If you do the RTV thing, make sure that you have a recess for the RTV to ooze into, but really, you shouldn't need it. The ends of the rope seal can be offset even now, all you have to do is get the upper to move a little, and then do the same amount of rotation of the lower... reassemble and smile at your own trick. tom

 

The rear main seal will seap, if you have a flowing leak I would guess that the expansion plug at the rear of the cam shaft has blown out.You say that you haven't pulled the fly wheel ,pull it and look.