I have a 1973 F-250 4WD pick up truck that started life as a fire dept. vehicle. It only has about 35K on it BUT it has what clearly appears to me to be a rear main seal oil leak. The oil comes out around the rear/back flange of the oil pan. The leak seems to increase when you inrease rpm (drive it around). When you stop the vehicle you get small (20-30 drops) of oil on the ground which is not all that consequential BUT looks like a small lake on a clean drive way . Because of the height of the truck I can drop the oil pan without putting the truck on jacks, I just have to disconnect the oil pump pick up. So far I have dropped and replaced the rear main seal 3 times!!!! I am using FELPRO rear seals (installed with the lip facing the right way) and rubberized oil pan gaskets. I follow the directions in the Chilton's book re offsetting the rear seal 3/8"between the upper and lower seals, where to put RTV, etc. It seems to be a very simple staright forward job and each time when I get it back together I think to myself, "That wasn't so bad, I'm sure I've fixed it this time" and EVERY timeI am wrong. 10-15 minutes after I start the truck this (relatively) little leak comes back. When I take the oil pan back down I note that the bolts from the rear bearing cap have been pressed into the rear of the oil pan gasket. After having read all the posts i replaced the intake manifold gaskets and valve cover gaskets, but still the leak! What is the trick here?! I have been turning wrenches on cars for 35 years and while most/all of my experience is with Chevy I don't understand why I cant stop this leak! When I pull the new/old rear seals back out they look fine, they have not been crushed on the ends/distorted/torn or cut. It is really starting to annoy me. Before I declare myself defeated ("3 strikes and you are out" is a pretty good rule of thumb) I thought I'd see if anyone has some trick or idea for me. As I said the removal/installation goes routinely (though I hate those little "nails" that I have to fight to get in place in the side seals). I have read all of the previous posts on FE motors and rear main seal leaks/oil leaks and I don't see any "ahas". Thanks Scott

 

maybe that's not your problem? maybe it is misleading you. and you only think its a rear main. maybe pressure wash engine and see exactly where leak originates from??? i dunno just a idea

 

I had a 1975 with a 360 in it and I could never get the rear main to stop leaking. I replaced it 3 times and after each replacement, it would stay dry for about 6K miles and then it would leak again. After the 3rd replacement, I gave up and just drove it. It my case, the oil pressure was low. Maybe the crank in mine wobble enough to ruin the seal?

 

Do confirm it's the rear seal. Easier said than done, I know, but if it is, you might try this. Ford did on the Ford GT engines with bad crank finishes...

http://www2.vsm.skf.com/usa_english/node930.aspx

How does that shaft look and feel when you replaced the seal? It doesn't take much for a leak methinks.