My mom has been having a problem with the main seal on her 1991 for explorer. It has a 4.0 with about 130,000 miles on it. It was leaking from the main seal really badly so she sent it to the shop to have it replaced. The problem is that it slowed the problem but just barely. Our mechanic, a close family friend has redone it twice using a bigger seal to try to make it stop but that hasnt helped. He's been out of action for a couple months for surgery but when he is better hes going to try to get it fixed again. I think that the block is so wore that its not sealing right.
Does anyone have a suggestion on how to make it stop leaking? It leaks about 1/2 quart every 150-200 miles.

 

Are you talking about the "Rear" main seal. It is a 1 piece seal on the rear of crank shaft. Had mine replaced and still had some leakage. On subsequent tear down for transmission repairs, inspection revealed that the seal was not leaking, but the oil pan gasket at the rear was seeping oil.
If you are sure it is the seal, I saw an article on 4.0 rebuilding that mentioned a seal kit that inclided some kind of sleve that went on crankshaft seal surface to beef it up for better seal face contact.

Excerpt from article:
The rear seal on the 4.0L has always been prone to leakage. It appears that the original factory finish on the seal surface was too slick, so the seal just couldn’t control the oil. Ford offers a repair kit that comes with a sleeve and a seal (p/n F5TZ-6701-A), but it costs about $50 at the dealership. Rebuilders should definitely install a sleeve over the factory seal surface, but there are better alternatives in the aftermarket; Micro Sleeve (800-475-3383) and Classic (800-393-0544) both offer sleeves for the 4.0L cranks.

Hope this helps
Dialtone