I have a 2004 Duece FXSTD. Drops of oil was found on the floor under clutch area of the primary. After some inspection, I found that it was streaming from the inner primary.Got a seal set from Motor Factory, changed all the seals, including a genuine part for the bearing race and seal, but oil is still leaking. Tried an geniune part inner primary seal, still the same, got a double lip inner primary seal and thought the problem was solved but after a second warmup to test one more time, oil leak again! I was asked to check for cracks on the inner primary, but could not see any. Anyone can help me out with this problem?

Did you check your front belt pulley? These pulleys are made of relatively soft cast iron as compared to the final drive gear. This is so that the easily replaced and relatively inexpensive front pulley wears before the difficult to replace and expensive final drive gear wears. The pulley will wear in the splined inner bore and will get loose on the final drive gear. Then transmission oil will run out of the pulley and down the inner primary. This is because of the lose of pressure on the "Quad" ring behind the pulley spacer. This may be your problem.

Another mistake a LOT of people make when replacing any seal is putting the seal up dry. On the inner primary seal, you want to put a bit sealer compound around the outer edge of the seal where it goes into the case. Although there are a lot new sealants out there, I still prefer good Permatex non-hardening. Once you get the seal in and straight, oil the inner seal surfaces and the bearing race. If you put the seal up dry, you start the engine, the mainshaft turns about 1000 rpm. This will burn up the delicate rubber lips on the seal almost instantly because of friction before the oil can get to the seal. You MUST lubricate the seal or it WILL burn up before you can blink an eye.

Also check the position of the inner primary bearing race. It must be 0.100" from the final drive gear. If it's way out of position, the seal may not be riding properly on it.

If you are in doubt about whether the transmission is leaking or the primary, refill the primary with a quart of automatic transmission fluid, the red stuff. It will work long enough for you to determine whether it's the transmission leaking or the primary leaking. If it's the transmission, check the front belt drive pulley for wear, You'll usually find that the large nut that holds it on is loose.

Good Luck
Steve