I'm trying to replace the oil seal between the 5th / main drive gear, and the main shaft, part # 12035B. Is there a trick to removing it, or does the main shaft need to be removed to replace it?

 

Thread a small sheet metal screw into the seal, one on each side, then pull on the screws with a pair of pliers, one side then the other until it walks out.

 

I found it totally impossible to remove that seal with a couple of screws, as you can with the shift shaft seal. The slot is too thin and it is impossible to get a drill close enough to the shaft to drill holes in the seal body and beside, that seal is extremely thin so I found it absolutely impossible to use screws to remove it without badly damaging the mainshaft boss on which the seal rides. If you nick that, it is very likely the new seal will leak.

The only way I was able to get it out of mine (and it took me 3 days of fiddling with it!) is to VERY CAREFULLY!!!!! run a very small screw driver through the black rubber seal (stay away from mainshaft boss) and put it under the very small metal lip of the seal and simply prize it out by going around and around and around . . . prizing it out little by little. It is not easy. For part of it, I damaged the small lip and actually had to use a sharp awl to get between the seal body and the inner fifth gear and wedge some of it out so I could get a bite on it. It was a huge PIA.

The pic below shows what these seals look like so you can see how thin the metal is. But mine sure was in tight!!! Just take it slow & be very careful to avoid damage to the mainshaft sealing boss.

 

OP are you talking about the shifter shaft seal or the main shaft seal. If the mainshaft then the drill and sheet metal screw trick works good. For the shifter shaft I'd use a 90* pic.

 

That PN is the 5th gear/main shaft seal and I have had the same experience as M&P340. The technique takes a bit if trial and error but once you see that you are moving the seal, you will know you have it mastered. Takes me about 15-30 minutes to get one out. You do have to be careful with the mainshaft as you don't want damage the surface where the seal rides but the mainshaft is not as "soft" as one might think. Be careful, I have gotten in a hurry, zigged when I should have zagged and actually pushed the seal in a bit instead of moving it out bad juju.

Every time I remove one, I swear that I will find some sort of hard plastic tool, or make a purpose built tool, to remove that seal. However, once the seal is removed and I can get on with the task at hand, the urgency for an alternative tool burns off and doesn't return until I have to remove another one.

It is a PITA but be patient and you will get it out.