MrMcD

MARK.
i read this and im confused as to RR .will the teflon work for RR and FR. Mine started leaking today Its probably the orig one. The boat sat for twelve years but had a hard 360 hours on it. Im finding more crap nto fix every day. 1980 sn with 351 commander./pcm

If you determine it is the rear main seal it is not that hard to change and I would highly recommend running a National Rear Main seal, a Teflon if available. The Teflon seals are a lifetime seal good for a million miles. They come with a plasic installation tool you just slip the plastic over the tail of the crank and push the seal up and in with the plastic protecting the Teflon on the way in. Teflon is easy to damage on install but not if you use the provided plastic installation tool.
It will seem odd because the tool is large enough on one side to slip on the crankshaft and the other is smaller and fits in the seal. When you press it onto the crank the tool helps the Teflon lip expand up and over the crankshaft.
Put these in clean and dry. Dry because you want the Teflon to start transferring to the crankshaft. Once it does you have Teflon rubbing on Teflon and zero leaks for life.
They are a really cool seal, I have used several. They originally were designed to dry up leaking rear main seals on diesel engines. They work great.   NOTE: if your engine is reverse rotation they will leak, the Teflon is threaded for standard rotation.
I bet your 1998 has a standard rotation with the 1.23 drive reversing the prop only.
The tranny is much easier to pull than the engine. Pull the tranny or slide it back out of the way and install your seal.   Should be 2-3 hours work tops.   If my memory is correct the 343 is the older 5.7L engine?   If it is the newer 5.7L LS style block Fel-Pro offers the Teflon Seal for that engine if it is the older 5.7L vortec style block National offers the Teflon rear main seal. These are only available in full round standard rotation seals.

Keno is right on this. I did not catch the year or even make of your engine.
I don't think Ford went to the full round rear main seal 1 piece seal till around 1986.
Same for GM. The older engines all used a 2 piece seal.
Two piece seals are in the Rear Main Bearing Cap. I think if you need to change one in a boat it would involve lifting the engine up and maybe out of your boat to do this job right.    Later model One Piece seals are pushed in from the rear of the block and could be changed by removing the Tranny.
The Teflon seals are the best seal out there but will not work in any reverse rotation engine. The seal is threaded for standard rotation engines.
If you have the 1.23 tranny you should be able to use the Teflon seals but yours in 1980 would not have this as an option. Sorry to misguide you and Thanks Keno for the assist.

Gary, I have not seen that but will look, the 2 piece seals work on a shaft size that is the same as the bearing size. The one piece applications all increase the seal surface to about 1.5 to 2 x the rear main bearing size. I am not sure why since that would increase the actual contact speed on the seal by the shaft.

Joby, the pan would need to come off and I don't know that you will have room for that in the boat. Then the rear main cap is dropped and the seal can be installed. I have done them under a car and on the engine stand but not with 6 inches of room under a boat. Does not sound like a good bet for success. They have to be perfect or will leak even more.   Have you really looked hard at the back of the engine to see if the oil may be coming from some other place, maybe the oil pan?