I found my port engine inboard mount loose; the engine was visibly mis-aligned. It had to have happened last time I ran the boat.
Pulled the drive off and looked into coupler- splines OK, but there is a rubber doughnut/washer on engine side of coupler splines that seems to have slipped down? Is this just a cushion for gear shifting, or does it need to be slipped over the drive shaft?
First time I've had a drive off this boat!

That is probably an 0 ring off the drive shaft. Check the shaft to see if the 0 rings are there. I replaced the shafts on my 1990's and they had 1 0 ring. The new ones had two. If it is off the shaft you will easily see a groove on the shaft, that is where the ring goes.

All the o'rings are accounted for and on the drive shaft- this is a large rubber ring about 1/4" thick & 2" dia. floating between the coupler splines and engine block- it is concave in shape on the side I can see.

There is a rubber washer just behind the gimbal bearing. It is between the gimbal bearing and the engine coupler.

It sounds like it is the gimble bearing seal. It is a rubber seal that is mounted to a steel carrier. Seems it came apart and the rubber part walked up the drive shaft. The part is cheap (18.00) The bad news is you need to remove the drive, Gimble bearing and then replace the seal.

It is not a critical seal but it prevents water and moisture from entering the gimble bearing from the engine compartment side.

How is it possible that this seal got on the other side of the coupler splines, between the block and coupler? It is much larger in diameter than the I.D. of the coupler splines. To the best of my knowledge, the engines have never been out of this boat.
Finding the coupler still in good condition (glad I caught the loose engine mount quick enough!), I'd hate to have to pull the engine just to get this seal on the right side of the splines.

You dont have to pull the engine to get to the seal, After the drive is off you need to pull the gimble bearing and the seal is pressed in behind that. I am assuming it is a bravo drive, The seal that is on the splined shaft will fall of when the drive is removed. Maybe I am missing something here as you said the drive is off, Possibly the seal is stuck to the coupler via grease? and just hanging there. I dont know for sure but the size of the seal you stated is about right. It could be the seal was put in backwards and that's how it ended up there.

You said between "Block and coupler" what do you mean by that? The coupler mounts to the fly wheel. Do you mean between the block and the fly wheel??

Ray- It is a Bravo drive mated to a 350MagMPI-
Sorry for the confusion; it is between the block and flywheel, on the back side of the coupler spline as viewed from the the gimbal housing, just floating there. Impossible for it to have worked its way through the female spline- it's far too large.

If it was between the block and the fly wheel. than its the block seal.(rear main seal) That should of made an oily mess though????

Sorry for the confusion; it is between the block and flywheel, on the back side of the coupler spline as viewed from the the gimbal housing....

I am not understanding you. If this seal is floating between the block and the flywheel, then how can you see it through the gimbal housing?? The space between the block and the flywheel is very small. The flywheel bolts directly to the back of the crank. The only seal that can be there is the rear main engine seal. I also can't figure how you can get to see this small area with the bell housing on and the motor still in the hull.

I don't mean to sound critical, I/we just don't understand you.