Well, as you know we were having problems with an oil leak at the bell housing and we wanted to get this repaired before heading off to Mexico and beyond. We were loosing about a quart of oil every 40 to 50 hours and were concerned that it might get worse over time. After doing some research I was pretty confident that the leak was coming from the rear main seal on the crankshaft. The only way to get to this seal is to remove the transmission/bell housing assembly and I was concerned that it would mean removing the engine to do it. I had a local mechanic come by and take a look and he said it could be done without removing the engine. I had my doubts but was happy to hear his plan. We came up with a deal that I would do most of the work myself while he would come by periodically to check the work (this kept the labor rate down). So here is what we did.
1. Drain engine oil and transmission oil
2. Disconnect the propeller shaft coupling and slide the shaft aft through the dripless seal as far as it will go
3. Removed the top nuts on the rear motor mounts. Disconnect the mixing elbow by removing the four bolts that hold it in place
4. Jack the rear of the engine up one inch and block with wood. I used a small car jack and it was placed just forward of the bell housing.
5. Remove the two motor mount brackets on the bell housing
6. Disconnect the shift lever cable
7. Remove all the bolts that hold on the bell housing. The transmission will not drop as four of the are nuts that thread onto posts that hold the transmission in place (see picture). Two of these bolts are low and can be challengine to get to. If you can't reach them a young, nimble mechanic/friend is nice to have around. I had to call the mechanic back to get the two bottom nuts off. I really didn't think he would be able to reach them so in anticipation of having to remove the engine I disconnected all the hoses and wires. He got them off no problem.
8. Slide the transmission off and lay in the bilge. No parts will fall out and it's not really that heavy.
9. Unbolt the dampner plate and set aside.
10. Unbolt the flywheel collar and set aside.
11. Unbolt the flywheel, an impact driver will help.
12. Carefully remove the flywheel using a puller. It is heavy but can gently be lowered into the bilge.
13. Clean the entire area before removing the seal.
14. Remove the seal (about $14)
15. Replace the seal. Gently tap into place so you don't tear it.
16. Re-install the flywheel and torque bolts accordingly
17. Re-install the flywheel housing and torque bolts accordingly
18. Inspect the dampner plate and re-install or replace
19. Clean the inside of the bell housing that is still attached to the transmission. Inspect the front transmission seal and replace if necessary.
20. Lift transmission into place and bolt back together.
21. Re-install rear motor mount brackets
22. Lower engine back onto rear motor mounts, remove blocks and jack
23. Tighten rear motor mount nuts, reconnect shaft and check alignment, re-align if necessary
24. Reconnect gear shift cable and replace transmission fluid and engine oil.
25. Run engine and check for leaks.
It took a total of about five hours spaced over three days (waited for mechanic to come and check my progress). Though it seems intimidating it really isn't. Some of the tools really needed that helped along were the impact driver and flywheel puller. I did apply some penetrating oil on all the bolts the night before, don't know if it was necessary but I didn't want to chance breaking any bolts off.
Here are some pictures we took during the work.
removing bell housing bolts. As you can see I taped a reminder that the engine fluids were emptied
Transmission/bell housing unit removed and laying in the bilge. Notice the oil all over the inside of the bell housing. I had taped reminders where all the hoses and wires were disconnected in anticipation of removing the engine (didn't happen, trust your mechanic)
With the dampner place removed and the flywheel unbolted the flywheel was ready to be pulled. Notice the placement of the jack. You can also see the studs that keep the transmission/bell housing from just falling off when it's unbolted (the upper starboard stud came out with the nut but was later replaced) 
You can see the entire area behind the flywheel covered in engine oil, a pretty good indication that the rear main seal was leaking (no other way for oil to get inside the bell housing) 
Clean everything up before removing the seal so you don't contaminate the inside of the engine. This is also a good time to start cleaning up the engine compartment. 
The rear main seal is now ready to be removed and replaced. The stud sticking out where the flywheel bolts is the aligning pin for the flywheel.
Copyright 2004, 2005, 2006 S/V Bruadair. No reproduction without written consent.