Rear Main Seal Options

boataddict

Well-Known Member
May 19, 2020
163
32
Point Pleasant, NJ
After 15 years in storage, suddenly my new boat is leaking like a sieve. I strongly believe it is the rear main. It is a steady stream of oil now while running. It slows to a drip when not running. Comes from the rear of the engine on the starboard side, believe the flywheel is oily as well.

A little background about the boat first. I was lucky enough to obtain it from a neighbor and thankfully do not have much into it. I got it running after seeing it sit for 15 years (See my full thread: BIG 1996 Shamrock 22 Project). Now that it's finally running well, I notice this huge leak which appears to be worsening.

I don't think any sort of stop-leak solution will work on something this size, and I'm skeptical that it will improve enough from this current sorry state to be usable. Currently I have Bar's Rear Main Leak Stop in the oil and I'm trying to run it on the dry to warm and circulate the oil as much as possible, in hopes to rejuvenate that seal. If I could somehow get it to a slow drip I would be thrilled with that and live with it.

The engine otherwise has been running well, however I really won't know what I'm working with until I launch it next spring and see how it does. I suppose it's possible it throws a fit the first time I try to push the RPMs- really no way of knowing or putting a load on her on land.

So for this reason I am very hesitant to put in a couple thousand dollar repair on a motor (5.7 Indmar TBI) I really don't have a lot of faith in just yet. If I was going to spend that kind of money to pull the engine I'd prefer to have a new engine going in altogether. I'm a student though and right now that's not an option.

I've done all of the work myself so far, but I'm not sure if I can do the rear main. I don't think pulling the engine is an option for me. My question is this: I saw someone pulling the transmission out of their direct drive Mastercraft ski boat, and it didn't look like something I couldn't manage myself. Is this possible in our boats? Could I possibly replace the rear main by removing the transmission to get to it, rather than pulling the engine?

Sorry for the longwinded post, a lot of thoughts right now
 

Jonnydime

Active Member
Sep 7, 2021
38
1
It should be a 1 piece rear main oil seal so once you get the trans out of there it should be easy enough. You will need to drop the pan though.
 

Ship

Well-Known Member
Supporting Captain
Dec 15, 2008
11,930
301
Atlantic City, NJ area
If you have access to an engine hoist, a fairly cheap fix. And can be done in one or two days of time. If you know what you're doing it it can be done at about 6 hours.
 

Gary S

Well-Known Member
Feb 4, 2010
1,596
156
Algonquin Illinois,Key Largo Florida
Not being a Chevy guy I'm not sure you have to drop the pan. Fords with 1 piece seals you do not but 2 piece you do. Here is a video I ran across changing a GM 1 piece seal,might be a tad earlier than yours but I think they they are all very similar. Should be able to remove the trans and bellhousing and change it right in the boat. Start pulling engines- you'll get caught up in the might as well mode......
 

redneck

Well-Known Member
Mar 2, 2009
1,434
58
Middletown DE
There's also three oil gallery plugs and a cam cap that could be leaking. Is the rear china wall dry? There are oil pressure ports back by the distributor it is usually were they thread the pressure sender into I've had those leak. Support the back of the engine pull the trans degrease the area and run it to see where its coming from.
 

Ship

Well-Known Member
Supporting Captain
Dec 15, 2008
11,930
301
Atlantic City, NJ area
The input shaft that goes into the trans has a spline that goes though the damper plate bolted to the flywheel. It rusts and is a bear to remove. You MAY have to drill a hole in the bell housing in line with the bolts in the flywheel and remove them. That's may have to. Then the bell housing needs to be unbolted too. Remember to apply antisieze when reinstalling.
 

fishineer

Well-Known Member
Jul 31, 2011
198
26
Wilmington, NC
Another opportunity for some all thread rod to assist you when you slide transmission from the bell housing or the transmission and bell housing from the engine. Maybe someone with experience can advise you on which is preferred.
 

Mistress

Well-Known Member
Staff member
Supporting Captain
Jan 22, 2005
7,373
218
San Jose, CA
The input shaft that goes into the trans has a spline that goes though the damper plate bolted to the flywheel. It rusts and is a bear to remove. You MAY have to drill a hole in the bell housing in line with the bolts in the flywheel and remove them. That's may have to. Then the bell housing needs to be unbolted too. Remember to apply antisieze when reinstalling.

I almost had to do that on my Cummins...but they already have a hole to access the damper bolts. I ended up making some wedges. Damper took some abuse.

I would plan to replace the damper anytime I remove a trans.

Removing the trans is not trivial (IMHO). Id do everything I could to eliminate other possible leak sources before committing.
 


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