Trying to replace gasket between powerhead and shaft housing

I'm pretty sure the gasket where the powerhead couples to the driveshaft housing is blown on my 84 Mariner 25hp. I don't have a service manual for this motor but in looking at the parts schematics, it appears there are only 6 bolts that hold the powerhead on. I've disconnected the throttle cables, shift linkage, all 6 bolts. There are still some wires connected and the rubber fuel line but those are not the problem. I can't get the powerhead to budge and I can't figure out why unless it's glued together with gasket maker/sealer. I'm afraid to put any more pressure on it for fear of breaking something. Anyone got any suggestions on what to do to get them separated without breaking it? Wish I had a service manual so I could be sure there's no more bolts that need to be removed.

Parts schematics. https://www.marineengine.com/parts/mariner-outboard-parts/25c-thru-prop-exhaust/695250101-thru-253861

Many gaskets had a sealer on them from the factory. That stuff is like super glue. Make sure there is not a hidden stud/nut somewhere you are missing. When sure then I occasionally use a pair of long screwdrivers to pry with. Gearcase off? Sometimes the driveshaft splines will hold the powerhead on.

I removed the lower unit at the start. I've been trying to use 2 large flat screwdrivers to pry with but the only thing I found to pry against so far is the lower cowl and I'm afraid that is what is going to break. Would it be a bad idea to use a block of wood against the powerhead and cautiously hitting the wood with a hammer?

When I removed the 6 bolts that go into the bottom of the powerhead, the ends of each one had some sort of material on them that I suspect was gasket sealer. It popped of the end of the bolts like a solid disk, the same diameter as the bolt. For lack of a better description, it looks like a layer of thick dried lithium grease and it's hard and brittle.

What you were looking at on the bolts is a bonded on lock patch......from the factory. The patch typically comes off with one or two cycles.
If you have a flywheel puller and a hoist you can apply some pull on the powerhead. Make sure you have a good solid closing hook in case it comes loose and flies up! Ask me how I know........

Why do you think the gasket has failed?Water present in lower cowl?if so more likely exhaust cover or water jacket gasket failure.If exhaust side is leaking internally will cause damage to powerhead.

Well I got it off with a little help from one of those flat pry bars. Slipped the 90° end under teh bottom head bolt and a little pressure with that and it started to move. Picked it up with my hands from there and the gasket separated in the middle like it was glued on both sides.

muddywaders said:

Why do you think the gasket has failed?Water present in lower cowl?if so more likely exhaust cover or water jacket gasket failure.If exhaust side is leaking internally will cause damage to powerhead.

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Water was flooding all through the lower cowl and coming out every little hole on the shaft housing. Started out I was only getting water out the telltale intermittently. Figured it was probably impeller. Pulled it apart to install a new impeller and found it looked good. Further inspection found the tip of an impeller blade stuck in the pump housing where the tube connects. Removed that and even though the piece did not come from the current impeller, I went ahead and installed the new one. Fired it up and water was still running everywhere and would only come out of the telltale when I rev it up. Blowing air back through the telltale hole and I could hear the air coming from under the powerhead. Figured the problem with the water not coming out the telltale was lack of pressure/volume to overcome the leaks. This motor has been leaking inside the lower cowl since I got it. At the end of last summer it started having the problem with the telltale. This is the first impeller I've replaced in this motor. The broken piece had to have come from an impeller that was changed before I got the motor.

Pappy, when I install the new gasket, should I use gasket sealer on one side, both sides, or none at all?

You have what I described in the first reply.....good luck scraping that gasket....its going to take a while.
If the new gasket has a shine (looks like it has been coated with clear coat) on it then it is probably one of the coated gaskets and requires no sealer. Second way to tell... the gasket will have something that looks like Saran Wrap on it or something that will peel off of the gasket prior to your installation.
If the gasket comes as a plain gasket with a dull gasket material look to the surface then it can be coated with sealant.

OK, thanks Pappy.

If both surfaces are clean (meaning NO old gasket left on them) and flat, there is no need for any gasket dressing or silicone or anything. Just put the gasket on and nothing else. This also eliminates the possibility of the silicone preventing the gasket from bonding to both surfaces as it is designed to do. When I sold these motors, I had a tech that worked for me that used to put silicone (or any other "dressing") on every gasket he had to replace. I found it odd that many of these motors that had been previously repaired were back sooner or later with gaskets leaking. Found on a bunch of them that the gasket would not bond to both sealing surfaces because the silicone would get between the surfaces and the gasket. I don't ever remember a single Merc/Mariner/Yamaha service manual that specified using any type of sealant WITH a gasket. Many times they would call for a certain type of sealant where no gasket it used (blocks and center cases specifically) but when a gasket was used to seal 2 surfaces, I can't recall seeing a note to add anything else. Even today, I always find that adding anything other than what the service manual specifies is usually a waste of materials, and a guarantee to get someone to bring it back for future repairs-which is IMO shoddy.

I agree 1000% on the silicone !!! Shade tree mechanics at it's finest!

Gasket finally came in yesterday afternoon. I was surprised to see it looked like it was made of the same material they used in the old automobile exhaust manifold gaskets we used back in the 70's. I had to put a small dab of gasket sealer on the corners to hold it in place as I lowed the powerhead back on. During reassembly, I discovered a melted spot on the side of the fuel line. It looked like a wire got hot and melted it. I know the previous owner had to replace some wires due to a short that had happened. Since it was within an inch of the end of the line, I just cut the section off and connected the line. Closer inspection of that piece showed it had a pinhole in the line. Started it up and adjusted on the idle and mixture a bit and she's running great now. No more leaks and the stream out the pee hole is the strongest of any motor I've ever owned. Now if this storm system that's starting on us today will just get out of here, maybe I can get some fishing in. Thanks for the help guys.