Hole in water jacket just under flywheel

[DangarStu]

I have been helping a friend with their Mercury 90 HP four stroke 2008 recently and when we looked at the old thermostat is had almost completely corroded away (just the top flange remained). I cleaned all the corrosion out and put a new Mercury original in along with a new gasket. Now it has sprung a leak in the top of the water jacket (photo attached). It looks like perhaps the increased pressure of a working thermostat has caused a weakness in the cast to finally fail and I'm wonder what is the best way to fix this. Getting it welded is obviously an option but I was wondering, is it an option to just drill and tap then put a plug in? Anyone repaired a hole like this before? I've heard this can sometimes be caused by just a fault in the casting, but I am wondering if something more is going on to corrode the old thermostat and cause this hole. (The old thermostat may not have been a OEM part, don't know its history).

I recently changed the four internal anodes in a Yamaha I rebuild. I am presuming this outboard will have something similar? If so I will change those too (if there is anything left of them!)

Any advice greatly appreciate,
Stuart

 

[quicktach]

Drilling and tapping may not work because of not enough metal in the area of the hole. Epoxy sealants not the best either. I'd take it to a good welder and have it looked at. The problem is that there will be corrosion and weakness below the opening you can't see. Nasty little problem.

 

[gm280]

The fact that it is in the water jacket area and not the intake or cylinder sections is probably the only good thing I can see. I highly doubt that any welder would touch that repair. And if they did, you would most likely have to completely disassembly the engine and clean the area so they could try. Personally I would try some JB Weld after seriously cleaning that area. If that doesn't work, looks like you will need a new/used casing if you want that engine to work again... JMHO!

 

[Faztbullet]

I would clean up the area, and cross thread a nylon plug into hole and cap it with a quality epoxy like Devcon...

 

[DangarStu]

Thanks guys I appreciate the advice. I think I'll pop the flywheel off and see if I can drill if out and assess how bad the corrosion is. You can almost touch the hole through the thermostat housing which is also good as it gives me access to the inside as well. If it was a tiny bit smaller I think I'd try a rivet, but I'm thinking for now I am going to go with the plug and epoxy method. If it fails in time I can always jump up to something more serious then. I'm guessing surface prep really is going to be the key here along with some good degreaser. I'll post some pics when it is done.

Thanks again,
Stuart