6R69M Thermostat Cap Leak

[cprodave]

On my 1969 Johnson 6HP 6R69M after replacing my Thermostat and doing a thorough cleanout of saltwater residue/crud (due to decomposed Impeller, which I also replaced) there is a leak between my Thermostat Housing Cap 312714 and the Cylinder Head. (Good news is that the Impeller is pushing a strong stream and powerhead is no longer overheating). I installed a news Thermostat Cap Gasket Sierra 18-0679 which I found local supplier (vs. OMC p/n 311145). The leak occurs midway (crosswise--2 screws 1 side, 1 screw other side, i.e. an elongated triangle) between the screws which fasten the Cap to Head. The leak is linear in shape and above the Gasket, I.e. between the Gasket and the Cap (not between the Gasket and the Head. I can see the cap is dished ever so slightly--laying a machinist's straightedge along the cap I would guess the Cap is "dished up" (i.e. away from Head mating surface) about .006-008" (about 1/2 of my thinnest feeler gauge which is .014). The mating surface on head is flat when checked by using same machinist's straightedge.

A couple questions: 1) are the OMC Gaskets better at filling the Cap/Head gap in case either the Cap or Head is dished wrong way?
2) is it possible to undo the wrong-way dish on the Cap by using 100/200/400grit silicon carbide paper on a piece of glass?
That is, by crude machining I could get the 2 longest mating surfaces on the underside of the Cap to be flat, or better yet slightly convex.
3) Is Gasket Compound a viable solution, either with or without Gasket? I do realize that next Thermostat replacement becomes more difficult, requires scraping away Gasket Compound...

I see Used Covers for sale on-line, but no New ones. A Used one might have same problem as my existing cover,

Thanks in advance for any help.

 

[Vic.S]

I would grind the underside of the cap flat in the manner you describe.

New caps are unavailable

 

[cprodave]

Vic, thanks for advising on the grinding and thanks for confirming that I am not overlooking a supply for New Cap. I did find 1 supplier that listed Cap at a reasonable price ($17) but item is listed as Unavailable. I am afraid that is going to be the situation everywhere.

 

[oldboat1]

The last one I did was for a mid-70s model, as I recall. The gasket was considerably thicker than typical gaskets -- think I would try to find a BRP/OMC gasket. The seal is tricky. You might try loosening and retightening the screws. If the old gasket is intact and in decent shape, you might try reinstalling it and see if that works for you.

 

[cprodave]

Sorry for delay in getting back to this problem--Fall fishing and hunting got in the way!


Past couple months as time allowed I continued to do my Crude Machining operation to get a convex surface on the underside middle of the Thremostat Cap. All to no avail, it was still leaking. So I ordered an OMC Thermostat Cap Gasket. It too leaked. I was at wits end, wondering if next I should attempt to make a convex surface on the head when I thought, "what The Heck, try it with both Sierra and OMC Gaskets in-place", i.e. double thickness Gaskets. It worked! No more leakage.

So...only questions is...is there any downside to running this motor with 2 gaskets in-place? I haven't done an extended (10-15minutes at least) sea trial yet.

Any inputs are welcome. Thanks,

 

[racerone]

Put part # into E-BAY and see if one is listed.

 

[F_R]

So it isn't leaking with two gaskets? What the heck, count your blessings and run it.

 

[cprodave]

Racerone, you're right there is a Used thermostat cover on eBay now and a New one too although these might have become available since I started working on the problem several months ago (I am quite sure but not 100% that I would have checked) . I might buy the less expensive one and keep it on-hand in the event that my double-gasket fix stops working. Thanks for the remionder.

F_R, yep I am counting my blessings!