OMC 40hp Gearboxes - late 1960s

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LitchboroughLitchbor
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Re: OMC 40hp Gearboxes - late 1960s

Post by LitchboroughLitchbor »

We took the '62 Johnson 40hp to the Hamble Rally recently and had a great day with everyone. The engine misbehaved intially, starting on 1 cylinder, but when the 2nd cut in, it ran very well.

Thanks for the advice on marine 2-stroke. I gave it a barrel run when we got home, and as you can see from the photo, the water is much cleaner. Correct marine 2-stroke, a new cork float, and clean plugs, firing on both cylinders seems to be the answer!

I've also attached a couple of photos of the cleared water drains in the gearbox. Out of 3 gearboxes, all had blocked drains and 2 had splits in the casing. I found an HSS drill no use at all on compacted aluminium oxide, and ended up using masonry drill bits 5-6mm dia and up to 300mm long. Now all the boxes are draining properly.

Taking the boat out onto open water and opening the throttle, the engine was very lively, but after a couple of minutes the overheat light came on. This has been a perennial problem with this engine. I hoped that fixing the head gasket would solve the problem, but it hasn't.

The seal between the powerhead and exhaust leg is completely failed, which perhaps explains this problem. Both castings have a poor surface. I've now managed get a reasonable flat surface on the exhaust leg, but there's a low spot on the powerhead, which I cannot fix. You can see that a feeler guage slides through.

I suppose the correct approach is to dismantle the powerhead, take out the crankshaft, and re-surface the powerhead casting. But I can't really face this task at the moment, so I'm hoping a sealer might work. Last time I tried Loctite 5980 flange sealant, but it didn't work at all. I've had more luck with Loctite 5660, which is rated to about 200°C. But this time I'm planning to try Loctite 5990, rated to about 300°C. (They seem to suggest it can be used on head gaskets).

Is there anything else that might work? It's a challenging setting - hot gas, hot water, oil, fuel and pressure. (I don't like the idea - but is exhaust pipe paste something to consider?)

Many thanks.
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LitchboroughLitchbor
Posts: 45
Joined: Sat Apr 25, 2020 1:08 pm
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Re: OMC 40hp Gearboxes - late 1960s

Post by LitchboroughLitchbor »

A few pictures of the powerhead on the '62 Johnson 40hp, now reassembled with the exhaust leg.

I managed to achieve quite a flat finish on the exhaust leg, but the powerhead is in a sorry state. At some stage in the future, I'm going to have to take the crankshaft out and re-surface this casing.

But for now I'm hoping Loctite's 5990 product will work. It seems to be a silicon rubber formulated for high temperatures. In the list of applications they mention exhaust manifolds, so I'll just wait and see whether this works. I'm planning to leave it for a week or two to cure. I was probably much too hasty, last time I attempted to seal this joint.
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