Upper Shaft Splines stripped (1974 Stringer)

I have a 1974 OMC 120 h.p. with the usual or inherent faulty upper shaft. Every 200 hours, like clock work, they corrode and then strip out. They also destroy the upper splines of the water pump shaft, as well. (Always in pairs.) I have been working on a permanent solution for this. It involves reworking a used water pump shaft (free) with a good set of lower splines. I am doing 3 of these and it works out to about $500 each in machine work. The cost of both shafts and seals can add up to the same (not including labor). Since they could last 1000 hours or more (30 years) then they will pay for themselves.

Here is a picture and a drawing for those who are interested in doing the same. I can supply better pictures if you wish to email me.

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I have a 1972 OMC electric shift high profile in my Skipjack, I recently took it apart and replaced the seals, ball gears, and water pump and water pump shaft. I also noticed the wear on the pump shaft as well as the upper shaft that you are talking about. Even with the new water pump shaft, I still felt some "play" in the splines. I guess for some reason OMC didn't make the upper shaft out of stainless and the splines corrode. The weakest link in the drive.

Would this work for a high profile? The low profile has a much shorter shaft, I don't see how there is enough length available in the water pump shaft for a high profile conversion.

Also just wanted to say it's nice to see someone trying to re-engineer the problems with this drive, mine is 48 years old and still going, never had the lower unit apart, need to stay below 600 rpm to shift without breaking the springs. Everyone says its a dog but all my problems have been self-inflicted like running low on oil and not replacing the seals, I like mine and I'm sticking with it.

The lower gearcase / electric shift is actually very. good as you say.-----These upper gear boxes and splines are the problem.-----They need regular greasing of these splines to make them live.

Yes, I believe his should/could work for a high-profile upper unit provided you started with an equivalent high-profile water pump shaft. (An 8-inch P/N 22411 High Profile OMC #908087). However, the ones I fabricated have been, done (in stages) by 3 different machine shops. My dilemma was, Vancouver, B.C. area has few machine shops and I couldn’t find anyone that could replicate the 20 internal spline sets. If you reside in the USA then you should be able to locate a shop and have a couple replicated from scratch. (Do you have a spare stern drive? Outside of EBAY; these parts can be, found cheap.)
I believe a 404 stainless is salt water resistant. Just beware some types such as 17-4PH can be heat treated but then they are not resistant to salt water which defeats the purpose.
Like you, I’ve had mine for decades and have gone through 3 shafts now. Therefore, if you can get a few made for $500- $600 each then they should pay for themselves since they should save you from also replacing the WP shafts, seals, and always inclusive labor. (Also, you need to replace those (5) top cap bolts (¼ x 2 with a ¼ x 2 1/4 bolts) and also install Heli coils inserts. I had mine strip out and the entire lower half of the drive (65 lbs.) was dangling by the shift wires.)
Lastly, if you noticed ANY wear on your pump shaft then your upper shaft is just about gone. Replace them in PAIRS (always) or else you will ruin your new WP shaft in about 10 to 15 hours or so.
Are you aware that OMC has a stainless-steel replacement for the original 12390-pump housing (aluminum)? They last indefinitely and even after 10 years the impellor shows little wear. With these three modifications then these OMC stringers are great and dependable drives.
I hope this helps.

Castrol Pyroplex blue or similar grease stays in well for trailer boats. Mine after 5 years and maybe 100 hours use was still greasy. But my boat was never moored.

hystat said:

Castrol Pyroplex blue or similar grease stays in well for trailer boats. Mine after 5 years and maybe 100 hours use was still greasy. But my boat was never moored.

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I agree that grease helps but it wears away eventually, especially in salt water. I submerged an old shaft in fresh water and 2 weeks later it was beginning to rust. When these upper splines rust out they also take out the impeller shaft. The repair bill usually runs around $2,000 and up. By reworking an old impeller shaft I now have a rust free upper shaft that should last 30 years or more in any environment. Fix it right once and you never have to worry about it. Just check your oil levels every month or so.

Bayliner18 said:

I agree that grease helps but it wears away eventually, especially in salt water. I submerged an old shaft in fresh water and 2 weeks later it was beginning to rust. When these upper splines rust out they also take out the impeller shaft. The repair bill usually runs around $2,000 and up. By reworking an old impeller shaft I now have a rust free upper shaft that should last 30 years or more in any environment. Fix it right once and you never have to worry about it. Just check your oil levels every month or so.

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I was in there every 50 hours or so to do the impeller anyways, so I would just grease it up