Does the prop shaft need to touch the back of the transmission? - SailNet Community
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post #1 of 6 Old 08-02-2012 Thread Starter
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Does the prop shaft need to touch the back of the transmission?

I'm installing a new engine, prop shaft and coupling. I've never done this before, and I don't know if the prop shaft is supposed to be pushed far enough through the coupling to touch the back of the transmission? This makes sense to me as the forward pushing shaft would push up against the engine and transfer that load, but I don't know. The coupling has two 1/4" set screws that I need to drill dimples for as well, just want to be sure I have the prop shaft setup right before I start drilling.

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post #2 of 6 Old 08-02-2012
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Re: Does the prop shaft need to touch the back of the transmission?

The coupling should be fitted on the shaft the holes drilled and faced and at a machine shop as a unit to work correctly

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Re: Does the prop shaft need to touch the back of the transmission?

As Tom said, the face of the shaft coupling should be trued in a lathe by a machine shop. Otherwise if it's not absolutely square to the shaft you can never align it properly. They can also drill the dimples.
It doesn't matter if the shaft is all the way in. The force is transferred by the coupling. After all, if the shaft can push against the transmission flange independent of the shaft coupling, it could also move back in reverse. That's a bad thing.

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post #4 of 6 Old 08-03-2012 Thread Starter
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Re: Does the prop shaft need to touch the back of the transmission?

OK - so I really need a machine shop to true up this stuff? This is only a 14hp engine, coupling and shaft are new. The coupling slips over the shaft fairly easily, it's a good fit but I didn't have to force the coupling on. Not sure if that's a bad thing.

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Re: Does the prop shaft need to touch the back of the transmission?

The hp rating doesn't matter. If the shaft isn't aligned properly it will wobble which will put undo stress on the stuffing box and tranny as well as causing annoying vibration. And you can't align the shaft if the coupling isn't true. The fit should be pretty tight, actually a press fit. If it's too loose when you tighten the set screws in may force the coupling off square. Installing and alignment is certainly a DIY job but I let a shop check the shaft for straightness and square up the coupling.
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post #6 of 6 Old 08-09-2012 Thread Starter
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Re: Does the prop shaft need to touch the back of the transmission?

Thanks for the responses. To follow up, I took my new shaft and coupling to a good prop shop and they said the shaft diameter was undersized by .003 which was enough to make the coupling loose and the shaft basically not usable. Shaft had other issues too apparently, not well made. So I had a 2nd new shaft made that was correct, fitted and faced good to go. Now I am in the process of trying to get 1st new shaft makers to give me my money back. 1st shaft makers don't understand why having a coupling fit loose is a problem ... hmmm.

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