300 Shaft Nut Torque Spec.

[Bill Goetz]

Just replaced the seal in our prop shaft coupling. Does anyone know the torque spec for the large nut on the coupling end of the shaft. Mercruiser manual doesn't show it.

 

[dtfeld]

There probably isn’t a spec published and it’s a subject of debate on how much torque is the right amount. It’s a pretty complex calculation with a whole lot of variables. I found several articles when I re did mine, and well, it was in conclusive.

https://www.followingseas.media/blog/2017/2/24/quick-update-on-prop-nuts

https://stevedmarineconsulting.com/wp-content/uploads/2019/06/ShaftCouplings179FINAL-PBB.pdf

I treated them just like a prop nut. Long story short assemble the coupler dry or with a very very thin coat of oil to prevent seizing, and tighten up as tight as you can using hand tools. Even if you get it wrong, the first time you put it in fwd gear, the taper will be seated.

I used a 18” breaker bar on mine with a large socket from Harbor Freight. Biggest and limiting issues are space to swing any wrench and ability to prevent the shaft turning as you tighten. I also bought new nylon lock nuts. I think from General Propeller.

 

[Njlarry]

How best to lock the shaft? Is the torque low enough to just put it in gear?

 

[my3sons]

How best to lock the shaft? Is the torque low enough to just put it in gear?

On a v drive I just put wrenches on the coupler bolts and tightened down the shaft nut. On our straight drives, I put a long bolt through the coupler half and held it against the floor with a pry bar. Tighten as tight as you can get with an 18” bar.
See post 4620 in the 400 db thread.

 

[Bill Goetz]

On a v drive I just put wrenches on the coupler bolts and tightened down the shaft nut. On our straight drives, I put a long bolt through the coupler half and held it against the floor with a pry bar. Tighten as tight as you can get with an 18” bar.
See post 4620 in the 400 db thread.

Can I assume this method was sucessful? I guess that would be the bottom line.

 

[my3sons]

It worked fine and is easier than holding the shaft some other way.
Note to @Njlarry, on the 400 DB, the Tides seals can be changed without pulling the props, there’s enough room to slide the shaft back without the prop hitting the rudder to get the seals and hose installed. I discovered that after pulling my props. You could always use a wood block to hold the prop and shaft, but would require a helper under the boat.