Merc 500 Driveshaft Question

Please forgive me if I am covering old ground... but I couldn't find the answer.

I have a 78 50HP Merc with a bad lower unit... case busted and nmissing parts.

I aquiered a complete late 60"s 50HP with a complete lower unit.

I removed both units and the look the same... the only difference I see is the drive shaft. The shaft on the 78 eng has a nipple about 3/4 in long that sticks out of the power head end of the shaft... The 68 Drive shaft does not have this nipple.

My question is.. What purpose does this nipple serve and is there any reason I can't install the older unit that does not have this nipple? I also wonder if I could just swap the shafts?

THank you so much for the help.
Reaney in NH

Re: Merc 500 Driveshaft Question

the pin is spring loaded its for drive shaft pre load and if the shafts are the same length go and swap them.

Re: Merc 500 Driveshaft Question

Thats great... Thank you for the info. I'll be installing it later this week.

Re: Merc 500 Driveshaft Question

let us know how it goes... oh... are both prop shafts in good shape?

Re: Merc 500 Driveshaft Question

Yes... both Props and shafts are fine. But the 78 is missing pieces... like the gears and a large collar. I havn't pulled the Prop on the older unit to see if the spline count is the same... someone told me it could be different. I hope its the same... If it is I'll have a cpl spare props.
I'll know more later in the week.
Thanks