Two different drive shafts? 1972 800 80 hp

72 Merc 800. Replacing the lower unit and I see there are two different drive shafts for this model. One with a pre-load pin and one without. When I install the one with the pin, the gears lockup. This power head came with out the pre-load pin drive shaft. Are these shafts interchangeable?
Best regards

Re: Two different drive shafts?

The pre load pin is used on lower units that have a tapered roller bearing on the driveshaft. Other lower units using a 'straight' ball bearing do not require any kind of bearing preload.The straight ball bearing lower unit will not accept the tappered bearing style driveshaft as vise versa.But both types of lower units will fit your application.I have used three different lower units, the original lower unit which on my 80 hp had the straight ball bearing style then went to the tappered style bearing with pre load pin 1976 115 hp lower unit and a 75 hp 1985 tapered bearing style with the pre load pin all worked fine except the gear ratio was off and had to compensate with a different prop.

Re: Two different drive shafts?

Thanks Steve! Big relief hearing this. I wonder about the gear hangup then. I suppose it could just be some crud buildup in the coupling at the power head? Causing too much pre load? I'll clean that up and see what happens.
Best regards
Dennis