lower unit interchangbility

[chris650]

Good evening everyone. I am wanting to change my mc-1 lower unit,which is a 78, with another unit that i have aquired. The new one i have is either an MR,or an alpha 1. Is this pretty much a matter of just bolting it up? Of coarse, i would do some maintenance on the one going on,just wondering if they are interchangable? Also, the serial numbers on these old merc outdrives seem to overlap....6152290, falls into two different gear ratios,according to the service manual. Seems that theres no rhyme or reason when it comes to going by them to try and figure anything out. How would i know what gear ratio i have? Thx

 

[haulnazz15]

Re: lower unit interchangbility

The MR/Alpha I you got will bolt up fine. Dunno about the gear ratio, but I usually just unbolt the top cap (4 bolts -12pt 3/8 socket) and count the gear teeth.

 

[chris650]

Re: lower unit interchangbility

does the gear ratio even matter if im just swapping out the lower unit?

 

[Bondo]

Re: lower unit interchangbility

does the gear ratio even matter if im just swapping out the lower unit?

Nope,... That's the Upper unit that determines the ratio,...

Donno 'bout yer '78, but the really older drives have a different spline count at the top of the lower unit input shaft, 'n of course at the upper unit output spline,...