1984 Merc 9.8 lower unit swap

[willgmr]

Hello all!
I have a 1984 Mercury 9.8 that had enough issues with the lower unit that I figured it would be cheaper and easier to swap out the whole unit. I found a used one locally at a Mercury dealer that was a bit newer. The original bad one has an O-ring below the splines on the drive shaft and the driveshaft splines are about 1.25" long. The new is identical except it has no O-ring and the splines on the driveshaft are about 3/4" long. I tried installing the new unit and found it wouldn't go all the way into the powerhead - there was about 1/8" gap between the lower unit and the midsection. When I removed the new lower unit I found that the splines are likely not long enough because there was a burr at bottom of the spline grooves.

So, can I safely dremel the spline grooves so they are 1/8" longer? I was assured by the seller that the lack of the O-ring wouldn't be an issue, but is that true? Can I just dremel out a space for an O-Ring there too? I'd really like to not remove the driveshaft - that seems like a big undertaking...

Thank you!

 

[willgmr]

Alternatively, what if I just fabricate an 1/8" rubber gasket to go in-between the lower unit and the midsection?

 

[Scott Danforth]

Define newer? 1985 is the last of the 11 cube 7.5/9.8s

You dont need the oring

 

[willgmr]

Thanks for the reply, Scott. I'm actually not sure what engine this lower unit came off of, but it was advertised as fitting 7.5/9.8s. The guy assured me that since all the lengths of the rods are the same, it should fit.

 

[Scott Danforth]

side by side are the two lowers identical?

 

[willgmr]

Yes, they really are identical except for the splines on the end of the driveshaft

 

[racerone]

Some motors have the spline that is open to the crankcase.------Those do need the o-ring to seal crankcase compression !

 

[willgmr]

Some motors have the spline that is open to the crankcase.------Those do need the o-ring to seal crankcase compression !

Hm okay, I don’t think I see how this small O-Ring could seal the pressure of a crankcase. Anyway to determine if that’s the case for my engine?
Any body have thoughts about dremeling the splines ?

 

[racerone]

Some will argue.-----Some 7.5 / 9.8 models have the spline that goes right into the crankcase.----They need that o-ring to seal the crankcase compression.-----Of course if you have never seen the inside of one of these then you will doubt when I try to teach !