Alpha Drive Shaft 8 vs 13 spline

I have sort of painted myself into a corner. I'm looking for some advice as to what my options are.

A couple of months ago I purchased a 1972 18' Bayliner Discovery with a 110 Mercruiser. I have not been able to find any numbers on the drive, but it has a 4.75" diameter gear housing so I've read in the manual that it is above serial # 1684188. The engine serial # is 3849449.

As was to be expected from a 37 year old craft there were any number of things that were in need of attention that I've been working through. The latest thing I am attempting to do is replace the LU. Reverse gear is shot in the old LU and when I finally got the prop off I found a crack line on one side of the propeller shaft that runs straight down the shaft from about 5 threads in all the way to just where the shaft enters the oil seal. In any event the Old LU is toast.

I purchased what appears to be a good used Alpha One Gen One LU, but when I pulled the old LU off everything was exactly the same as my old LU except the replacement one has a 13 spline drive shaft, while my old one has a 8 spline shaft. Major problem!

Any ideas? Changing the gear in the upper unit is probably out of the question? What lower unit model should I look for with an 8 spline shaft?

Thanks all.

Re: Alpha Drive Shaft 8 vs 13 spline

What lower unit model should I look for with an 8 spline shat?

Click to expand...

Ayuh,..... really, Really, REALLY Old 1s....
That shaft went away back in the Very early 70s...

Change out the Whole outdrive...

Re: Alpha Drive Shaft 8 vs 13 spline

Thanks Bond-o. Any idea what gear ratio I should look for with that old 110? 1.98 maybe?

Re: Alpha Drive Shaft 8 vs 13 spline

Ayuh,.... You've got it...
It probably also uses the Short input shaft,.... 8" something, vs. the later 9" somethings...
I honestly don't know how New a drive will fit your transom...
That's pretty Old iron ya got there...

Re: Alpha Drive Shaft 8 vs 13 spline

If your input shafts are the same length you might get away with pressing out the ujoints to swapping the shafts. I'm not even sure if the ujoints are the same but it's worth a shot. Problem is that there are so many changes over the years it's hard to say for certain what fits what, sometimes you just have to look at it and try it.

Re: Alpha Drive Shaft 8 vs 13 spline

JustJason: Good idea. I think changing to my old input shaft would be an option. I had the drive off a couple of weeks ago and replaced the u-joints & gimbal bearing. I didn't measure the shaft length, but judging from the "what they fit" description for the u-joints I'm pretty sure the u-joints would be same. Guess I'll be looking for a 1.98 upper unit! Is it likely that a late 80s to early 90's UU would have the 13 spline drive shaft?