Swapping a 25" with a 20" Drive Shaft

[bm1961]

I have 2 Merc Lower units, one from an 83 200 with a 20" drive shaft where the prop shaft splines are slightly twisted and the case is in rough shape. I also have a newer 25" Merc Lower unit (90's) from a 200 which looks new. Are the drive shafts interchangeable so I can swap shafts and run the newer unit as a 20 inch?

[tj309]

The splines are the same but the lengths differ by 5". You also need a shift shaft and a water tube to complete your swap.

[bm1961]

I'm interested to know if the actual 20" shaft can be pulled from the older unit and installed in place of the actual 25" shaft of the newer unit, converting the new unit. Are Bearing sizes the same, etc

[shooter1]

Your 1983 lower unit will be a preload pin shaft and the 90's lower will have a driveshaft nut. They aren't interchangable. You can probably look around and find another shaft or swap the longer lower.

Shooter1

[bm1961]

Thanks Shooter, thats the answer I was wanting to know. I looked at the breakdown on my merc manual for the 83, but was unsure of the later 90's unit. I have rebuild a mercruiser outdrive before, been into the gearcase and stuff, but just did not want to tear into both outboard lower units just to find out in the end that the internals were not the same. Are there any definitive differences (externally) between a preloaded pin unit and a unit with the threaded shaft w/nut?

[shooter1]

Not that I can tell. I have put a preload case on a newer motor without any prblems and vice versa. All of the 2.0, 2.4,2.5 motors with 8 spline shaft should work. There are some different ratio's that you can play with on these cases. The 135, 150,and some 175's had 2.0-1 ratio and the 200s had 1.87-1. You can use the 2.0-1 on a 200 as a high elevation option but you usually have to prop up with more pitch to keep it in the curve. Hope this helps. Also you might be able to put the prop shaft in the preload, I think it is the same shaft. You might be able to find a used driveshaft for the newer case also. You might list it in parts wanted.

Shooter1

[chris_lacey]

Preload and alpha type cases will not interchange internal parts. The drive shaft will not exchange.

To change the alpha style (newer) case from 25" to 20" you will need to get the bearing up style shaft.

The gearcase assemblies will exchange as a unit.

[camaro7081]

there are some shops who cut the shaft down for you and respline the shaft . i dont know the whole process but this is what i heard on S&F if you are interested i have one number somewhere that was sent to me thru S&F i dont know the price either but it sounds like it will save us a lot of troubles . just cut the shaft and done

[graemers]

I have the same question. I have a 25" preload lower and was wanting to swap it with a preload 20" shaft. Is that possilble? I have some old inline cases that have the preload shaft, could i use one of them?

[shooter1]

You shouldn't have any problem as long as it's preload to preload. As far as the inlines are concerned I don't think they are beefy enough for the V-6. I've purchased 20" driveshafts on E-bay fairly cheap.

Shooter1

[graemers]

What would the difference be if they were the same diameter? Did they use a weaker metal?

[shooter1]

I've read that the XR4 150 Had a small gearcase diameter with a driveshaft out of an inline, but it won't hold up to the pressure of the 200 HP. I looked up the part# for the small diameter case pinion bearing and it was 31-32898 and the pinion bearing for the preload large diameter is 31-35934a 1. I would probably go ahead and tear the gearcase down and check the internals because you might be beating on a dead or dying horse.

Shooter1