Going from a XL shaft to L shaft (25" to 20")

[Racer67]

I have a 21' Center Console boat, the lower unit had hit something when I bought the boat, so it was already destroyed. I bought one on eBay (1982 115HP mercury). The description stated it had a 25" shaft.. same length has the one I removed. But when it got here, it was just 20". When I removed the lower unit, there was a spacer per say between the lower unit and the power head as the shaft was longer. I can put the lower unit in without that spacer. I'm trying to find a picture of how far the prop was below the boat and if 5" will make a huge difference.

Any suggestions? I can't switch the two shafts out because the 25" shaft has butchered the splines up from whatever it hit.

Thanks!

 

[gm280]

If I am reading correctly you are wanting to replace a 25" lower shaft assembly with a shorter 20" section? If this allows the cavitation plate to be about even with the lower edge of your boat, then yes go for it. It will actually be the better option. But like you already stated, you will need a shorter 20" drive shaft as well. My only question is what splines are messed up? If it is the splines on the drive shaft that goes into the power head assembly, then you honestly need to examine the crankshaft on the engine to see if they are still useable. Otherwise you will need to replace the crankshaft. That is a huge effort and money option too...

 

[Racer67]

Thanks for the reply. I'll have to size it up and see where the plate will sit at. The lower unit I bought already had all the shafts including the 20" - was suppose to be a 25", that's why I'm hoping it'll still sit low enough. The spline that is messed up is from the lower unit and it's the lower part of the drive shaft that goes into the lower unit where it makes the 90 degree connection to the shaft that the prop is attached to. The part that goes into the power head is fine.

Thanks again

 

[gm280]

Sound like a deal if it is the correct length then...

 

[Chris1956]

I did that conversion. If I remember correctly, the two studs in the midsection needed to be shorter, as they passed thru the extension in the 25" gearcase. I think everything else from the 20" gearcase fit fine. I took the studs off an old '73 motor I had. They fit just fine on the '93 motor.

 

[jbcurt00]

IF the 25" shaft was the correct length shaft for the boat, and the cav plate was about even w/ the keel, then NO a 20" shaft won't work.

You mentioned a spacer when you pulled the 25" lower. Are you saying that the lower WAS a 20" lower, it just had an added space to convert it to a 25" length?

If your new lower fits the spacer (if the new lower fits the upper, and the spacer was installed correctly when you pulled it apart) then why don't you order a new longer drive shaft only to fit your lower.

Use the spacer, a new drive shaft and any/all additional parts that need to be sized for an XL leg and put an XL back on. IF the boat needs the XL shaft length.

If it doesn't, that may explain why the lower got damaged: It was sticking down about 5" too far to be setup correctly.

On a vintage Johnson the water tube/shift linkage is matched to the shaft length too so adding/removing the available 5" extension requires other changes then just the drive shaft. Not sure if that's the case w/ Mercury's.