1978 Merc 20HP Driveshaft R & R Procedure

[DaveHawkins]

I have a late 70's Merc 20hp short shaft that needs a lower unit. I found a good long shaft lower unit for a reasonable price. Can I just swap the drive shaft from my bad lower unit to the long shaft lower unit and be good to go? What is involved in removing the drive shaft?

 

[Laddies]

Re: 1978 Merc 20HP Driveshaft R & R Procedure

Get the housing number out of the two housing you have (the number is cast in the housing in the area above the cavitation plate) and serial number goto the BAM Marine site and see if the casting numbers are interchangeable if so remove the carrier and pinion nut, the water pump and housing the drive shaft will then pull out the top. Make sure you keep the shims from the two housing separate and in the right positions so you won't have to re shim the housing just return all shims to the right position and your go to go.

 

[DaveHawkins]

Re: 1978 Merc 20HP Driveshaft R & R Procedure

Thanks Laddies, great information. I will do exactly that.

 

[Mac uk]

Re: 1978 Merc 20HP Driveshaft R & R Procedure

Before you strip it check the shift lever works the same as youre old one. Lower units before 1970 work in the opposite way so I found out a little to late, and got stuck with a unit a couldn't return. Got to change my drive shaft as well.

 

[DaveHawkins]

Re: 1978 Merc 20HP Driveshaft R & R Procedure

I decided not to go this route. The fellow with the long shaft lower unit could not provide the housing number even after I gave him explicit directions on where to find it. So I figured if he couldn't or wouldn't be able to provide the number for me...it was enough for me not to trust him. Also, I didn't realize it, but the shift shaft would have to be changed over too and I wasn't sure I wanted to do that. I'll just be patient and wait for a short shaft lower unit to come along .