Swap lower unit

[DAWGIT10]

I have a 71 thunderbolt 500 4cyl and the lower unit is shot I was wondering if I could use another LU from a motor close to that year to see if they would match up or am I just going to have to rebuild the entire unit cause they don't make those anymore

Any help would be appreciated

 

[racerone]

Re: Swap lower unit

I believe many lower units will fit 35 hp 40 hp and even the 3 cylinder 65 / 70 hp units.

 

[carholme]

Re: Swap lower unit

First, what is your engine s/n and is it a long or short shaft?

Gerry

 

[hotrod53]

Re: Swap lower unit

Yes, I did it on my 1977 Merc 500 and got a good unit for $120 on Ebay. Remove the lower unit and look on the top in the casting near the water pump, there will be a number there, it will look something like 1643-6037. You will need to know your serial number, how long the shaft is, does it have a preload pin on the top of the drive shaft, and how many spline the drive shaft has. Any reputable boat mechanic can help you with that, many are interchangeable. I'd look on Ebay, there are many parts resellers advertising there. I emailed one of the sellers and told him what I wanted and he matched it up for me.