Re: Differences between a ’73 and ’81 25 HP Rude??

[DAN ANDERSON]

Instigator; Just so you are not confused (or maybe I am). Not much of anything off the ’81 motor will fit the ’73 motor. Some pieces look similar, but are not the same. As FrankR mentions these are two different family of engines. Without going into every detail (you can compare part numbers on ME) the powerhead won’t fit your exhaust housing. Not sure your lower pans & motor cover would even fit over the larger powerhead? The earlier powerhead was before the interim ignition system used before CD so not sure you could adapt the later ignition to the early motor. Anything can be done, but it is not a bolt on. The late gearcase has a different driveshaft than the early one. I think the water pump is even different. The late gearcase is a three lug shifter which would be good, but again not sure your old driveshaft can be made work with early driveshaft? The ’81 has a TR style carburetor which is a larger mounting flange. About nothing is the same.
As far as power output. The 32 cu in motors didn’t become 35hp (30hp after prop rated) until they got the thru hub exhaust. That requires a totally re-designed midsection. The old split line gearcase never put out that type horsepower. Even though they marketed a 28SPL with that old style gearcase.
Just wanted to make sure you understand what you have. Hope that helps.

Dan in TN