What I can find on the web is that lower units from 85-140hp 1978-1991 will bolt up fine to a 1981 115hp Evinrude. My question is would the internals be different on a 140 vs a 115 hp lower unit. I looked at parts numbers on boats.net for both motors and all the internal numbers are different between the two. Will it run okay on either motor. Reason I ask is I got a chance to pick up a lower unit off a 1982 140hp and would like to put is on my 1981 115hp. Mine is leaking and it would be cheaper to swap out the units than to get the lower unit repaired. Appreciate any advice.

would check shaft size and count splines

there all 2/1, 13/26 and all the same splines, your good. 77-78 was the change over (shifting was reversed) and only the early 140's had the smaller gear 13/25

Only thing you have to do is use the shift shaft out of your old lower and make sure the replacement has the same length main shaft. Take note of number of turns it takes to turn the shift shaft to all in from its original position. Normally 5 to 7 turns out. I put a 1991 V6 lower on a 2006 E-Tec. had to change the main shaft and the shift shaft