Re: Will a 1989 115hp powerhead swap to my 1993 90hp lower unit?
That is a difficult question because right around that time, 1993, Mercury started monkeying around with things.
Physically, the 4 cylinder block will mount in the same way as the three cylinder 90. HOWEVER: There is a possibility that Merc changed the crankshaft splines. If you have the option, count your 90 HP drive shaft splines and the 1989 crankshaft splines. Obviously, the 4 cylinder block is taller and you will need a bigger hood to cover it. While the electrical system is similar, you will need a 4 cylinder trigger and another coil unless the four cylinder block is complete with ignition.
AND---If the 1993 has Merc control cables, you may need to jury rig them on the 1989 or buy an earlier US Marine control box and cables. If you have the option, examine and compare both blocks before starting.
Lastly, did Force Make a 115 in 1989? I thought they only made 120 and 125 in the 4 cylinder engines