I've had a 4HP 1981 Johnson twin for many years (converted to fuel pump and external tank) and have bought backup parts as I can find them inexpensively to ensure it has a long future. (I use the motor on the back of an 18' aluminum square-back canoe.)
Recently I purchased very cheaply a 1976 Evinrude 4HP twin as both a backup and for my brother's canoe. The seller said it was a 1981 Evinrude and thinking parts would be pretty much interchangeable it seemed a good idea.) But it is a 1976 and the 1976s regrettably have quite a few differences. The 1976 had a frozen piston in the bore, broken ring and a tiny slit in one piston from who knows what? Pistons are not available for the '76 but I have new ones for for the '81. After a quick hone I installed the '81 pistons in the '76 block (ring gap and all was good). Even though I had measured from wrist pin to piston crown as being identical the new pistons hit the head on the old block. (I can find no dimensional difference between the pistons other than the rings are in a slightly different place.) So, I put an '81 head on the '76 and it turns over fine. (All the mounting/cooling details are the same between the two heads just the internal profile is different giving the pistons clearance. I have not finished putting things back together or fired it up yet but wanted to throw a post up here and see if anyone has been down this road before. I know head clearance (bumper clearance) is a big deal and I have no idea what it should be and even how to measure it on something like this. At this point I'm just glad it turns over but....
Any thoughts???
Recently I purchased very cheaply a 1976 Evinrude 4HP twin as both a backup and for my brother's canoe. The seller said it was a 1981 Evinrude and thinking parts would be pretty much interchangeable it seemed a good idea.) But it is a 1976 and the 1976s regrettably have quite a few differences. The 1976 had a frozen piston in the bore, broken ring and a tiny slit in one piston from who knows what? Pistons are not available for the '76 but I have new ones for for the '81. After a quick hone I installed the '81 pistons in the '76 block (ring gap and all was good). Even though I had measured from wrist pin to piston crown as being identical the new pistons hit the head on the old block. (I can find no dimensional difference between the pistons other than the rings are in a slightly different place.) So, I put an '81 head on the '76 and it turns over fine. (All the mounting/cooling details are the same between the two heads just the internal profile is different giving the pistons clearance. I have not finished putting things back together or fired it up yet but wanted to throw a post up here and see if anyone has been down this road before. I know head clearance (bumper clearance) is a big deal and I have no idea what it should be and even how to measure it on something like this. At this point I'm just glad it turns over but....
Any thoughts???
