22cu inch head swap 20hp and 25hp???

[noonespecial]

Hello, everyone. I have a 1972 oachita flat bottom with a 20 hp 1972 20r72r i bought from the original owner about ten years ago. This boat is the best 350 dollar boat ever bought! Well, i had the misfortune of stripping a spark plug hole by overtightening, which was a half turn past hand tight. A couple years ago, i found a 1975 25hp 25r75b at a garage sale for 10 dollars, which had 60psi on bottom and 120 psi on top, and an electric starter which went right on my 72 20hp!! Since i did not/do not want to do a helicoil, I swapped the head from the 25r75b onto my 20hp motor. previously, my engine(20hp) had 125psi on both cylinders, and now with the different head it shows 133psi on both. I have read that on these motors the crank,pistons,head are different, but the block is the same. I have also read about a swap which involved swapping the entire rotating assembly, but not just the head. The pistons do not touch the plugs, but i have not tried to start this motor, i do not want to ruin it. The only visual differences I can see are the mounts for the exterior coils, but obviously there must be a difference in the chambers.

So, the question is, can any of you seasoned veterans tell me if I am a fool? Can the entire rotating assembly be swapped? What about just the heads? Also, the pistons on the 25hp look better than the ones on the 20hp, and you can still see cross-hatching on the cylinder walls they are so smooth, any ideas where the compression went on this $10 motor?

 

[Daviet]

Re: 22cu inch head swap 20hp and 25hp???

Did you check the compression using the electric starter after you swapped the head? I have found that I will get a little higher compression reading with an electric starter verus manually pulling the engine over on the same engine.
Both engines use the same head gasket, looks to me like you will be OK swapping the heads.

 

[noonespecial]

Re: 22cu inch head swap 20hp and 25hp???

Yes, I did check it with the electric starter, it had been on for a couple of years. It was 125 on both since i got it, it never changed, until the head swap and new gasket. I was thinking a new head gasket might explain it, but I have read that there are subtle differences in the head. The info i am really looking for is to hear a "seasoned veteran" (my nice way of saying, an older guy who was around in the 60s,70s) tell me they have firsthand experience with either seeing or doing this kind of swap. I have read someone swapping the crank pistons and head for 5 more horsepower, but i do not really need more power, I just want to avoid a helicoil type repair. Back then, people were more likely to swap out used scavenged parts to save money, my dad does it all the time. I just would like to hear a more experienced person tell me "sure, go ahead, Ive seen that done, it won't blow up if you put it together right!!"