Re: Can I swap a 1980 mercury 402 powerhead with 1979 merc 402 powerhead?
OK, I see why you are keeping the lower end intact, you should still change the impeller though. For the power head make sure you have all the bolts removed, there should be 8, 3 on each side of the main block and 2 under the front intake. Often the front ones are not noticed on the first attempt. Remove the defective power head first, learn on it. The gasket can hold on to the block as well. You may have to run a knife around to separate the gasket. you can tap the knife blade in as a tiny wedge to break the seal, do not try to pry the head off. Go slowly, use lots of penetrating oil or liquid wrench on the studs, turn the engine upside down and let the fluid run down the bolts.