Re: powerhead or piston 68 3.9 mercury
Considering that you have to remove the existing powerhead in order to work on the innards, it would be easier to just swap it for the replacement. You'll do additional work replacing the piston in the bad powerhead, in addition to crankshaft seals, bearings, and any other bad parts you find.
Just swap all the external parts over to the replacement powerhead and reinstall with a new base gasket. You might also consider taking a look at the upper grommet for the water tube, since the only time you can access it for replacement is when the powerhead is off.
Check the tube and grommet for obstructions, it's very common to have salt/debris buildup around the copper water tube and on the outside of the rubber grommet; this squishes the grommet until little water will flow. The answer is to pull the water tube/grommet out of the exhaust plate, clean out all debris, and reinstall a new grommet. Grease it up real good with waterproof grease, or a thin coating of Permatex #3, and it'll keep salt out of there.
HTH.........ed
