77 Mercury 700 replace piston?


Jul 15, 2010
Hey everyone,
I recently bought a boat with a 77 Merc 70 HP and one cylinder has low compression. The other two are firing at 140 but this ones only at 40 psi. I'm about to pull it apart and see if its a ring or head gasket, but are there any tricks to putting it back together? Will i mess up the timing by pulling this piston or will I be able to pull it, replace what I need to, and put it back? Thanks for your help.


Jan 16, 2004
Re: 77 Mercury 700 replace piston?

There is no head gasket on this motor. To get at the internals you'll need to remove the powerhead, remove all the accessories off it (flywheel, carbs, electronics, etc), then split the crankcase halves.

The entire rotating assy (pistons, rods, bearings, crankshaft) pulls out of the block as a unit and is installed the same way. Mercury sells special ring compressors for the reinstall but they're quite $pendy.

I have had success in the past by hanging the crankshaft/rods/pistons from the overhead on a winch then lowering into the block, working in one piston-at-a-time. You'll have to be specially careful feeding the rings in, as it's easy to get them out of position and/or catch & break them on the block going in. Takes some practice but possible.

Others have used very thin shimming material wrapped around the piston and secured with tie wraps at each ring location, to hold the rings in place until slid into the cylinders. Or beg, borrow, rent a set of Merc ring compressors.

At any rate, you won't know much until it's torn down. If you're lucky you'll be able to hone the cylinder, install new piston/rings and be done with it. However, if the cylinder is scored you'll need to bore that cyl oversize and install O.S. piston/rings to go with. At minimum you should re-ring the other (2) cylinders and replace the upper and lower crankshaft end cap seals.

The above's just an opening shot, lots more involved of course on the rebuild. Recommend you purchase a good manual so you'll have access to all the procedures, torques, specs, etc.

HTH & let us know what you find when you get 'er apart......ed