99 force 120 rebuild

Have a long go with an engine problem, which was originally to be a simple exhaust gasket replacement. Ha,ha. Broke bolts off in taking off plate. Any, took to a mechanic, and found that I also had a damaged piston(scared) and I needed to address it. This was in June of 09. Was told that that engine was worth nothing and I should look at replacing it. After looking at the cost of repairs of $2500 we went looking for mercury to keep wiring harness,ect. Prior to this, was told could not get a rebuilt powerhead, parts hard to find. After searching on my own, found that not to be true. I have boat back now after 7 months with all components. Had to have powerhead sent to machine shop prior to get out some twisted bolts out and sleeves inserted. I am thinking about rebuilding this engine and need to know if I can replace only the piston that is scared and then do all rings. Also, do I need to do other items, other than gaskets. I only have 45 hours on this engine and all internal areas look like new. The piston that is damaged on the top. Have priced engines, but hate to trash this one due to being in almost new condition. Block was honed while at machine shop. Any info on this matter would be appreciated.

Re: 99 force 120 rebuild

I know other folks have replaced just one piston in their 125's 120's etc with success with no negative impact to the motor.

I replaced the pistons in my 1988 125 with the forged Wiseco ones. Very good quality and they have floating wrist pins, not the pressed in wrist pins like the OEM ones. Much easier to assemble/disassemble.

Check the connecting rod bearings, crankshaft journals and crankshaft roller bearings for discoloration (blue color) that would indicate they were overheated. Replace if necessary. Also make sure the surfaces that the roller bearings ride on are smooth, no galling or groves or deep scratches. I also replace the rod bolts just for good measure. They're relatively cheap.

I also get new crankshaft seals. The metal seals that look like piston rings that go in the crank. I usually get a new set plus a couple of extra. It's not uncommon to break one when dropping the crank into the block while trying to line up the locating pins in the block with the holes in the crankshaft bearings.

Good luck!

Re: 99 force 120 rebuild

Yes, you can replace only one piston. And, if the cylinder is not scored or scratched, you need not overbore it. These engines get so little use compared to automobile engines, that it is not necessary to replace the rings on the other three pistons-- as long as the compression is still good.---you will not gain any power at all, just spend extra money.

Re: 99 force 120 rebuild

pnwboat said:

I know other folks have replaced just one piston in their 125's 120's etc with success with no negative impact to the motor.

I replaced the pistons in my 1988 125 with the forged Wiseco ones. Very good quality and they have floating wrist pins, not the pressed in wrist pins like the OEM ones. Much easier to assemble/disassemble.

Check the connecting rod bearings, crankshaft journals and crankshaft roller bearings for discoloration (blue color) that would indicate they were overheated. Replace if necessary. Also make sure the surfaces that the roller bearings ride on are smooth, no galling or groves or deep scratches. I also replace the rod bolts just for good measure. They're relatively cheap.

I also get new crankshaft seals. The metal seals that look like piston rings that go in the crank. I usually get a new set plus a couple of extra. It's not uncommon to break one when dropping the crank into the block while trying to line up the locating pins in the block with the holes in the crankshaft bearings.

Good luck!

Click to expand...

Thanks for your quick response on this issue. Your help is greatly appreciated. Jerry

Re: 99 force 120 rebuild

Frank Acampora said:

Yes, you can replace only one piston. And, if the cylinder is not scored or scratched, you need not overbore it. These engines get so little use compared to automobile engines, that it is not necessary to replace the rings on the other three pistons-- as long as the compression is still good.---you will not gain any power at all, just spend extra money.

Click to expand...

Your input has been very helpful on this matter. Thanks for your help, is greatly appreciated, Jerry

Re: 99 force 120 rebuild

Don't know if I am doing this reply right, but I appreciate all the information that has been provided to me about this matter of piston replacement on my force 120 outboard motor. Thanks again for your help, Jerry.