Johnson 1987 pistons

[Tim T.]

Hi everyone, once again I need your help. I bought a 1987 50 hp Johnson for $50 with the intention of a rebuilding it. It took me 3 days just to get the power head off (very stubborn bolts), and once I started tearing it down I took off the head (got lucky and did not break any bolts) I noticed the top piston wiggled slightly in the sleeve and the bottom did not. I tore the rest of it down and found a chip in the top piston right between the rings. I plan on honing the cylinders and replacing the rings. My question is is it ok just to replace the top piston (which are not cheap) without replacing the bottom? The bottom piston is in very good condition and both sleeves I can still see cross hairs and maybe just a quick deglaze on both cylinders may be enough? Where the chip was there is a small line you can barely feel with your fingers. Also I broke off a few bolts on the exhaust cover, will heli-coils work in exhaust cover? please forgive me for the long post, Tim

 

[Scott Danforth]

use a bore gauge and measure the bores (top, mid-stroke and bottom). you will most likely find that a re-bore is in order as you will find the bore egg shaped and tapered.

 

[Tim T.]

I do not have an bore gauge but I will see if Harbor Freight has one. Do you know if heli-coils will work in the exhaust cover?

 

[Scott Danforth]

You won't find a bore gauge at harbor freight, but you can get a cheap one from Amazon for about $125

 

[stresspoint]

OP , if you had seen some of the hack jobs i did on those old motors back in the day you would not be concerned about just swapping one piston ,
since its an old motor ,just look around for a used piston without a hole in it and plonk it in there , button it up and go fishing .
as posted , check that the bore is round "or close to it " first.

 

[racerone]

Not a good idea to put new piston rings on used pistons.-----Yes the motor will run.----Many worn out motors run.----Doing a 1965 Mercury " panther " to help a dealer.----Found that crankshaft was worn , thus allowing crank to wipe out labarynth seals.----Resulting in the poor idle.----Owner of motor want this motor fixed !

 

[stresspoint]

i used to pull the good piston assembly ,rings, bearings, etc from a motor that had a dead cylinder and banged out crank and stick it in the one i was repairing ,
just to point out how slack you can be with these motors and they will still run fine ,
sometimes even checking the bore was an afterthought , too late for that after the motor was back together and running.

people used to just want the motor running again , this practice served that purpose many times.

 

[airshot]

Why not sell it for parts and buy a better/newer motor ?
If your gonba rebuild it, do it all and do it right or don't waste your time and effort. My two cents anyway !!

 

[salvageyard saviour]

i used to pull the good piston assembly ,rings, bearings, etc from a motor that had a dead cylinder and banged out crank and stick it in the one i was repairing ,
just to point out how slack you can be with these motors and they will still run fine ,
sometimes even checking the bore was an afterthought , too late for that after the motor was back together and running.

people used to just want the motor running again , this practice served that purpose many times.

"All engines run on used parts "

I have more time than money, so I use what works.

 

[Tim T.]

Not a good idea to put new piston rings on used pistons.-----Yes the motor will run.----Many worn out motors run.----Doing a 1965 Mercury " panther " to help a dealer.----Found that crankshaft was worn , thus allowing crank to wipe out labarynth seals.----Resulting in the poor idle.----Owner of motor want this motor fixed !

I have it all torn down and I could not believe how pristine the crank and bearings were, I have rebuilt a few 4 stroke motors before but never a 2 stroke. both upper and lower seals look almost new but I am going to replace them anyway so I am sure this crankshaft is in great condition.

 

[Tim T.]

Why not sell it for parts and buy a better/newer motor ?
If your gonba rebuild it, do it all and do it right or don't waste your time and effort. My two cents anyway !!

because I have an 1988 on my boat now and is running great and I just want a spare motor and I am retired and live on a fixed income. So I do not the 4 or 5 hundred dollars to do a total rebuild.

 

[racerone]

Tim T----bearings on a Mercury are mounted different than on an OMC .-----Rarely find crankshaft damage on an OMC motor.

 

[Tim T.]

I have always trusted your experience and knowledge, is it worth it to spend the money on a bore and new pistons?

 

[tphoyt]

Try looking for a local machine shop to mic the bores and be sure they are within spec. If they are not round new rings will not seal and could break as well. You mentioned there is still crosshatch so you might be fine but better safe than sorry. Jmo

 

[Tim T.]

Try looking for a local machine shop to mic the bores and be sure they are within spec. If they are not round new rings will not seal and could break as well. You mentioned there is still crosshatch so you might be fine but better safe than sorry. Jmo

I ordered a gauge and it will be here in a few days. I will post results, thank you everyone for your expertise Tim

 

[ct1762@gmail.com]

I ordered a gauge and it will be here in a few days. I will post results, thank you everyone for your expertise Tim

if you cannot feel a "lip" at all were the rings stop at TDC, it shouldnt be worn more than .0015'' in my experience and thats max. I NEVER just quick ball hone and new rings on a motor though. not that it cant be done to get a few hundred hours out of the motor and you dont mind some power loss down low.

 

[Faztbullet]

Being able to see crosshatch in a cylinder means nothing.