Need advise in rebuilding 87 90hp johnson

[wdm2]

Bought a 1987 90 Johnson crossflow engine that was running, but had low compression on one cylinder. Bought it for parts, but I'm now thinking about rebuilding for the expirience and education. Have removed power head and the two pistons on the bad side which only one was bad. The bad piston had only one ring left, the second ring was completely missing, I was unable to find any pieces inside the engine, including the exaust port. The only thing I can think of is that it was blown out through the exaust ports. The cylinder wall appears to be in good shape, no major scaring. The cylinder wall where the bad piston was, has a very minor 1 or 2mm furced scratch that I can remove with a very fine emory cloth. I showed it to a friend who used to be a mechinist and he told me that he didn't believe that it would require horning or re-boring. I would like some advise since this is the first time that I'm attempting a rebuild of an outboard motor. I have done a car engine.
If I decide to rebuild this engine, I'm planning on replacing both piston along with rings on the bad side, as well as the rings only on the good side. I'm wondering if this is the way to go, and if its worth it...Thanks for any help..Walt.

 

[Dhadley]

Re: Need advise in rebuilding 87 90hp johnson

The damage was probably from coking, a build up of carbon on the rings. probably a result of, at least in part, of low top rpm (lugging). All this creates abnormal heat in the combustion chamber & cylinder. Once you measure it you'll probably see it needs boring from being out of round.

The expensive part of a rebuild, per cylinder, is the cost of the piston. Since you are going to buy at least one new piston it doesn't make sense to not bore it. Plus you'd need new rings for the others and new rings come with a new piston so now the price gap is narrowed even further.

 

[iwombat]

Re: Need advise in rebuilding 87 90hp johnson

If you're re-ringing you're honing. The new rings will never seat right in the cylinder if you don't hone them.

If you're replacing the piston anyway you should seriously consider a rebore.

 

[wdm2]

Re: Need advise in rebuilding 87 90hp johnson

The two cylinders on the left side looked exellent and were giving me 125psi each, don't even know if I should re-ring those since compression is good there. As for the right two cylinders the upper one was about 118-120 psi, and the lower damaged one was 30psi. One piston is damage and absolutely needs to be replaced. If I decide to rebore the right side only, and replace that side with two new pistons and rings, is there any down side to going that route. Don't really want to spend too much on this engine, it will only be used as a spare. I would appriciate any thoughts on this. Thanks Walt.

 

[iwombat]

Re: Need advise in rebuilding 87 90hp johnson

If it were mine, I'd re-bore the one hole for a new piston. I'd hone and re-ring the rest.

Ultimately, the decision is yours to make. I'd certainly try and talk you out of re-boring the good hole though.

 

[wdm2]

Re: Need advise in rebuilding 87 90hp johnson

Thanks Captian, it sounds logical. I will go that route. Mind you its been quite a while since I rebuilt an engine, but if my memory serves me correctly, when you rebore a cylinder you put in a slightly large piston for that cylinder. Please correct me if I'm wrong. What would I ask for piston size when I order the piston for that cylinder, or would the mechine shop tell you what size when they rebore it. As far as the re-honing of the other cylinders they should still take the same size rings. Is that correct. Thanks walt.

 

[iwombat]

Re: Need advise in rebuilding 87 90hp johnson

I would find a good _marine_ machine shop and take the block and existing pistons in. You'll want them to measure up the good cylinders to the good pistons and make sure everything is still really in spec. They'll tell you what oversize you need to get for the bad hole. Chances are, they'll be able to source one for you too.

Start another thread and ask for a good shop in your area. Someone will chime in.