Bad Pistons

[115johnson78]

I have a 78' Johnson 115hp. Two low compression cylinders on the starboard side. I pulled tlhe head and the pistons look like someone took a hammer and a screwdriver to them and the head around the edges. The cylinder walls look good and smooth there is bairly a ring lip at the end. What could cause this?

 

[HybridMX6]

Re: Bad Pistons

Broken rings, which comes from either overheating or poor lubrication of the cylinders, i.e. carb blockage.

 

[emdsapmgr]

Re: Bad Pistons

These crossflow models came with the special high ring pistons. The idea was to move the rings up as high as possible on the piston to increase compression. With the change in today's modern day fuel formulations, the entire crossflow family of engines is more susceptible to carbon accumulation around the piston skirts and ringsets-causing heat buildup and poor lubrication, stressing the rings. When the ring cracks, it breaks into pieces and rattles around in the combusion chamber till it is smashed into bits, then exits through the exhaust system. Bombardier makes a number of special fuel additive products and XD50 oil which addresses carbon buildup in the engine. Personally, I would not reuse the head.

 

[115johnson78]

Re: Bad Pistons

Thanks for the info. I hadnt planed on using the head. Would it be possible to rebuild those two cylinders. The other two cylinders are at 100 psi on compression.

 

[emdsapmgr]

Re: Bad Pistons

Yes, if the cyls are stilll smooth. (not gouged) I'd put 2 new pistons in it with 4 new ringsets, that way you won't have to tear it down again, should the other two rings decide to go. Hone them all before reassembly.

 

[115johnson78]

Re: Bad Pistons

What about the crank and the rod bearings should they be replaced if they are smooth also? Thank you all for all the help.

 

[emdsapmgr]

Re: Bad Pistons

Any marine shop will probably replace the bearings. As long as they are not blue from an overheat, they are reusable.