Merc 90 Inboard Jet trouble

[Bob K]

While out last time the motor died, I got back to the ramp and weet home to find out I lost the top cylinder. Broken piston, ring and ?. What the heck happened and is this repairable or do I look for another motor? Would there be pieces in the sump of the motor? Any ideas? Thanks, Bob

 

[Bob Bourget2]

Pictures / serial numbers would be helpful. I'm going to assume it is a "sport-jet 90", which is the older Force / Chrysler powerplants. Sounds like it detonated......

If the block has not been damaged, they can be repaired. Parts are becoming NLA very quickly though......

 

[Bob K]

The motor is a Merc 90 hp 3 cyl. #OE112619. I don't understand what caused this to happen, I've had the boat for about 10 years and I have never had any trouble. Could the piston & ring parts cause more damage to the bearings or crank, or where did the pieces go?

 

[Bob Bourget2]

There is obviously fragmented steel atop the piston. Likely from a piston ring. Common cause of failure is called "coking", where carbon build-up in the ring lands prevent the ring from sealing properly, sometimes resulting in ring / port snagging then breakage. I'm sure the piston has the same puck marks as the head. Most of the time, the metal is blown out through the exhaust port. What I can't see is the cylinder walls. If there is minimal damage and scoring, the ONE cylinder can be bored and an oversize piston installed. They are all the same weight as the standard sized piston, so reciprcating weight is not an issue. A thorough inspection should be done on the other two cylinders, crank, and bores. Be sure to bring it to someone you trust, have the failure analysis drawn up, {which requires powerhead disassembly}, then choose your course of action. Merc only has re-man powerheads in the 4-cylinder Sport-Jet 120.