what causes a ring to catch a port

[mt.sledder]

One of my piston rings caught a port and took a chunk out of the piston, scored the cyl a bit and wrecked the head. What is the usual cause of this happening? The motor is a 1996 johnson 90 outboard.

 

[emdsapmgr]

Re: what causes a ring to catch a port

Which model? They made both a crossflow and a 60-degree looper that year. The crossflows have a tendency to crack rings with age. A common failure scenario is when the rings accumulate carbon around them. The carbon buildup prevents the cooling fuel/oil mixture from reaching the top ring and it overheats and cracks. Once it cracks, it will eventually catch on one of the ports and disintegrate into the combustion chamber. A good remedy is to use the Carbon Guard product in your fuel mixture, or the Bombardier XD50 oil which has that additive in it. Or you can run a can of Bombardier Engine Tuner through it annually.

 

[mt.sledder]

Re: what causes a ring to catch a port

It's a crossflow, and the piston that caught the ring is 2 years old, probably around 200 hours on it since it was done. Thought maybe not enough chamfer on the port. I tore it down tonight and it doesn't look like any carbon buildup on the rings. The plugs have been burning perfect and engine running awesome. The cylinder in question had been sleeved and had a std piston in it, the other 3 are bored 1 size over. From the looks of the cyl right at the edge of the port where it caught, looking at boring oversize and a possible resleeve again if it cracked the sleeve. Head needs replacement also. Only took 4 hours for the complete teardown this time.

 

[ondarvr]

Re: what causes a ring to catch a port

What happened the first time for it to need a new sleeve and rebuild?

 

[emdsapmgr]

Re: what causes a ring to catch a port

It's unusual for a recent rebuild to snag a ring on a port. Any chance that the cylinder was running hot? I have seen engines catch rings if the water diverters were not replaced or were not put in the correct position. When they are missing or out of position, one cylinder can overheat by itself and not sound an overheat horn. Were the diverters replaced correctly during the overhaul?

 

[ezeke]

Re: what causes a ring to catch a port

Lugging the engine (overpropped) will cause that, too. That's fairly common with the V4 crossflows.

Also, the more recent crossflow manuals require doubling the oil to a total of 25:1 during the 10 hour break-in period.