Scored Cylinder

[jlshields]

Here's one for ya. '97 Mercruiser 5.7LX purchased new, less than 50 hrs, never abused, well maintained, Formula Shell 40W oil, had a knock. When I tore it down the #5 cylinder was scored. The piston showed signs of being very hot around the wrist pin and wrist pin was loose in the piston, there's the knock! The factory installed spark plug in #5 was loose! I'm guessing it was able to pull enough air for that cylinder to run very hot and also not able to transfer heat away from the plug. Valves and top of piston are not burned. Camshaft and all bearings OK. Opinions Please Thanks, John

 

[Walt T]

Re: Scored Cylinder

Piston pin overheating and failure is usually caused by lack of lubrication in that area. Oil supply through the crank to that journal is suspect, the rod is suspect, and the rod bearing is suspect. The main bearings next to that rod on either side are suspect as is the crank. You have an oil supply problem localized to that area. One thing to check, and I know it is far fetched, but I have actually seen this, is the crank is ground .010" on one rod journal yet standard bearings are used creating low oil pressure on that one rod, thereby damaging the pin area and scoring the cylinder. Yes I know you saw no oil pressure problems. The loose plug was not the cause. Keep looking and use a micrometer.

 

[jlshields]

Re: Scored Cylinder

DielelWalt, Thanks. I was looking for trouble on top of piston but it makes sense that when the piston, around the wrist pin has been hot, the problem has to be lack of lube. I'll look it over REAL good. John