Monday October 21, 2013

Greetings and welcome!

Yesterday brought the first real accumulating snow to our region. I did not see any here in Silver Cliff, but an hour or so to the northwest it was making white ground. This was posted on Facebook by a friend near Eagle River..

firstsnow (photo credit to Holly Leusink)

The ground around here isn’t even remotely frozen, but it is getting a lot colder and would probably hold snow like this for a little while.

Sunday also brought a little time in the shop. It wasn’t much between getting organized and knocking off early to watch the Packers, but it was enough to figure out why a friend’s snowmobile stopped last winter.


Sometimes it is challenging to figure out why an engine died. Often it is not. This one was as clear as glass. Bad gas ate the piston, no question about it. I will add pictures once I pull the piston off of the rod, but it was a classic hole in the top with a four point seizure.

I talked about this a little last year. The four point or four corners seizure happens when the piston overheats. The thickest part of the casting expands the most, and that is the thick area around the piston wrist pin. The corners of the pin casting expand and you get a 4 point seizure.

This happens with low octane gas or old gas. The octane rating is a measure of how evenly the gas burns under pressure. Lower octane gas can burn hot and unevenly, and in this case caused detonation before the piston was at top dead center. The extreme heat and pressure eventually melted the hole into the piston.

You can also get this if the timing is too far advanced, but unless someone has been adjusting the stator timing, the failure analysis points directly to low octane or old gas.

In this case the owner got lucky and the cylinder survived it, so it can be fixed with a new piston and some gaskets, and a flush of the crank case and fuel system.

There are some articles put there that suggest that a four point seizure can happen from a lean fuel condition, and I disagree. A lean condition usually burns the exhaust side of the piston, not the top. They also suggested that it can happen from hammering a cold engine and the piston expands faster than the cylinder. The four point can happen in a cold seizure, but you won’t get the hole in the top from the detonation.

Well I am off to get a little more sleep before the alarm goes off. Have a good week and thank you for visiting!