My blaster engine seized on me during break-in of a new top end. I have done top-end break-in before, and I believe I was doing it correctly.

Anyway, we pulled the engine apart, and one mechanic told me that it was "cold-seized", but then I took it to a shop and they said it had been "lean-seized."

I thought I understood these terms: cold-seize means improper warm-up, or riding at low rpm and then getting on the throttle all of a sudden, right?
And lean-seize means too much air in the air-fuel mixture?
Would these 2 problems leave the piston and cylinder looking the same?

I'm getting ready to rebuild the bike, and would appreciate any input... don't want to ruin the next top end, like happened to the last one!

 

I "think" cold sieze is from when you run a cold motor hard and the piston warms up/expands faster than the cylinder so it freezes. Think its mostly a water cooled motor problem, but guess it can happen to any. Lean sieze as far as I know is caused when the motor is overheated from a lean jetting condition. I would assume a lean sieze would show signs of overheating but I wouldnt know, I havent had the luck to encounter either yet (Thank god) and im not a mechanic.

Since it was a fresh rebuild, you might also consider piston clearance etc...

 

A cold seize will look different than a lean seize.
A cold seize will be all graffed up on the side of the piston and bore. It will not look melted. It will usually seize on two sides of the piston. A piston is not round and the sides of a piston will expand at a different rate and it will be seize up on these sides. The top of the piston should look fine with a cold seize situation.
With a lean deal, the piston top should look melted and show heat marks. The top of the piston should look VERY dry. It will melt into the first ring land if it gets bad enough. The sides will get scratchy but not like the cold start deal.

Then you can get into the lack of lube seize, or the cause of most; the lack of clearence seize.
If you just rebuilt it my guess is the last one.
Measure the bore and find out how much clearence they gave you.

 

Doctorturbo is on the money with his response

if it lean siezed as it sounds make SURE to have your motor "lpressure checked" when you rebuild again
if a crank seal is bad air can leak into the crancase under the piston causing the same thing...often times 2 strokes get rebuilt without fixing the problem that caused the issue

know what I mean man?




BRAAAAAAP

 

Thanks for the replies.
I think that you're both right, and that the piston clearance was wrong. These people telling me that I seized the bike are just trying to cover their own mistakes. There is certainly no sign of melting on the piston.
Oh well, time to find a new shop to send my bike to!

 

how did u go about breaking it in? did u give it time to warm up slowly and put around under 1/2 throttle for a tank of gas or 2? or did u gun it as soon as it started up? i know alot of people that just stomp on their kicker and fly all ***** out(2 stroke engine). i always cringe when i see people do that cuz there is always a possibilty of it seizing and them goin over the bars.

 

My buddy rebuilt his YZ250. Thing is, he didn't realize that there were two kinds of pistons available. One type expands more that the other when it gets hot, so you need to bore the cylinder, even though the size cold is the same as a piston that doesn't expand. Of course, you know what happened on his first ride..............