piston failed, whats the cause?

[nashport_native]

I picked up my 86 R a few weeks ago.

unshure of the time on the engine, stock porting, 38mm airstyker,lrd pipe and silencer, 94 octane fuel, running amsoil dominator 50:1

engine was checking at 156 psi.

160 main jet, just did a plug check on it befor riding sunday, still a little rich, but i left it there.

the engine ended up locking up with all the alum falling down in the crank. the bore looks fine. any ideas?

http://i110.photobucket.com/albums/n99/nashport_native/250r/piston1.jpg
http://i110.photobucket.com/albums/n99/nashport_native/250r/piston2.jpg

[LONG-ROD]

appears you had some piston slap going on, I believe that is the skirt that broke off. 160 seems lean, but there are a lot of unknown variables, and 50/1 is not good. 32/1

[bluetrx]

i dont know anything about amsoil but i run 40:1 with redline, 50:1 seems a little dry.

i go back and forth between a 185 and a 188 main at sea level depending on the weather, could see a build and elevation being the diffrence to a 160 but it is quite a jump.

when you're checking cylinder pressure make sure you kick the piss out of it, i checked mine once 190 psi then the guy who knew what he was doing hopped on it and got the gauge to tach out at 310 psi, although mine burned a hole in the top of the piston so i guess yours dosent really look like a pressure problem.

[05LSR250R]

Looks like you had a crack from old age (fatigue).
I've seen a few crack like that on the intake.
But were caught before they broke!
Then it finally let loose on ya.

50:1 seems really lean as well as your 160 main!

[nashport_native]

ok, thanks for the info guys, i did a wide open throttle chop and the plug was still black.

i ride/race in tight woods so the bike is hardly every reved all the way out, mostly mid range 2nd threw 4th gears.

I agree on the piston slap, it's what it sounded like befor the engine stoped.

i will knock the mix back to 40:1, see if that helps after the rebuild

gotta split the case to clean it out, so i might as well put a new oem crank and new crank bearings in it for peace of mind.

[jcs003]

cold seizing a motor is synonomus with skirt fracture. the piston heats faster than the bore and with low lubrication it will be a common failure in two strokes.