HAPPY PISTONS!!!!!!!!!!!!!!!!!!!!!!!!!!!!

LOUD MOUSE · Post by **LOUD MOUSE** » Wed Mar 03, 2010 12:18 pm

These are a set of pistons I recently removed from a 305 engine sent to me for a rebuild.
Notice the little time they ran from the pics of the inside.
The rest of the inside is just as bad.
Looks as someone didn't time it correctly and the shift drum rollers were installed upside down so didn't rotate which destroyed the shift forks ends.
Ya got to love it. ............lm

Spargett · Post by **Spargett** » Wed Mar 03, 2010 4:42 pm

So what exactly happened to those pistons?

68Cb77 · Post by **68Cb77** » Wed Mar 03, 2010 11:14 pm

Oh Dear God!!! ...OUCH! That's a mess.

teazer · Post by **teazer** » Thu Mar 04, 2010 12:52 am

Looks like detonation, but what caused it? Was it just really bad timing or was it also running lean enough to raise running temps up to dangerous levels too?

Spargett · Post by **Spargett** » Thu Mar 04, 2010 12:54 am

I thought pre-detonation was apt to burn a whole right through the top of the piston, since it's the thinnest part. What would cause it to hit the sides like this?

teazer wrote:Looks like detonation, but what caused it? Was it just really bad timing or was it also running lean enough to raise running temps up to dangerous levels too?

teazer · Post by **teazer** » Thu Mar 04, 2010 1:10 am

duplicate removed

teazer · Post by **teazer** » Thu Mar 04, 2010 1:33 am

Pre-ignition is the mixture starting to burn before the spark. In and of itself does not necessarily cause damage but typically causes the mixture and the motor to get very hot very fast.

Detonation is often caused by pre-ignition and is a shock wave traveling through the mixture at much higher speed than regular flame propagation. For all intents and purposes, the mixture is exploding. The shock wave is what punches a hole in the top of a piston.

On a 2 stroke that mode of failure is usually in the middle of the piston crown. On a 2 stroke that's typically the thinnest and hottest part of the crown. But failure is not always in the center. Failure usually takes lace where the shock wave "crashes".

In a 4 stroke the hot spot is often an exhaust valve and that starts the detonation event at the exhaust side resulting in failure on the intake side of the piston.

Other times the hot exhaust side of the piston is the weak spot and failure occurs on that side instead.

If the problem is a "cold seizure" where the motor is still cold and the piston gets too hot too soon and expands into a cold bore, then typically that will show a 4 corners scuffing and may fail at a "corner".