Read my piston if you can

[Another Bucker]

It's factory Dirko HT

I stand corrected. Thanks, it was worth a shot.

 

[PES+]

Tom,
I have several detonation failures in my collection. They can all be related to lean running.

This one (deto1) had 2 lean faults, an air leak at the clutch seal, and trash in the fuel filter screen in the carb. Lean means less fuel. So what is in the combustion chamber burns faster, so the flame front speed becomes very fast. This sets up detonation, where some of the fuel lights off after the plug has fired, and two or more flame fronts collide. It often happens at the same place, near or under the plug, so you may get a hole hammered through the top of the piston, like this one (deto2).

The location of the hole in your piston kinda makes me think that maybe someone used a rigid screw in piston stop that caused a fracture in the top of the piston, where it is thin, and then it blew out later. That is why you should use a plastic stop that gets pinched in the squish area, where the piston is stronger.
Did you find any lean faults on the saw?
Just curious. I have yet to prove that low octane or timing can kill a saw, and I have tried to do it. No luck!

Just for clarification did you mean that the fuel ignites at the hot spot and then the plug fires and the two flame fronts meet?

It is called "pre-ignition" I have never heard of post ignition causing a problem.

 

[Mr.]

Detonation is post ignition!

 

[Mr.]

And preignition and detonation look completely different on the piston.

Fred

 

[Stihl #1]

Just for clarification did you mean that the fuel ignites at the hot spot and then the plug fires and the two flame fronts meet?

It is called "pre-ignition" I have never heard of post ignition causing a problem.

Preignition is where a little flake of carbon or an overheated spark plug ignites the fuel before the plug fires. Detonation is what I explained earlier, where 2 flame fronts collide at extremely fast speeds, faster than the speed of sound.
One will usually cause the other, and at 12,000 RPM the plug is firing 200 times per second, so everything melts down real fast.
Normal combustion flame front takes place slower than the speed of sound. A fast (lean) burn will tear off the boundry layer of molecules that are protecting the piston from the high temperature of combustion, and then it melts.
HTH

 

[PES+]

There is no second front as far as I know but an instantaneous burn.

They can be related and the pistons you show look very different from the Thall piston

I'll bow out since you folks have all the answers covered and I know so little of engines

:biggrinbounce2:

 

[Mr.]

Why are chainsaw guys such pansies?

Of course you know alot about engines Pest, but nobody knows everything. If that's what you're after, then you will only disappoint yourself.

I like alot of your posts and I know I can learn from your experience. Extend the same sentiment.

Fred

 

[PES+]

Why are chainsaw guys such pansies?

That pretty much sums it up:hmm3grin2orange:

*painting toenails*

:deadhorse:

:welcome:

opcorn: opcorn: opcorn: opcorn: opcorn:

 

[romeo]

These pics are of a stihl ms310 piston. The rings are seized on the exhaust side, the rest of the piston looks fine as does the jug . Can I get away with just replacing the rings (I havent tried to remove the old ones yet)

View attachment 48806

View attachment 48807

View attachment 48808​

in picture of piston top ,exhaust side is on top of picture

All of the missing metal on that piston is stuck to you cyl. wall. Get a cyl/piston kit. When it runs for 30 seconds then dies its because that transfered metal is expanding and pushing in on the ring / losing compression.

 

[romeo]

Worth saving? I don't think so.............

timing too far advanced?????????

 

[Lakeside53]

Nope - metal object used as piston stop....

 

[Mr.]

After you said that last time I thought you were right. In part.

Probably dug a burr that caused some "glow plugging."

Fred

 

[joatmon]

no eBay?

First thread I've seen on AS with saws this sad without a reference to eBay. :hmm3grin2orange: