What caused the piston carnage? With Pics

[Spokerider]

So I am rebuilding an early model 266SE that I bought from a fella cheap.......as it was a non-running saw. Ah yes.........those "cheap" saw projects......

I noted it to have low compression via pull coed, and decided to tear into at that point. Well, upon taking the cylinder off, I see the piston has two missing chunks out of the skirt at the bottom edge, and a crack that goes from one of those holes all of the way up the piston to the ring groove. The missing chunks are on the intake side, one on either side of where the port would be, but not "in" the port, if that makes sense? I can flex the whole wall of the piston with thumb / forefinger and get the crack to open up. No wonder the compression was poor.

Surprisingly, the cylinder didn't look bad. The exhaust side of the cylinder looked even better than the intake side. Some metal transfer noted on the intake side, no scoring, and it cleaned up well Mastermind's method of "no acid" cylinder clean up. I am waiting on the Meteor piston to arrive so I can check piston / cylinder wall clearances with feeler gauge.

So what caused the piston to fracture and crack like that? Just worn too thin from use?

 

[1Cut]

With out pictures… kinda difficult to tell.

 

[nmurph]

...So what caused the piston to fracture and crack like that? Just worn too thin from use?

A very plausible explanation. Another could be a piston slapping the case because of a bad bearing.

 

[Spokerider]

No discernible radial play in the big end bearing. The wrist needle bearing seems fine too, but it will replaced for sure anyway.
The inside of the CC is clean, and the bearings are bright and shiny. No discernible radial play in either the FW or clutch side.
I have new aftermarket crank bearings, seals and O rings coming. I'll only replace the crank bearings if necessary.

Wondering if others have seen this type of carnage before.

 

[nmurph]

It could be thinning of the skirt due to wear, or it could just be a manufacturing flaw. I've seen skirts break for no apparent reason.

 

[Mad Professor]

Piston slap.

What is the clearance on the fubared parts? about 0.003" would be new, at the skirt. Note that pistons have a rather large taper top to bottom. The rings keep the top away from the cylinder. If bottom is close to top piston was junk. You WILL hear a slapping noise when things get bad. You can run it more or ..........well you know.

Check your new piston clearance, see above. Too tight and it will seize and too loose.......well you know