Time to get familiar with Dolkitas... picked up a 6401 today.

There may be oil in the gas but it could be regular motor oil or a synthetic 5W which would look mostly amber in color @50:1.

 

This is a stretch, but if a piston was a bit out of spec, and was run hard from cold, could the dis proportionate heating of the aluminum "out of round" piston start a hard rub snowball effect?? The lean condition only exasorbating the problem??

 

It looks like there is a small chip missing above the piston ring to the right of the centering pin.
Could have been one where the transfer occurred as well

 

I'm thinking that because the impulse comes in high and inside the transfer port, the fresh air is confined to that one channel and blows into the combustion area at the end of the burn, overheating just the local area where it emerges into the combustion area rather than mixing evenly into the mixture in the crankcase.

 

Sounds like it's time to push my camera's macro settings a bit further...

There's actually both buildup on the piston AND a gouged area, but the gouge looks like nothing so much as the path left behind when you're rolling up a big ball of snow to make a snowman. The bottom of the trough is rough and choppy, not cleanly cut or smeared like other areas. The trough also starts out very small at the bottom of the piston, and gets wider and deeper as it goes up, terminating in a flattened bump that's raised above the original piston surface and above the surrounding damaged area as well. This line crosses the intake port, and there are no nicks whatsoever in the intersecting top or bottom edges of the port to imply an object hitting them.

Above the flattened lump of displaced material at the top of the large trough is another, much smaller trough, and the material from that appears to jumped over the ring and been deposited as another lump at the top edge of the piston.

 

Nice, but I looked at the ring with an 8x loupe and there are no such nicks in it. The surface is scuffed in a narrow area and there's one shiny spot of aluminum stuck to it, but no grooves going across. It's actually in pretty nice shape.

 

Just wondering...
What did the plug look like?
Was it missing anything?
Could the Item that caused the damage have been harder than aluminum, but softer than ductile cast iron?
Just spit balling here..

 

Perhaps we should consider gremlins or PFM (pure ___ magic)?

 

Plug looks good - a little carbon at the end of the threads where it emerges into the combustion chamber, clean insulator, brownish at the tips of the electrodes. No chunks missing. No scratches at all that I can see on the cylinder wall, just burnished areas and varying amounts of aluminum transfer. One thing, and maybe this is normal, is that most of the transferred aluminum is in a really thin layer, as if it just a few molecules' thickness rubbed off, but then distributed randomly over that area are small clumps of thicker stuff, as if this was the school desk under which students had been discarding unwanted aluminum chewing gum. I've never looked so closely at top end damage, so maybe this is typical.

We can stop this if it's getting boring. I'm still fascinated, but I realize we may not be able to pin this down conclusively. We can always turn our attention to the question: 6400 or 7900?

 

That would sorta depend on if it's a keeper, or a flipper..
Keeper? 7900...
Flipper? 6400...

 

Well, at this point all I've got in it is the $15 I paid for it so I'm not losing money no matter what I do. If I keep it a 6401 then I'll almost certainly sell it, but if I upsize it then I'm inclined to think I'd sell the ms460 and keep the Makita for a while.

 

Will you be modifying it? If no, then get the 84cc top end.. If you intend to modify it, use only OEM 79cc P&C...
That's if you decide to keep it to use as your "Big saw"..

 

I'm unlikely to modify it. I'd swear I read somewhere (AS?) the opposite recommendation, i.e. that OEM was the way to go if *not* planning to modify, but the rationale wasn't too clear.

 

The 84cc jugs run with a lot of grunt for pulling big bars out of the box and are a lot cheaper than OEM79cc.
OEM79cc have material in places that the aftermarket jugs do not, allowing the builders "who know how" to modify the transfers and get good gains. They'll grind right through the AM jugs to daylight... Only a few builders have a good recipe for improving on the Dolmar 79cc OEM jug...