So what caused my piston edge to melt?

[Jeff in Pa]

I got a 122 earlier this summer, got it running ( it smoked ) and then parked it. It was at the business the friend had and it sat unmoved for the 10 years he was there. ( Back up plan when I rebuilt the 125 if I ran into major problems rebuilding the engine)

As I was tearing down the engine, I noticed some new tins ( the muffler heat shield, a 3x3 square ) and also a newer, low boss head. That had studs with spacers. When I went to remove the head, only two of the nine bolts resembled "tight enough". That gave me reason to believe whoever had the engine apart never retorqued the head after one hour and again after 5 hours.

It seemed to have "deposited" portions of the top edge of the piston against the top of the cylinder wall, on top of the block at the exhaust valve and a slight bit on the head. The head gasket appeared to have a slight leak also.









I did clean up the top of the head ( flat filing and a large stone ---It's ok, I'm a machinist with over 30 years in the trade)


The bore still looks great and measures with in spec.

Any ideas of what caused the melt down?

Thanks
Jeff

[mgonitzke]

I think a lean mixture will cause that. This is why air-cooled engines should be run slightly on the rich side.

[Jeff in Pa]

I think a lean mixture will cause that. This is why air-cooled engines should be run slightly on the rich side.

Thanks Matt

The plug was oil soaked so I couldn't get a good read on it ( other than excessive oil )

[vince_o]

I say too much air.

[Farmallgray]

Could also be from overheating caused by plugged cooling fins. I have seen that in a lesser degree on quite a few engines I have rebuilt.

[Jeff in Pa]

Thanks Vince

Could also be from overheating caused by plugged cooling fins. I have seen that in a lesser degree on quite a few engines I have rebuilt.

The cooling fins were clogged "somewhat" with oily grass clippings so the overheating is another possibility.

Thanks

[J Hayes]

the studs with spacers should be correct as it holds tin up off top of engine for air flow
someone may have been using too hi of octane gas ? just a guess? may have melted piston near the Exhaust side .

[Racenitro]

Matt is probably correct and the lean burn situation was enhanced by the clogged fins...
Would suspect that you better do a valve job also as lean situation tends to hurt the exhaust valve seat.

[Jeff in Pa]

I took my Jeep Cherokee over to Phoenix Performance to get it inspected this afternoon. I took the piston and talked to Larry, their engine builder. ( they build engines/race cars for SCCA racing and have won numerous national championships ) I showed him the piston, told him it was a Kohler K-301 with a cast iron block with an aluminum head. I asked him what caused the piston to erode like that.

He looked at it a bit and said two possibilities, wrong heat range plug or a head gasket problem. Considering the lack of tightness on the head bolts, the head gasket problem is most likely since the plug was correct heat range ( Autolite 216 iirc )

[rweaver]

I cant see very well but it looks like that is a high comp head that might have some bearing on it as well

[Jeff in Pa]

I cant see very well but it looks like that is a high comp head that might have some bearing on it as well

Here's a couple more pics of the head, I think it's just a low boss "regular" head. there was a casting mark of ?-93 on it

[rweaver]

hi yes its a reg head not hi comp sorry could not see