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After speaking with the machinist/mechanic who rebuilt this Engine from the bottom end up, I'm assuming this was caused by having too high compression. I neglected to run a compression check after the rebuild. This was only the fourth trip I had her out following the rebuild. I was running 20:1 ratio with Maxima Super M premix and premium gas 91 oct. with 2 ounces of Maxima High Test octane boost per gallon (mfg. claims 4 points increase per ounce per gallon). I'm confident the jetting was correct. I had just started using a NGK BR9EIX Iridium plug. My mechanic believes I can get by with honing the cylinder and a new piston, but obviously want to avoid this happening again. The cylinder is on its last bore, so I need to sort this out. I'll probably switch to 50/50 mix of premium gas and race gas in the future. Any other recommendations?

More pictures, bigger more detail pictures please.

Pics of the head too

List all modifications including any to the fuel system.

I hate when this happens.

Running a 38mm Keihin PWK (210 main jet at sea level), mild cylinder porting, aftermarket pipe but I'm not sure what brand, Boyeson power reeds. My mechanic pointed out that extreme detonation will burn in the center of the piston, which can be seen in photo. On the trip that it seized, I could tell by the smell of the exhaust something wasn't right. I'll post some more pics tomorrow.

I would like to see a pic of the bottom of the piston dome(burn pattern) if its not obliterated by the hole and alum spray. I agree, detonation burn issues. I am about to begin race fuel/ aviation fuel experimenting. I would love to know more about how you desided on your octane combo and what compression you had.

I ASS-U-ME you do not run an EGT? When you start building up engines, EGTs are key to monitoring Engine burn temps. I do not rely solely on the gauge,after getting plug chops to set jetting,use the guage to know what "normal" are. Then you can see a change in temps while riding. Not trying to cause bad feelings, just pointing out. Been down the rebuilding road too many times before I became an EGT believer! LOL

GARY

Pics of your intake system?

210 main jet is a big red flag to me, if you had to run a main jet that big something was wrong.

You can melt a piston like that with a few drops of water in the float bowl of the carb I would not blame compression or low octane just yet we need to look at the whole picture and CSI the whole machine.

Need to find the root cause of the 210 main jet work from their.

Stock gas tank and fuel hose routing?

210 is mighty big... could possibly have been an air leak, that required that big of a jet to conceal...
Did your mechanic do a leakdown test? have you done one???
I bet not since you never tested compression...

Any stubles off idol or when gapping it to WOT (Wide Open Throttle). The big main might be from low intake velocity or an air leak. I would lean toward a lean condition/air leak since it's on it last bore and usally with large over bore the intake velocity is greater and the main is usally smaller do to the increse in velocity pulling fuel throught the main.

Now if and I mean If this was a post recall head air cooled and any change and maybe no change in the head gasket and on an 82mmm bore this kinda showes what I was refering to in another post on the MSV and timing change which would be incountered due to heat release and location. The big main washes the cylinder untill there is a change in barametric pressure and tempature and then its too small and there just too much heat in the cylinder and even with the 9 its not enough heat being wicked through the head and disapated.
A pic of the head upside down and a mic layed across the squish area ( max OD of squish area) will tell me what I need to know. The lead or oxynated additive will reduce this effectt but will not cure it. It will only change the burn rate in time and still allow more heat on the pistons outer edge and then she will run away after that continuing to advance the timing untill a blow hole accures.
Lets just hope it was an air leak. a quick check of the export opening to deck will also aid the theroy if a post recall cylinder to help prove or disprove theroy for a maxincrese in MSV in a over bored air cooled head with after market exhaust.

Just a thought.

Put some grease on both sides of the head gasket and bolt the head back on pressure test the Engine before taking it apart further...