The front is nearly always the first to detonate, hence the reason for many tuning companies advising on 0.6mm base gaskets when doing the HRC head conversion.

I know the HRC conversion isn't applicable in your case, but I am just pointing out that the front cylinder tends to be the weaker of the two.

The jetting for the front cylinder would appear to be just over the edge with the race fuel, and well over the edge on pump fuel, regardless of the increase in one MJ size when you fitted the UL inserts. One MJ size obviously wasn't enough. My MC21 runs 2 sizes difference front to rear (it also runs the needles in different clip positions), but there is nothing cast in stone that says it needs to be this way. There is no reason why there can't be a 3 or even greater size difference, or that the top can't be richer than the the bottom, as I have seen in a couple of 300's.

If you are positive that the compression ratio is where it should be, and you are using an 030 UL card, and the motor is in otherwise A1 condition, then the jetting will almost certainly be wrong.

However, if there is an air leak, or an O-ring leak (allowing coolant into the combustion chamber and consequently increasing the compression ratio) for example, then that will have a similar effect on the piston.

Running lean, low octane fuel, high compression, and too much ignition advance will all produce similar failures. Even the wrong choice of spark plug can cause the piston to melt in extreme circumstances! Combinations of these issues will just cause it to happen more quickly.
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Andy.
NSR-WORLD.COM

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