Stewart,
I did that exact mod on my '79 maico 400 that I raced. Was combined with much other porting so hard to tell what its exact contribution was, but the combined effect was tremendous. From dog to holeshot heaven!!! That is actually one of the porting things that make sense since as the piston travels downward, the cylinder transfer ports open up but if the skirt of the piston is shouding the lower transfer opening then flow is reduced unnecessarily. On my maico tje shrouding was terrible. Good pics. I should mention that on my maico I had to cut away much of the reinforcement rib on the piston skirt which eventually caused to to crack. But it did last 50 races. That was a forged mahl piston, very strong. Not sure how well the stock cast piston would last. I'll have to look at the ribs on my old one. Cam.