Philip,
The piston skirt blocks the transfer passages when at bottom of the stroke. Triming away the lower skirt at the wristpin ends corrects this. You see this in many model engines. A radius is needed releive stress on the piston. Here is a picture. I added a Zenoah G62 piston for comparison. I usually remove the rod, use masking tape to protect the uncut areas. You don't even have to remove the ring when using tape. I mark the area to be removed, drill holes at the corners. Then I use a dremel with a cutoff disk to cut away the remaining material. a swiss file and fine sandpaper finish the job.


Dave