My RJL 40 head had a concavity in the center of the contrapiston with a flat (maybe a slight angle, but I don't think so) band around it. Unfortunately the groove for the O-ring was too close to the bottom of the contrapiston and the flat band broke off and welded itself to the contrapiston and top of the piston. No damage except to put a twist in the rod.

Gene Hempel made me a new contrapiston with the groove for the O-ring higher and the contrapiston a little closer fit in the head. The bottom of the new contrapiston was flat except for a shallow conical dimple in the center. No reason for that except that it was there in the piece of aluminum we used, and I saw no harm in it. So far as I could tell, the two contrapistons gave the same performance.

It is my understanding that the fuel/air charge in a diesel fires very rapidly throughout the combustion chamber, as the major source of heat is compression which is uniform throughout the chamber. If so, then I wonder if shape of the combustion chamber would make any difference. However, perhaps there is a "hot spot" somewhere in the chamber which initiates ignition, not a slow burn like with a glow plug, but just a little less uniform burn than would be expected if the ignition heat were from compression alone.

Jim