I found these pistons in a parts bike (73 cb750) that I bought along with a Kenny Harmond D grind cam. They measure 64.5 mm. Unfortunately, two of the pistons were installed backwards, and show the marks of valves "kissing" the pistons. I have scanned the internet looking for the maker, but have not found anything. Markings on the bottom start with the letters "A" and end with "S" (five letters total). The letters in the middle are worn off. Action Fours? They have one piece oil rings, so I'm of the impression they were installed some time ago. My resident old bike mechanic(He's a good one!) does not recognize them either.
