I bought a NOS STD B40 piston last year, part number 41-0242 was written on the box. But the actual piston in the bx has 41-243 stamped on the top? None of the B40 parts books list a 41-0243. Doing a web search and parts book search, this number 41-242 does come up as a STD B40 8.7 to 1 piston. Doing multiple web searches only brings up an old post of how BSA added a 1 to export pistons to the US, 242 becomes 243?? And a west coast site that lists the 41-243 as a 10 to 1 piston. Other then being 1mm OS, it(the 41-243) looks just like a B40 41-247(1mm OS) piston that I have. Any leads to a site with good piston IDs on it. Jeff
I started going through a bunch of old catalogs and after going through 7 old DomiRacer catalogs. I finally found a BSA piston 41-243 listed. It is a 8.7 to 1 STD BSA B40 piston. The web is great, but sometimes being a old hoarder has it advantages. Jeff
All of the numbers that I have mention are stamped on the top of the dome of the finished piston. They are above the size and date stamping and are the same size and style stamp. I have had a few others respond to this same question on other forums and they are saying that the 41-243 is the casting number and that the 41-242 is the part number. Going through many old catalogs, I have found BSA B40 STD 8.7 to 1 piston listed as either 41-243 or 41-242. I wish that I could find some publication from the era that could set this straight. Jeff
Trouble with these unwritten rules is that there is always an exception, the only thing you can say is that the cast in number, the stamped number and the number on the box should be close to being in a sequence.
When you spend the $$ on any NOS +50 year parts, it can be a gamble. But when the box has one number on it and the item has another number on. Then you need to take a long look at the part. This is why I learn a long time ago to trust my micrometers. Unfortunately there is probably not a lot of the old factory crew around from when these where new anymore. Jeff