Howdy all, does anyone have an aftermarket number and make for the air filter on a R51/3
Peter, I'm not quite sure what you're asking for. The air cleaners for the "/3" bikes don't have replaceable pleated paper elements, but have a sort of wire mesh element that is supposed to be soaked with motor oil.
The earliest type, which I've never seen in person, has several openings around the circumferance of the unit, and they can all be closed to choke it. The type that came with my 1953 R51/3 is a clamshell affair, with a separate element that goes in between the clamshells. There's an air openning in bottom rear, and a choke lever there as well.
I believe the paper element cleaners didn't come in until the R50/R60/R69 bikes after 1955.