Why is my 3rd housing a 13b.
All works perfectly but always wonder. ????
FYI: I did not build the engine.

 

Because 20b and 13b parts are interchangeable. It's obvious that 20b of yours blew at some point so the builder just used a 13b housing as a replacement. It's not something to worry about. Interchangable parts are one of the biggest advantages when it comes to rebuilding rotary engines.

 

Thank you.