Thanks Paul. Was thinking mostly of the inside cylinder version. I just know that many people would be happy with a steam loco that looks more or less right, given that no workaday RTR Irish steam loco seems on the near horizon ! I wasn’t even considering 21mm. If my experience in 7mm is anything to go by, when modelling 5’3 steam to scale gauge, you really need to build the whole thing as such from the get-go, such are the clearances involved. I retro-fitted a loco from 32 to 36.75 once and wouldn’t do it again .