Thanks Jamie,

Yes the lower casing is available in two sizes as its important to have a tight fit around the engine to hold the heat. The small casing is 59mm so any engine head which is 59mm or less needs this casing and any engine above 59mm needs the large casing its a simple as that

OS speeds, Vspec, Alpha, Go engines use the small casing, RB, Reedy, Novarossi need the large.