Radical R/C has a nice price on the TT 15. I have a couple of older Magnum GP 15s, which appear to be the identical engine to the TT GP15. They run very nicely. They have a 7/32x32 propshaft which is the same as the 15FP, beefier than the 5mm of the 15LA and Magnum XL. Bill Turner got a tap specifically so he could make prop nuts to fit the 7/32 shaft, it's an oddball size. So if you need a spinner nut, see Bill!