Actually the G-Mark prop screw is 3mm metric: M3x0.5 which makes it a bit smaller than the Cox 049 prop screw which is 5-40 thread-and 1/8" diameter. There are plastic bushings available from APC that can be used for bushings-but brass tube is just as effective-you may have to telescope a couple of sizes to match the prop ID with the screw OD. A 6x3 is a good match for the G-Mark-which peaks around 19, 000-so about 3000rpm less than a Cox TD 049-the APC with its relatively narrow blades is a very good match. If you haven't run it yet-my advice [and I own about 6 of them!] is to run it in gently-these are nice engines-but they take a significant amount of running in compared to a Cox. If you give it just a token Cox style run in, you will be disappointed with it in use.

ChrisM
'ffkiwi'