Spetz,
You have us bouncing around from one thread to another
if your 2 threads are for the same thing (which it does looks like it) here's some info.
3.81mm (3.8) = .150" and
4.75mm (4.8) = .187".
now, if you are looking for a flex cable with a prop shaft to fit on your Blue Streak 800 (in both thread/both questions) and you want to use an engine that is smaller than a .21 (like a .18) I recommend that you use a 3.8mm (.150" ) flex shaft with a 4.8mm (.187" ) prop shaft also called stub shaft.
any flex shaft bigger than 3.8mm (.150" ) is to big for a .18, and the reason you should use a 4.8mm (.187" ) prop/stub shaft is it will give you a much wider range of prop to choose from.
Can you cut the flex shaft? YES.
Can you cut the prop/stub Shaft? Only the threaded portion, the prop/stub shaft is not hollow (only a portion to allow the welding of the flexy) so you cannot cut the other end (where the flexy gets welded in).
Just incase you were wondering if you can drill the center of the prop/stub shaft.....Nope, The prop/stub shaft is tampered steel.
Dan.