This is the kind of coupler I am using on the single rotor. I never saw a problem with it so far. Since this coupler came with the redrive, I do not have an independent source for it.
One thing I like to point out for using this type of coupler, it is very important for the input and output forkes to be very precisely centered with each other. Insufficient centering will lead to immediate destruction at elevated power levels. I have seen that on automotive engine dynos quite a bit.
FWIW

Richard Sohn
N2071U
www.fairpoint.net/~res12/home.html

What idle speed do you use Richard?

Paul Lamar

Richard, How much does your 1-rotor engine weigh?

Harold Lunt