BTW; Rusty, what diameter barrels on your TB?  Mine are 44 mm, which could be great for automotive street or racing), where idle transition and throttle response are more important.  For aircraft I do believe that a bit larger would be better. 

Hi Al,

I've measured mine a number of times, and I recall that they're essentially 1-5/8, which of course works out to about 41mm.  The TWM site only lists 40 and 42 for the 2900 series, so I'm not sure exactly which one I have.  I got it from Atkins originally, with the wrap over manifold, so I got whatever he was sending out.  

Rusty