Our TB is not 66mm stock. My understanding is that it's right around 65, but tapers down to about 63. This isn't a mod done for dyno gains, people do it for throttle response. The FC piece apparently has the linkage set at a different angle, which opens the throttle proportionately wider for less accelerator. Meaning, if you floor it, you're not going to notice much difference - but part throttle acceleration feels peppier. Normally I wouldn't drop money on this mod, but the group buy price is low enough to make it worth my while. Various people port the TB by just grinding it out - which works, but the aftermarket piece also does the little trick of adjusting the attachment angle of the linkage. FWIW, I think it's worth doing.
