I hate to continue further off-topic with the butterfly valve issue, but broken throttle shafts are a common issue for Miatas that turn high rpms on a regular basis (like mine) due to a harmonic vibration throughout the intake manifold. I had 2 broken throttle shafts in 6 months - on my third throttle body I followed some forum recommendations and epoxied the throttle shaft to the throttle butterfly, making sure to cover the screws, and haven't had a problem since. The epoxy not only secures the butterfly and screws, but the extra mass changes the harmonic frequency of the throttle shaft which prevents the failure-causing vibrations. I'm thinking that this would probably be a great preventative measure on the DOHC intakes butterflys - I'm planning to try it out the next time I do my timing belt.