I've actually threaded the holes in the manifold and installed screw-in brass bushings, and then reamed the bushings for a stainless shaft and flap valve. That's a little bit of overkill, but it works. Just don't get the clearance too tight - - - -it needs to be a little sloppy.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!