I duplicate the shaft and flapper valve in stainless steel, and use a bimetal spring for a Jeep that's available on Ebay for less than half the price of an OEM stovebolt spring. The 3-bolt V8 heat riser isn't a good option, as it blocks off almost all of the exhaust flow instead of re-routing it.
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!