Alternative solution- - - -unless you're getting a noisy exhaust leak, ignore the sloppy fit. Carbon buildup will cause even a moderately close fitting shaft to lock up pretty quickly. If you absolutely must replace the shaft and flap valve, drill and tap the manifold for brass bushings made from 1/4" NPT thread fittings, and drill and line ream the holes for a stainless steel shaft and flap valve. I've reworked several heat risers that way. That's the time consuming, E$$$$PENSIVE way to do it, but it will outlive your grandkids.
Jerry
