/6 exhaust manifold heat control bushing

well, you could go full monte and remove entire butterfly valve, oversize drill the bores to fit a brass rod and then drill out the rod plugs to fit the valve. you would then tap these "new" brass bearings on over the installed valve. make sure they rotate freely as there is very little drag on a stock part. New manifolds are pretty cheap, I think they are under $100 on Ebay.