hi vint mow
as long as its all air tight it should work ok, just make Shure its not too close to muffler.
I would let the mower run for 10 minutes shut down and see watt happens. that's when your manifold will get to its highest temperature with motor stopped, as heat from the engine block and muffler creep into the manifold no longer cooled from atomised fuel.
maybe jb weld epoxy would have been a better choice as it can with stand highest of engine temperatures ,
good on you for having a go, hope alls good
