Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

This is the Teflon gasket material I use.  It's soft and seals easily.  It's thicker than the paper gaskets or the hard Teflon gaskets so two of them with the sight glass will put the boiler nut flush with the edge of the threads but that's never been an issue.

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Only downside with this stuff is that it's difficult to cut with a knife so you really should punch it out. I had to make a larger punch for this set for the boiler glass.