My 1992 certified 5" gauge Hunslet was last steamed in 2001 by the builder. I acquired the loco recently and am currently preparing it for a hydraulic test.
When the safety valves were removed, sighted within the boiler is a black crumbling substance that could be the remnants of a manifold gasket.
Sheet 5 of the drawings shows in the sectional drawing something that could be a gasket, but most of it would be exposed to steam and there would only be a small vertical surface providing the seal.
The build description by Don in my possession (from Locos Large & Small ?) in Part 6, Page 19, describes a 1/64" CAF gasket being applied to the flange joint in the conventional way on the horizontal surface.
It seems to me I need to remove the manifold to clean up any mysterious crumbling substance, but, can anyone shed some light on what this substance might be ? And what the gasket material of choice would now be ?
I had to do a search to identify that material, seems great for pipe flange sealing, but why is there a substantial bead inside the steam space ? It sounds like a fairly benign material, but I assume still better to remove manifold and clean it up.
sounds like you are removing the cover for the safety valves anyway, and to get rid of what ever that 'gunk' is, which is a good idea. Blackgates sell thin gasket material of modern composition. it is worth blueing the faces and truing up if necessary as it is quite a large diameter.
Had a discussion with my club Boiler Inspector. He suggested that the boiler should be tested as it last ran, who knows what other issues may arise. If it tests Ok, just ensure the blowdown is as thorough as possible.
A separate interesting issue has already come to light. The rear safety valve does not have a spring, it relies instead on the feathering spring to set the pressure.