Ever since the 60's we have used "silicone" O rings for steam engine pistons, and in Ewins type lubricators. So far so good.

Last lot I purchased from Blackgates are catalogued as "White silicone", but were actually brownish red – suggesting they are silicone rubber (not fluorosilicone).

I had reason to check material compatibility, having had some red ones fail quite quickly on boiler clack valve duties. See the compatibility chart here:

http://mykin.com/rubber-chemical-resistance-chart-6

Which indicates silicone rubber is actually not very good on steam. More worrying is that is not recommended for lubricating oil, tallow or rapeseed oil – the last two being the principal ingredients of cylinder oil.

My particular concern is that I am planning automatic lubrication (using a Ewins type lubricator) on lubricating oil and I think Viton is probably the way to go for these. Maybe red silicone for the cylinder oil, based on "that's what everybody uses"?

Is there anybody out there with more experience in this subject – chemistry is not my strong point (it doesn't even register as a weak point actually).

Thanks

Martin