Ian,
Normal Buna-N o-rings are only good for 230 ºF. They do make silicon o-rings, but they are pricy. Some catalogs say they are good for 500 ºF, but that must be an intermittent maximum, as most give the max as 400 ºF. The latter max is the same as Viton o-rings, and they are cheaper than silicon, but more than Buna-N. The problem with o-rings is they really do not suit the gland space right. When I looked at doing the glands on my F28, one was not enough to fill the available conical space, and two was really one too many. Plus the pair did not want to 'pack' properly when squashed. So I used gland packing cord.
You could try Teflon cord used in old style water pump and valve glands; it will take the heat of the intake gland. But Teflon cord is a bit soft, done up too tight and it would tend to just extrude into the gland or the port, or both, if it has anywhere where it can escape. If you have the original tin washers in place, that should keep it from extruding up into the gland nut. It tends to cold flow under persistent pressure, and with the thermal cycling in use; the glands would probably always need tightening up after every run. I think the reliable maximum for Teflon is 500 ºF, before it starts to break down.
However you can still get graphite packing gland cord, as used on steam valves. It is available in the US 1/8 square cross section, perhaps smaller if one were to look harder. Indeed it is available in quite a variety, but most variations are due to different lubricity and wear properties that do not apply to a static packing gland. There is even one with food-safe lubricant! The cheapest to consider are graphite impregnated Aramid, good for 600 ºF, US$6 for10'. Next is graphite impregnated graphite, good up to 800 ºF, US$17 for 10'. You may see higher temperature quoted for this packing, up to 1200 ºF, but that is temperature in steam, dry heat values are much less.
A bit smaller in cross section is PTFE cord, 3/32 diameter Goretex is US$20 for 10', and Value Seal is a bargain (sic) at US$14 for the same size, both good to 500 ºF.
But I wonder if you could not get away with some inexpensive cotton cord or manila twine? Probably the original material was just some paper or plant fiber based material. Later flange mount manifolds just use a paper base gasket on the intake ports, so how hot can it get? Worse it can do is char and need replacing with something better. Just make sure you do not use any string with any polyester in it or you will have a snotty mess to pick out of your gland threads!
-Doug
[fix corrupt text symbols. -Doug 17Feb14]
