by Doug Coulter » Sun Oct 03, 2010 3:22 pm
I would try with taper on both, as that alone might work OK without a gasket. Kind of depends on the Al alloy and heat treat state. If I knew I'd use a washer for sure, instead of a taper in the hole, I'd step drill the hole just a little larger, and use the knife-edge that creates to get a good seal. In softer Al, that whole trick might be moot, however.
You're most likely going to wind up with 6061-T6 as it's common. Midrange properties.
Re stripping, use a thick piece of Al and deep thread depth. Finding anti-seize that won't cause other troubles might be an issue there. If low temps are expected, a little beeswax/MoS2 mix might do well. If not, I don't know what you'd use for the "fluid" part of such a mix that would have low enough vapor pressure. I would anodize it to give a hardcoat surface, which is generally a good enough anti-seize.
The gaskets that come with the flat base plugs kind of stink. They are hollow so they can be crushed against both faces (some are made of two pieces of metal spotwelded together). But it's not soft enough metal to conform well to rough surfaces -- they solve a different problem in an auto, which can tolerate leaks we'd consider huge. If you had both surfaces nicely shiny (under 25 micron surface roughness) you could probably get by with an o ring or teflon washer.
This is one of those weird things. Such a washer can tolerate a wavy surface -- it can bend and flex into the dips, but tiny scratches leak as it won't get down into them!
My first try would be to just screw in a plug, and maybe paint the outside with Glyptal (the real thing) to seal. It would be easy enough to break when replacement was needed, not require high torque, and has vapor pressure of nil once baked a bit (it cross-links and thermosets at something a bit over 100c). If that worked, I'd further file a flat or grove down the plug threads to let trapped air in them get out easier so I could get to purity quicker on pumpdowns.
Those igniters are cool, Jerry. Are they from oil burners or something like?
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.