taping metal cap for spark plug

[singularity]

i have a threaded iron cap that i need to tap for a spark plug... but i have no idea how to do it. normally i would just drill a hole and screw the fitting into the pipe (1/4 nipples most of the time) and it would make threads on its own because the metal of the fitting was stronger than the plastic (PVC). but metal on metal i cant figure out and i don't have a set of taps... so do have no choice but to buy some or is there and easier way? any idea what the thread is on spark plugs? this cap is going on a hydrogen cannon so it cant be epoxied in place i think

[DYI]

For one thing, I would advise strongly against being near this hydrogen cannon when it explo...err "fires", and due to the way that anything that is designed to ignite hydrogen should be made to high standards, I would also advise that you buy a thread tap and make a home made sparkplug. Normal sparkplugs usually use oddball metric threads, so, unless you live in a country that uses metric, it would be a good idea to either weld in a sparkplug, or make your own using normal threads.
I was just kidding about your cannon exploding, I'm sure you have taken into account the extreme violence with which hydrogen detonates (~4000m/s) while designing this, and know what you are doing.

[frankrede]

Go to harbor freight, its a discount tool store, they have tap and die sets for real cheap.
As for a spark plug tap, I would call some local automotive repair shops or check mcmaster/.

[singularity]

yea i know hydrogen is incredibly powerful but with the remote detonator and the 1/2 black iron chamber i think ill be fine. ill have to check the local auto shop i would have never thought of that. my dad wont let me buy any of the stuff from harbor freight he says its all going to break after a few uses (which is probably true) and said he would buy me a real nice set of taps but hes been saying that for like 6 months now... if only i had anther one of those pre paid visa cards... you know if CVS stocks them?

[noname]

Use a regular old 1/4" tap with a homemade schrader spark plug. Tapping steel and iron is damn hard, try drilling the hole 1/32" bigger, it does help.

[mark.f]

If you use a large pipe size like 1/4", then tapping metal is hard. I'd go for as small a size as possible. You can tap the threads easier and there's not as much force acting on the smaller surface area.

If you could find the tap, they make 1/16" pipe nipples, (usually in brass or stainless steel). Simply run an electrode through the center with some sort of plastic spacer and a generous coating of epoxy, and you've got an OK spark plug. You can solder a small length of thick wire to the outside, (pipe), part of the plug and bend it over the center electrode, to make the spark gap.