Spark Plugs

Does anyone know what thread the top of an old spark plug is, i'm talking about the HT lead end. I'm after a nice knurled nut, iv found some on feepay but not sure which one i need, and cant find a die here that fit's to check. It's for a Champion 7com to go on my 1921 RH AP.
See these -

2Ba http://www.ebay.co.uk/itm/220891760209?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

And these -

8-32 UNC http://www.ebay.co.uk/itm/261011689598?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

Any ideas ?
Cheers
Luke

In the past I believe the standard thread was 5BA.
I'm not sure if this is still in use or if it was on every plug in the first place.
Hope this is of some use.
Phil

I'll check that, i'm not sure it's that small. Nearer a 3 or 2 BA ? I have a BA set.
Cheers.
Luke

The Green Sparkplug Company are listing... m4, 2BA and UNC.

http://www.gsparkplug.com/shop/