Nitro Glow Plug thread size?

[ED66677]

I know this has nothing to do with FE but some of you guys must know that...
Any idea of what is the Nitro Glow Plug thread size?
Need to machine my own water cooled cylinder head for a .21 nitro engine, as this is not very common tools in France, I need to know exactly before I order a tap bit!
1/4-28 or 32?

[G Doggett]

Hi Ed
The standard size is 1/4 x 32.
Graham.

[ED66677]

thanks Graham!