general rule of thumb "brass is fast" (theaded end of plug) silver is slow(aluminum) but, I recently got some 'stainless' end fast ones; also the tips (heat element) of fast are usually tapered, but slow are full dia to rounded tip
I suggest using a starter 'push button' in place of a relay, and an indicator light from buss bar, at least you will know when it's working 8} :?