check the helecoil and see if it has pulled out ali with it. It probably has.In which case you will need to go up a size. You could either tap the whole thing to m12 and use an m12 bolt or m12 studding and nut(Another option is to go to an af thread like 3/8 unc) or you could go for an the next size up helecoil--
M 11 x 1.5 pitch
You should be able to get a stud or at least a bolt from a good fastenings company but inquire first before you get the kit.
I did tap mine to m12 on my old et engine and it worked but if you mess it up or that thread goes you will need a new head or your head welded.
You can helecoil in situe if you use a 90 degree chuck air drill.
Oil has come out that stud hole on a few s12s that have been helecoiled badly in the past by SE nissan. Put some silicon in the hole before you wind your new stud/bolt in and it will be fine
Helecoils can be found at
http://www.toolsnextday.ltd.uk/Helic...icoil_kits.htm
If your lucky and the helecoil just wound itself out you could reinsert a new one the same size.Somehow i doubt thats happend. Have a look