CO2 front exhaust plug size

I recently had my bike at a dealer who checked the CO2 level, he had to drill out the old exhaust plug and replace it with a new one. After driving hundreds of miles around Scotland and returning home, I was changing my round cans back to the oval ones I normally run when the panniers are removed; and noticed a hole in the pipe where the plug should have been. I guess he didn't put thread lock on it, but have had to put a large bolt in the hole not threaded with a jubilee clamp to hold it in to stop the gases escaping.
My question is what thread and pitch size is this bolt as I can't find it in the torque setting of the service manual? this is an ST2 BTW.
I have one on an old set of down pipes I have but no amount of penetrating lub and an impact driver will shift it. The rear plug from the vertical cylinder is a different size as this one I could remove but it is smaller in diameter.
I'd like to fit one with a hexagonal head as I'd have more chance of removing it in future if it got stuck again.