It'* not a standard thread. Nor is it a tapered pipe thread. It has an o-ring to seal. Only the OEM plug will work right.
The GM part number is:
Or you can order a magnetic plug from www.canadiansupercharger.com
but it will require removal of the nosedrive, and drilling/tapping for a fit.
I installed one on Chadow427'* M90 last month during his porting/polishing/rebuilding.
I'll move your topic to the right section for you. Please keep all supercharger-related topics in Forced Induction.