Just offered a few bolts to mine. It looks to be 7/16" UNF............I just can't find the tap to confirm this 100%...........but I'm virtually certain of it.
7/16" UNF you'll need a bolt of correct length [I had to use an angle grinder to cut mine to length] as it is, its a swine of a job to get the bolt into bolt the inertia fixing into it's very limited space. Well it was on swmbo's car Good luck !
Arnie, defo 7/16" UNF. Shame you didn't mention earlier as I've only just re-fitted mine and could have told you the length. ISTR it took a bit of effort to tap the hole, what with powder coat and years of crud.
some people say you should take it easy and use lots of lubricant (for tapping that is), I find it's much more fun to wang it in and break the tap off. Bert