What size helicoil for the exhaust manifold flange to exhaust?

What size helicoil should I use for the exhaust manifold flange to exhaust?

The double downpipe exhaust from 63-74 used 5/16 course. I would think the later cars were probably the same but can not confirm. Bob

Remove one and go to your friendly NAPA store.
They should have a KIT for drill /tap for the helicoil size you need.

as stated, 5/16 coarse. A kit is about $30-40, with drill bit and tap

you can also use a nut and bolt, put the nut on top, bolt from underneath. if you can get all-metal lock nuts, so much the better (Nylock will melt).. Not much different in terms of awkwardness. Might want to run a 5/16 or an 11/32 drill thru there first, to get rid of the threads. Make it easier to get the bolt in.

Stainless might work best, with anti-sieze