5/16" is .312" OD.... so that can't be it. And .395" is logical for the largest diameter of a 1/8" NPT. OP, just pout various bolts in 'til yo find what fits.
Also look inside and see if the new threading pattern is different from the old one. The 1/8" NPT thread pitch is quite a bit finer than any standard size bolt thread up near the size we are talking about.
