What size threads are on a CVA breach plug?

Answer 1

That would depend on the type of rifle and the year it was manufactured since the model and year are needed to order replacements. The new CVA QRBP (Quick Release Breech Plug) is mated to the barrel at the factory, and replacements don't just drop right in. Fitting requires a Mating Tool be used so the plug can be removed with fingers only. The factory does not advertise the diameter or threads per inch (TPI) of their plugs, perhaps to discourage hardware store replacement parts, or tapered machine taps being used by shooters who try to save a few bucks. This pre-supposes that the plugs are standard sized threads, and I don't know that to be a fact.

In either case, normal machine threads have minimal tolerance between the male-female threads to allow torquing so bolts don't loosen or vibrate out. (of a plane's wings in flight for instance) A tight tolerance is unnecessary and in fact counter productive with a breech plug. Any escaping gas would be minimal and ordinary plugs will tighten as they heat and expand rather than loosen and back out. This is caused by the blasting away of lubricants and the buildup of cap and powder crud when fired, compounded by plug expansion which further minimizes thread clearances, squeezing lube out and fouling in.

Breech plug thread sizes may actually be proprietary, and used only in gun manufacture. For instance, the thread size of traditional shotgun front bead sights are a 3-56 screw thread (tiny #3 screw with 56 TPI) and used exclusively for guns. It wouldn't surprise me to learn that the diameter, threads per inch and thread types (there are many) of rifle breech plugs are non standard to minimize the chances of dangerous mismatching of replacements that aren't properly fitted and heat treated to withstand the pressure curve when black powder is detonated and slower, progressive burning smokeless powder substitutes are ignited. (Modern powders are classified as propellants while black powder is a Class 3 explosive because it burns so fast)