Breech plug thread engagement

[Underclocked]

How does one determine, for a particular rifle and breech plug, how much thread engagement there is?  How do you measure those internal threads so far as total thread diameter? Does one simply measure the outer diameter of the plug threads and compare that to the inner diameter of the barrel thread hole?

I'm really curious about this and do not know how to properly evaluate thread engagement.   It would seem to me that the two simple measurements I mentioned wouldn't tell a very complete story.  Length of the threads, depth of threads, surface contact between internal and external threads, engagement angle, and yield strength of both barrel and plug materials would need to be considered in any evaluation, would they not? 

Have any of you actually done any measuring of this nature and recorded the results? What measuring tools are required?

I know I've observed apparent deviations in plugs made by more than one manufacturer with regard to fit and wonder how one might determine what is acceptable vs what is dangerous.

ps:  I like to use Teflon tape with just wee dab of Slick50 ONE grease over the tape on the forward threads. Amsoil food grade grease is also excellent but much more expensive. 

[Swampman]

When I worked in quality assurance at a Jet Engine repair facility, we would have used wax impressions.  An optical comparator or dial calipers + a little math will provide the info you need.  I'm not sure what is an acceptable fit.

[mirage1988]

You could start with putting steel ink (magic marker in a pinch) on the threads and installing the plug. You should be able to tell how much thread contact you have when you remove the plug. I think the area that would be the most critical for a good seal is the front (cone) so mark that too and torque the plug like you normally do.
I think people misunderstand the way a breechplug seals- they are straight threads, not tapered like pipe threads. The plug seals when the cone bottoms out on the shoulder in the barrel and when the back side of the thread on the plug meet the front side of the threads in the barrel. Teflon tape or grease do little or nothing for the seal-they just keep the plug from seizing.

[Semisane]

Teflon tape or grease do little or nothing for the seal-they just keep the plug from seizing.

I've not found that to be the case with my Lyman Mustang mirage.  I agree with respect to the grease.  It does little to create a seal and gets blown back into the breech area a little.  But Teflon tape does indeed create a complete seal in my gun - no blow back at all through the threads.  I suspect different breech plug designs may behave differently.

[Underclocked]

Actually grease can assist a seal, depending upon the grease.  Anyone that has been at test depth on a submarine can probably attest to that.