As I said I have only measured these two screws. Actually, I did not originally take either of them out, they had both been taken out by someone else before I removed them for measuring.
Actually, the true determining factor for whether a thread is straight or tapered is Pitch Diameter. On both of these two screws, I did measure the pitch diameter & they were both straight & as I recall were identical. Also, the od of them both measured straight.
I have no doubt that some of these guns were built with tapered threads, there seems to be ample evidence of this, I have no idea which type is predominant.
Either method would require precise control of the tap for proper seating when the gun came on face. The tapered method would also require precise cutting of the screw. From a machinist's viewpoint, I would have to say using the straight screw would be the easier of the two.
As Keith has said measuring the pitch diameter of an internal is pretty well universally done by plug gages. I have made no attempt to measure this, can only say that on these two guns, just going by feel the screws run with seemingly the same clearance un till the taper of the lead of the tap is encountered. Understand when using the type of tap I am referring to if it were run to where the lead exited the hole the end result would be a straight thread. The lead is to distribute the load over the first so many teeth, rather than the first tooth carrying the entire load.
A taper thread tap, on the other hand, is tapered for its full length. Pipe threads are the most commonly encountered taper threads.
Keith, I may have the name wrong but seems I recall there were some of the Crossbolt design guns sold under the name Waverly. It has always been a Wonder to me, why no one else picked up this feature after the patents expired.
