metric threads

[Dan Fruth]

I pulled the locks on an English shotgun I just acquired, and saw some cracks behingd the breeches. So after much coaxing I removed the previously buggered tang bolt in order to see how far the crack goes, and the threads on the bolt are seriously scrubbed. I think when I was backing out the bolt, the wood was holding it so tightly it might have scrubbed off the threads. I assume the trigger plate is hardened....Non the less, I now need to figure out the thread of this thing...Looks like a 28 pitch thread, which is .907mm,  at least that seems very close. The mavor diameter , I'm guessing would have been about .185-.190, or about 5mm.....Anyone  ever fix something like this before....The piece dates 1800 or there abouts....Dan

[Clark Badgett]

Metric was still called the French system at the time and not very old, and if it's English it will be in English system. Nothing was really standardized anywhere at that time. I've got an 1848 Engineers and Mechanics hand book around here that list all the different European "inches" and non are the same anywhere.

[Dan Fruth]

 I think I can make a 28 pitch thread work...I'ts hard to tell exactly but it looks like 28, and if I have to I can fudge a little, I only need about .250 thread engagement.   I'm sure when I'm finished she'll be ready to go! I love fooling with these old guns.....Dan

[James Wilson Everett]

Guys,

Check out the topic "Making an Odd Sized Screw - HDTDT", this shows how to do a job like this.

Jim

http://americanlongrifles.org/forum/index.php?topic=33579.msg322062#msg322062

[J Henry]

 I found out one day that there is also a thread angle to be looked at!!something about the angle being 125 or 150  don't ask me to explain it,.need to check with a person who works with them .

[jerrywh]

 Go buy a metric thread gauge and check it out. Maybe your local machine shop can check it for you. There are two different metric types.
  This sounds like a 10/28 thread yes there are special threads
http://drillsandcutters.com/10-28-hss-tap-set/?gclid=CNHkoszbuscCFQqQaQodgd4Bog

[Pete G.]

If it is truly English, then it is probably not metric. Might be Whitworth instead, but that would be a little later than 1800 time period.

[Dan Fruth]

Well I found the right pitch thread...28 worked, and I did something that worked rather well. I triged out the depth of thread for the pitch, but I figured out the correct dia by turning the screw shaft at a slight taper, chased the threads on the lathe, and screwed it into the trigger plate until it fit correctly, and then measured the dia at that point. Ends up at >182 major dia, and 28 TPi...And best of all it works well! I think I'll draw it to a blue and then won't have to worry about any more....Dan