I'm working on my first build. Recently picked up a new GR Douglas barrel in 45 cal. Has a breech plug that is not fully seated. I ordered a different plug to install. The depth of the shoulder in the barrel is .394. The thread length on the new plug is .625. My question is this, is .394 deep enough for the plug to seal and be safe? I don't own a lathe or have ready access to one so any work will be by hand. I think it wold be easier to shorten the plug threads.
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Breech Plug Install
- Thread starter wapiti22
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R.H. McCrory's book The Modern Kentucky Rifle page 8....(45 cal) "The breech end was then counterbored to 1/2" deep". You are too shallow for my thinking. You do not need a lathe, just a drill bit and a tap. Too bad it is so close. Maybe someone else will think it is OK, but I would want deeper. PM Zonie, he will know for sure!
Good Luck
Flintlocklar
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I would cut off the .394" and start over. You will have a section of messed up thread if you try to deepen it. You can not just ram a twist drill in then and then tap it. Maybe a piloted drill but that is really specialized. Sorry, but you really need a lathe to do it right. This might be a good thing to farm out.
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No need to cut it back.
A COMPETENT machinist/Smith can bore the minor diameter forward about .150-.200, pick up the threads and extend them forward.
Hardest part of the operation is setting up and dialing in the barrel.
Well worth the money spent.
A COMPETENT machinist/Smith can bore the minor diameter forward about .150-.200, pick up the threads and extend them forward.
Hardest part of the operation is setting up and dialing in the barrel.
Well worth the money spent.
Needs to go back in a lathe and run a boring bar to proper depth and bottom tap it.
Depends on how it was done.
I cut a thread diameter relief where the shoulder will land. The threads are then bored up to the shoulder. Then the threads are finalized with a bottoming tap. By adding the relief cut, the shoulder will be free of burrs in the shoulder. It is not possible to thread up to the end of the hole. So, a relief cut or removing threads from the end of the plug is necessary.
If a relief cut was done then there will be no threads there. So you'd have to deepen it bit more to get the same number of working threads.
0.394" is nothing to the length of the barrel. Starting fresh will avoid head scratching and graduate level machining that costs if you have to hire it out.
As to 0.394 being sufficient threads it depends on how well the jobs was done. You will not have 0.394 of perfect threads. Of you had 3/8" that would give only six threads @ 18 TPI. Seems sketchy to me.
I cut a thread diameter relief where the shoulder will land. The threads are then bored up to the shoulder. Then the threads are finalized with a bottoming tap. By adding the relief cut, the shoulder will be free of burrs in the shoulder. It is not possible to thread up to the end of the hole. So, a relief cut or removing threads from the end of the plug is necessary.
If a relief cut was done then there will be no threads there. So you'd have to deepen it bit more to get the same number of working threads.
0.394" is nothing to the length of the barrel. Starting fresh will avoid head scratching and graduate level machining that costs if you have to hire it out.
As to 0.394 being sufficient threads it depends on how well the jobs was done. You will not have 0.394 of perfect threads. Of you had 3/8" that would give only six threads @ 18 TPI. Seems sketchy to me.
Wapiti22
You didn't say what the thread size was in your barrel but the roughly .400 depth sounds like it is on the shallow side to me.
As a ballpark estimate, the minimum length of the threads in the barrel for smaller threads should be about 1/8" shorter than the diameter of the thread. The maximum depth needed to get the full strength of the thread would be equal to the diameter of the thread.
For a 9/16" thread breech plug, that makes the depth of the needed hole, 7/16 - 9/16" (.44 - .562) deep.
For a 5/8" thread breech plug, that makes the depth of the needed hole, 1/2 - 5/8" (.50-.625) deep.
For larger threads the minimum length can be about 3/16" less than the thread size.
For a 3/4" thread breech plug, that makes the depth of the needed hole, 9/16 - 3/4" (.562-.750) deep.
For a 7/8" thread breech plug, 11/16 - 7/8" (.688-.875).
I know many of the 3/4" and 7/8" breech plugs come with thread lengths as short as 1/2" and I have never heard of any problems with them but I think the reason for this is because the thread size for the large bore guns these are used in is a sever case of over kill. Large bore guns usually have lower breech pressures than the small bore guns, even with some pretty large powder loads.
You didn't say what the thread size was in your barrel but the roughly .400 depth sounds like it is on the shallow side to me.
As a ballpark estimate, the minimum length of the threads in the barrel for smaller threads should be about 1/8" shorter than the diameter of the thread. The maximum depth needed to get the full strength of the thread would be equal to the diameter of the thread.
For a 9/16" thread breech plug, that makes the depth of the needed hole, 7/16 - 9/16" (.44 - .562) deep.
For a 5/8" thread breech plug, that makes the depth of the needed hole, 1/2 - 5/8" (.50-.625) deep.
For larger threads the minimum length can be about 3/16" less than the thread size.
For a 3/4" thread breech plug, that makes the depth of the needed hole, 9/16 - 3/4" (.562-.750) deep.
For a 7/8" thread breech plug, 11/16 - 7/8" (.688-.875).
I know many of the 3/4" and 7/8" breech plugs come with thread lengths as short as 1/2" and I have never heard of any problems with them but I think the reason for this is because the thread size for the large bore guns these are used in is a sever case of over kill. Large bore guns usually have lower breech pressures than the small bore guns, even with some pretty large powder loads.
Thanks Zonie. Threads are 5/8-18. I'm going to have it cut and re-threaded.
My engineer nephew and his engineer son says anything over 5 threads is for over safety for lawyers.
Sidney Smith
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This thread had me thinking because when I did the breech plug for my .32 caliber, I had to file off some of the rear of the barrel and cut off some of the plug in order to get it to mate up with the flat I had wanted. It too was a 5/8X18 plug, and after getting it seated and tight to the shoulder of the bore, it measures .420, or about 3/32 less than half an inch. IIRC it is right around 6 threads deep. Is this sufficient for a .32 caliber rifle?
Plenty
Col. Batguano
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Aluminum scuba tanks with about a 1" neck hole are coarse threaded with 10 threads (3000-3300 psi working pressure). In order to pass annual visual inspection they need to have 8 full uninterrupted threads on both the cylinder, and the valve (made of plated brass). My filter cap on my high pressure filter holder (5000 psi working pressure) is about 3" diameter, is aluminum, threaded very coarsely, and also has 10 threads. Aluminum isn't as tough as steel to resist pressure.
Firearms work at higher pressures than this (for MUCH shorter durations), but it ought to give you something of a guide to go on.
But as always, consult a real expert in the matter, rather than just a bunch of internet guys that may or may not know what they're talking about.
Firearms work at higher pressures than this (for MUCH shorter durations), but it ought to give you something of a guide to go on.
But as always, consult a real expert in the matter, rather than just a bunch of internet guys that may or may not know what they're talking about.
There is a lot of information on what length of threads give what percentage of maximum strength. However a length of 1/2” is very much on the long side on original barrels with non-patent breeches.
3/8” long breechplugs are fairly common.
I’m not about to start shortening breechplugs on my personal builds to make a statement but I do wonder how all those barrels survived, and for many of them, passed proof testing.
3/8” long breechplugs are fairly common.
I’m not about to start shortening breechplugs on my personal builds to make a statement but I do wonder how all those barrels survived, and for many of them, passed proof testing.
Col. Batguano
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Easy Rich. They didn't have lawyers back then telling them they couldn't!
Fitting a plug was a chore since getting flats to align was tough and to seal the plug at the same time would make a sane person spin. I used brass disks that I filed to seal as I tightened the plug to align to flats. Gun makers leave threads long and crush fit with a million pounds of torque. If you ever want to remove a TC plug with the tool you can buy, you will twist the barrel for a new twist rate. You can break your vise or destroy the barrel. Once I get a breech plug fit I never, ever remove it. You can file the plug to fit as you turn in the plug but you can't see in there and spotting dye can help. You will be fitting what you can see and another out of sight.
