I just bought another barrel, its 15/16 x 42.Its a new barrel old stock numrich. It has 5/8 by 18threads cut in breech, so I bought aplug,5/8 by 18 and it seems alittle loose. Ive built a few guns and the plugs all felt snug, this one wiggles around a bit. Do you think a local machinest could thread this,I dont want to have to ship this out again. I use taps and dies at work alot, but I heard somewhere here that it should be machined.
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breech plug issue again
- Thread starter billymas
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Standard threads are usually machined to a Class 2 tolerance.
A 5/8-18 UNF class 2 thread can have over .010 clearance on the major diameter, .0162 clearance on the minor diameter and .0074 clearance on the pitch diameter.
Although these are the extreme limits they can occur and if they do the fit will feel rather sloppy.
When the thread is tightened the male thread will centralize itself with the female thread because of the 60 degree V thread form and once tightened it will be just as strong as any other 5/8-18UNF thread.
The clearances I mentioned are common with straight threads of this size and it is the reason most builders will tell folks that their breech plug should be seated against the shoulder where the threads end and the bore begins.
It is this seal at the shoulder that prevents leakage of powder gasses thru the threads when the gun is fired.
In answering your question about having a local machinist do something about this, from the sounds of things your threads are already machined at their minimum material condition. Further machining would only make it worse unless you are thinking of having the breeches threads cut off and the breech totally rethreaded.
Even if you do this there is a chance that your new threads will also be at the extreme limits of tolerance and you will be right back in the same boat.
I would work on fitting the breech plug so it is tight against the shoulder at the end of the bore when the breech plug is torqued down to 30 ft/lbs.
A 5/8-18 UNF class 2 thread can have over .010 clearance on the major diameter, .0162 clearance on the minor diameter and .0074 clearance on the pitch diameter.
Although these are the extreme limits they can occur and if they do the fit will feel rather sloppy.
When the thread is tightened the male thread will centralize itself with the female thread because of the 60 degree V thread form and once tightened it will be just as strong as any other 5/8-18UNF thread.
The clearances I mentioned are common with straight threads of this size and it is the reason most builders will tell folks that their breech plug should be seated against the shoulder where the threads end and the bore begins.
It is this seal at the shoulder that prevents leakage of powder gasses thru the threads when the gun is fired.
In answering your question about having a local machinist do something about this, from the sounds of things your threads are already machined at their minimum material condition. Further machining would only make it worse unless you are thinking of having the breeches threads cut off and the breech totally rethreaded.
Even if you do this there is a chance that your new threads will also be at the extreme limits of tolerance and you will be right back in the same boat.
I would work on fitting the breech plug so it is tight against the shoulder at the end of the bore when the breech plug is torqued down to 30 ft/lbs.
matt denison
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Zonie.
As usual you have come through with real answers in quantifiable numbers.
You mention torquing the plug to (only?) 30 f/p. For some reason that sounds very light. Not argueing. Where did/does that number come from.
I don't know the exact f/p's I am using to install breech plugs but am sure it is more than that. Could I be doing something wrong? Maybe I should rethink the effort I put into tightening plugs. ??
And thanks for the info on thread tolerances, this is the kind of information I'd like to know but don't have easy access to.
As usual you have come through with real answers in quantifiable numbers.
You mention torquing the plug to (only?) 30 f/p. For some reason that sounds very light. Not argueing. Where did/does that number come from.
I don't know the exact f/p's I am using to install breech plugs but am sure it is more than that. Could I be doing something wrong? Maybe I should rethink the effort I put into tightening plugs. ??
And thanks for the info on thread tolerances, this is the kind of information I'd like to know but don't have easy access to.
A 5/8-18UNF thread when tightened to 82 ft/lbs of torque will develop a clamping force of roughly 10,550 pounds.
While this sounds really neat, for a breech plug IMO it isn't.
The breech plug's job isn't to clamp a static load. Its job is to take the force of a sudden high load when the gun fires so it needs to keep most of its strength in reserve for those occasions.
Although I give a higher value in this link where I discuss this issue, if you read it you will notice that the values I recommend are maximum torques. Lower torques reserve more of the threads strength. http://www.muzzleloadingforum.com/...p?tid/253924/post/948621/fromsearch/1/#948621
The maximum torque values I recommended in this link are based on saving enough thread strength to withstand a breech pressure of 15,000 psi as I recall but don't quote me.
IMO, 30 ft/lbs for a 5/8-18UNF thread is sufficient to prevent loosening of the plug from normal handling. After all, 30 Ft/lbs on a 5/8-18UNF fastener will produce a clamping load of 4,800 pounds and it is high enough that the plug cannot be unscrewed with ones fingers. :rotf:
While this sounds really neat, for a breech plug IMO it isn't.
The breech plug's job isn't to clamp a static load. Its job is to take the force of a sudden high load when the gun fires so it needs to keep most of its strength in reserve for those occasions.
Although I give a higher value in this link where I discuss this issue, if you read it you will notice that the values I recommend are maximum torques. Lower torques reserve more of the threads strength. http://www.muzzleloadingforum.com/...p?tid/253924/post/948621/fromsearch/1/#948621
The maximum torque values I recommended in this link are based on saving enough thread strength to withstand a breech pressure of 15,000 psi as I recall but don't quote me.
IMO, 30 ft/lbs for a 5/8-18UNF thread is sufficient to prevent loosening of the plug from normal handling. After all, 30 Ft/lbs on a 5/8-18UNF fastener will produce a clamping load of 4,800 pounds and it is high enough that the plug cannot be unscrewed with ones fingers. :rotf:
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Thanks Zonie for your helpful information. It felt a little sloppier than other barrels I breeched,but I get what your talking about, its kind of like what they call a crushed fit, where the threads move a bit into position. I did put teflon tape on just to try and it did tighten and left a good imprint with no slop. So Ill give it a try, and yes I was going to cut first then machine. Thanks again
ohio ramrod
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Before I retired from tool and die making I would custom thread all of my breech plugs with oversized threads to give a slight interference fit. And heat the barrel to insert the plug. But not having access to the precision machines I now just use a good coating of antisize to fill gaps in the threads an seal against the shoulder. :hmm:
Folks who are really, really concerned and want the best standard fit available should look for taps and dies that are Class 3.
They are harder to find and often cost more than the common Class 2 items but the Class 3 threads pitch diameter clearance between the internal and external threads can be as small as .0000 clearance (.0080 maximum clearance for a 5/8-18UNF-3 thread).
The major and minor diameters still have quite a bit of clearance but that's the nature of straight machined threads.
Then for the really paranoid person who is concerned with high stresses in the threads they can look for taps and dies that are made to produce the UNJ threads.
The male thread has a controlled root radius rather than the uncontrolled root radius that can be as small as .000 in a regular male thread.
These threads are used on aircraft engines and parts where a fasteners failure can cost lives.
They require a special tap for the female threads which cuts a larger minor diameter to clear the male fasteners fillet radius.
For what it's worth, the UNJ threads are a reality but they are not needed for the breech plugs in a muzzleloading firearm. The Class 2 threads have more than enough strength. :thumbsup:
They are harder to find and often cost more than the common Class 2 items but the Class 3 threads pitch diameter clearance between the internal and external threads can be as small as .0000 clearance (.0080 maximum clearance for a 5/8-18UNF-3 thread).
The major and minor diameters still have quite a bit of clearance but that's the nature of straight machined threads.
Then for the really paranoid person who is concerned with high stresses in the threads they can look for taps and dies that are made to produce the UNJ threads.
The male thread has a controlled root radius rather than the uncontrolled root radius that can be as small as .000 in a regular male thread.
These threads are used on aircraft engines and parts where a fasteners failure can cost lives.
They require a special tap for the female threads which cuts a larger minor diameter to clear the male fasteners fillet radius.
For what it's worth, the UNJ threads are a reality but they are not needed for the breech plugs in a muzzleloading firearm. The Class 2 threads have more than enough strength. :thumbsup:
matt denison
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Thanks for the enlightenment. In other words, "tight is tight" and that is enough.
Yup.
I should add one thing to my comments.
There is such a thing as too loose when speaking of a breech plug.
Finger tight is not good. With the fairly large threads used on breech plugs even pliers tight isn't good enough.
Either of these will not cause the threads to become "loaded" against their mating thread.
If the plug is just lightly torqued in place when the gun is fired the unloaded threads will suddenly be impacted by their mating thread and this can overload all of the threads. Impact is only a good thing when one is pounding something with a hammer.
When installed using moderate torque, each thread becomes loaded against its mating thread so it is already carrying some load. Then the additional load from firing the gun will be absorbed by all of the threads working together.
I should add one thing to my comments.
There is such a thing as too loose when speaking of a breech plug.
Finger tight is not good. With the fairly large threads used on breech plugs even pliers tight isn't good enough.
Either of these will not cause the threads to become "loaded" against their mating thread.
If the plug is just lightly torqued in place when the gun is fired the unloaded threads will suddenly be impacted by their mating thread and this can overload all of the threads. Impact is only a good thing when one is pounding something with a hammer.
When installed using moderate torque, each thread becomes loaded against its mating thread so it is already carrying some load. Then the additional load from firing the gun will be absorbed by all of the threads working together.
