Breach threads ?

[bob in the woods]

I have a Bresein barrel which was gifted to me some 20 years ago. It's a 1 inch octagon .54 cal
I ordered a 3/4-16  plug which seemed correct, however the threads on the plug are "sharp"  [ sorry- I'm not schooled in the correct terms to describe threads ]  while the barrel has what I would call "rounded " threads
The 3/4-16 seems correct but it is definitely a force fit, and I don't want to ruin this barrel.  Any idea what this threading might be ?

[Smoketown]

Bob,

Your local machine shop or bearing supply house can probably tell you exactly what threads you have.

Cheers,
Smoketown

[Hungry Horse]

Sounds to me like another plug, or similarly threaded bolt has been run in,and out, of the barrel enough to roll the threads in the barrel. This often happens when the builder is trying to get that last  eighth of an inch needed to square up the breechplug with the barrel. This is where overtorguing happen the most often. The plug you have may be a replacement for one that had the treads similarly compromised. The same issue can occur when tapping the breech of the barrel with a home grown bottoming tap.
 I'd have a gunsmith, or machinist, look at it. But make sure they are aware of what a properly breeched barrel entails.
 Hungry Horse

[bob in the woods]

The tops of the threads are definitely rounded off. The new breach plug has sharp threads and although it looks the same re thread pitch, etc i.e. 16 by 3/4  it is a force fit. I stopped after a couple of turns. I think I'll just cut off the end, and re drill & tap it for the new plug. 

[JCurtiss]

Instead of cutting, drilling and tapping, why not first try chasing the existing threads with the appropriate tap?

[bob in the woods]

See what happens when I'm left to my own ideas.....   Thanks . I will try that 

[Hungry Horse]

Use a bottoming tap so the threads are restored clear to the bottom.

 Hungry Horse

[FDR]

You might find this link useful if you don't already own a bottom tap.

http://www.ebay.com/bhp/3-4-16-tap

Fred

[Acer Saccharum]

I wrap some silly putty on a stick, and press it into an unknown thread I'm trying to measure. Carefully remove the stick and check the thread impression with a thread gage or bolt. This will tell you if your plug is the same pitch as the breech threads.

I use Silly Putty because it doesn't stick to anything.

[Pete G.]

I wrap some silly putty on a stick, and press it into an unknown thread I'm trying to measure. Carefully remove the stick and check the thread impression with a thread gage or bolt. This will tell you if your plug is the same pitch as the breech threads.

I use Silly Putty because it doesn't stick to anything.

.....and he has been known to be silly (sometimes). Must have ingested some at some point.

[T*O*F]

You might want to check with your local mechanic.  Both Snap On and Matco sell  taps for cleaning up bunged threads.  It doesn't cut them, it reforms the bunged up threads.

[Ky-Flinter]

You might want to check with your local mechanic.  Both Snap On and Matco sell  taps for cleaning up bunged threads.  It doesn't cut them, it reforms the bunged up threads.

That is a good idea.  Using a thread cutting tap will remove material, creating a looser fit.

-Ron

[Acer Saccharum]

See if barrel and plug are the same pitch before you run the dies or taps.

Hello?

[bob in the woods]

I have double checked the pitch on the barrel vs the plug and they are the same. I believe that the problem is that the threads on the barrel were cut on a lathe, not with a die. They are the same pitch , but the peak of the thread is rounded over i.e. more blunt , while the plug has sharp peaks on the threads. I ran some molding clay down the breach, and the results showed the rounded over threads . The grooves are the same, so not deep enough for the higher, sharper  plug threads. This concerns me since I don't think I'll get a tight fit, even if I recut it. Whoever breached this probably cut the plugs in the same manner, so it wouldn't have been a problem.

[Hungry Horse]

Short flat threads can be the result of using a drill bit that is too large, for the initial unthreaded hole. If a bottoming tap of the correct size doesn't restore the threads, you will either have to go to a larger plug, or cut off the barrel and retread.

  Hungry Horse

[Acer Saccharum]

Bob, from what you've said, it sounds like the breech threads are undersized. You can check this with a bolt. If it's a tight fit with standard hardware(which is normally undersized), then you can afford to run a tap in the breech. AFter you do that, and the plug is still too tight, then you have an oversized plug. Not much you'll be able to do with that unless you have the split adjustable die for running the plug into.

It might at that point be better to get a new plug of standard size.

[bob in the woods]

The plug is one I just purchased from TOTW.  The barrel did not come with a plug , so if I can't resolve this easily I'm probably just going to revert to my initial thought of cutting and starting from scratch. From my measurements , it does indeed look as though an oversized drill was used, resulting in the flat or rounded top threads.

[Mike_StL]

It's the thread cutting device, either through thread tool in the lathe or the tap, the is responsible for the rounded thread vroove.  Is the drill was oversize, then the breech plug would thread in easily.   Measure the diameter of the threads,  minor diameter.   That will tell you the size of the drill used. 

MostI key you will need to run the correct sized bottoming tap to get the barrel to match the plug.

Go to a machines to get this job done.

[Smoketown]

The plug is one I just purchased from TOTW.  The barrel did not come with a plug , so if I can't resolve this easily I'm probably just going to revert to my initial thought of cutting and starting from scratch. From my measurements , it does indeed look as though an oversized drill was used, resulting in the flat or rounded top threads.

Bob,

If an oversized drill bit was used, the tops would indeed be flat.

But, the plug should easily screw in ... You would just have less than the recommended 75% depth of thread engagement.

If the threads were cut on a lathe, they were probably not cut to full depth.

Run the proper sized bottoming tap into the barrel and see if it doesn't clean up the threads.

You have nothing to loose and it would cost almost nothing.

Cheers,
Smoketown

[bob in the woods]

If the bottoming tap works, i.e. cutting the threads deeper, will I still have adequate thread engagement for safety ?

[Smoketown]

If the bottoming tap works, i.e. cutting the threads deeper, will I still have adequate thread engagement for safety ?

Bob,

If the bottoming tap does cut the threads deeper, your 'flat tops' should almost disappear.

At that point you would measure the major and minor diameters of the threaded parts, do the math and then, measure the depth that the breech plug goes into the barrel.

Machinists books and charts on the web show you how and also give recommended values.

Of course, your findings could also be verified by the knowledgeable builders in this site.   

Cheers,
Smoketown

[Scota4570]

If somebody bored the threads in a lathe, he probably left them undersized.  That way you could bring them to the correct final size using a bottoming tap.  By doing it this way the chance of misaligned threads is about zero

That is how I do it. 

Boring threads to the final perfect dimensions is no easy feat. I give boring a small thread about a 9.5 out of 10 difficulty level.   You have nothing to loose by running a bottoming tap in there. 

Trying to force the threads to fit with he plug invites disaster.  Even if you go slowly, at some point the mating surfaces gall and seize. 

[bob in the woods]

Bottoming tap is what I'll do.  I'll post how it works out.

[JOHN L. HINNANT]

Greetings All,

It has been my experience that the average breech plug threads are a bit too large in diameter. Even with a quality Hawken breech plug like the R. E. Davis. It has always been necessary to "chase" the breech plug threads with a proper size die.

Take a very close look at the barrel threads. They could be slightly flat, not rounded. Flat is not necessarily a bad thing. The average threads do not have a 100% thread engagement, more like 80-85% engagement, which is normal. Internal threads are usually flat on the crown of the thread.

Many times it I have had to chase both barrel and plug threads to get a fit. That is usually the difference in the tolerances of a tap and die coming from two manufacturers or the difference in two different classes of thread fit. There ARE also four different thread classes. You might have a mismatch difference in thread class.

My respects to all,

John L. Hinnant
"God and Texas"

[bob in the woods]

Thank you for that.  I still haven't resolved this issue , after trying the bottoming tap, so this was a timely post.