Bottoming tap breechplug--doesn't bottom

[ShutEyeHunter]

Breaching a 5/8 breechplug.  Threads in the GM barrel are about 1 1/2 threads short of the step the plug seals on.

Made a bottoming tap by grinding a plug tap back to full threads, but can't get the threads cut to the bottom. At least not with the limited force I want to put on the tap.

Thought about buying a new bottoming tap, but looked at the dimensions and it doesnt look like it will bottom within about 3/4 of a thread.

Didn't want to use undue force on the breechplug cause I figured that I'd just gall the threads.

Easy solution,would seem to be--rebate the plug threads  to the minor diameter by 1 1/2 threads.

Q1-anything wrong with the rebate?  Got plenty of thread engagement if I do that.

Q2-how do the experts tap to the bottom of the hole?  Is,there a secret science to grinding my own bottoming tap?

Thanks!

[shortbarrel]

I would just rebate the plug to it's root diameter, file it or machine it on a lathe.  The hard part comes when you start to fit the nose end and rear end of the plug to bear evenly, plus bringing the plug up to a flat on the barrel. I use prussian blue to spot in the two shoulders.  Takes time to do it. From reading your post, I think you are certainly up to the task at hand.

[ShutEyeHunter]

Thanks! I didn't want to blow myself up if the rebate wasn't a good idea.

Got Prussian Blue. I think one tube of the stuff will last a,couple of lifetimes.

Have a grade 8 SAE nut that I use as a thread chaser for such things.  Forms threads but doesn't cut like a die will.  Think I'll thread it on the plug before I file anything.  Will clean up the,threads and might be useful as a depth gauge.

Thanks again!

[Acer Saccharum]

You can taper the nose threads of the plug to match the nose taper of the tap.

I hate trying to tap all the way down to complete perfect threads at the bottom of the hole, because you always lose one of those wee partial teeth at the tap's end.

[Mark Elliott]

I never could get the threads all the way down either, but close enough to worry the plug in tight.  Has anyone gotten full threads all the way to the shoulder?  If so,  I would also be interested in the method.     

[Stophel]

Tap it as far as the tap will go, and then grind the end of the tap down to get rid of the taper/bevel and finish tapping.   

[shortbarrel]

Mark, the only way I have gotten the tap to go to the shoulder is hone off the lead tooth some, then the second tooth takes over, which is stronger. As Acer said the lead tooth will probably break off and sometimes  it will score the barrel seal. Not a foolproof method, but seems to work for me.

[okieboy]

 Bottoming taps aren't supposed to go completely to the bottom. All taps have a chamfer on the front that goes to or below the minor diameter in order to function. The usual chamfers for taps are: taper 7-10 threads, plug (most common) 3-5 threads, modified bottoming 2-1/2-4 threads, semibottoming 2-2-1/2 threads, and bottoming 1-2 threads. Please note that each of these different chamfers is at a different angle by definition (chamfers do not have to be at 45 degree angles). One should also note that as the chamfer gets shorter the torque required to advance the tool goes up, and as torque goes up, tool life goes down and the likelihood of breaking the tap goes up.
 To get the breechplug to seal in a tapped hole would generally mean chamfering the end of the plug to or below the minor diameter of the thread, but larger than the diameter of the bore.
 By threading with an internal threading bar in a CNC machine one can get close to the bottom, but the normal way in a machine shop to get threads "completely to the bottom" is to cut a relief groove at the bottom of the counterbore.  For the home hobbyist, this would mean owning a lathe of sufficient size, internal groove tools and being fairly skilled at lathe operations.

[Bob Roller]

Bottoming taps aren't supposed to go completely to the bottom. All taps have a chamfer on the front that goes to or below the minor diameter in order to function. The usual chamfers for taps are: taper 7-10 threads, plug (most common) 3-5 threads, modified bottoming 2-1/2-4 threads, semibottoming 2-2-1/2 threads, and bottoming 1-2 threads. Please note that each of these different chamfers is at a different angle by definition (chamfers do not have to be at 45 degree angles). One should also note that as the chamfer gets shorter the torque required to advance the tool goes up, and as torque goes up, tool life goes down and the likelihood of breaking the tap goes up.
 To get the breechplug to seal in a tapped hole would generally mean chamfering the end of the plug to or below the minor diameter of the thread, but larger than the diameter of the bore.
 By threading with an internal threading bar in a CNC machine one can get close to the bottom, but the normal way in a machine shop to get threads "completely to the bottom" is to cut a relief groove at the bottom of the counterbore.  For the home hobbyist, this would mean owning a lathe of sufficient size, internal groove tools and being fairly skilled at lathe operations.

This is the way Bill Large did it I did it when I helped him.

Bob Roller

[James Wilson Everett]

Guys,

Old time gunsmiths had no problem here, they were able to form threads to almost the very bottom.  The tool swaged the threads rather than cut them.  As can be seen in this photo of an original breech plug bottom tap, the thread would be formed almost to the very bottom.  This did not cut the threads, they were swaged or pressed without the resulting cutting chips.  Of course, they were working with the original barrel material, wrought iron, which is certainly a lot more ductile than the modern steel used today.

Jim

[Long John]

I do what shortbarrel suggests.  I use a standard bottoming tap to get the threads as far down as I can.  Then I file the first full thread on the plug down to the groove (root) diameter.  I coat the breech face with soot and turn it in.  From that point on its a simple matter of filing either the breech plug face down or the barrel breech down a few thousandths at a time until I get the flats to line up.   If you are careful you can do it all with hand tools.  Remember that removing metal equivalent to 1/8 the pitch of the thread moves the plug in one flat.    With 60 degree threads and a Class A fit 5 full threads are as strong as the round rod used to make the male threaded component.

Best Regards,

John Cholin

[jerrywh]

I use three taps. 1 regular ,1 bottom tap and then I grind one with no taper at all. The last one has full threads all the way to the end. Then you have to relieve the treads on the shoulder of the plug also.

[shortbarrel]

Good stuff here, lots of opinions. I have rebuilt lots of old wrought iron barrels over the years and never had one that was sealed at the front. This should tell us something. Next rifle I build will be breached this way.                 

[gunmaker]

In McCory's  little book on gunmaking he shows a tap with the bottom 3 thds ground down so it fits into the bore, allowing the tap to cut close to the bottom of the hole....

[Dphariss]

Breaching a 5/8 breechplug.  Threads in the GM barrel are about 1 1/2 threads short of the step the plug seals on.

Made a bottoming tap by grinding a plug tap back to full threads, but can't get the threads cut to the bottom. At least not with the limited force I want to put on the tap.

Thought about buying a new bottoming tap, but looked at the dimensions and it doesnt look like it will bottom within about 3/4 of a thread.

Didn't want to use undue force on the breechplug cause I figured that I'd just gall the threads.

Easy solution,would seem to be--rebate the plug threads  to the minor diameter by 1 1/2 threads.

Q1-anything wrong with the rebate?  Got plenty of thread engagement if I do that.

Q2-how do the experts tap to the bottom of the hole?  Is,there a secret science to grinding my own bottoming tap?

Thanks!

Be careful running the tap all the way to the bottom. If you run it into the shoulder at the end of the hole this will have to be repaired or the breech face will not seal the bore.
Once you are close its better to file the thread back a little on the breech plug.
But ONLY the thread. Don't do this.
This slop job allowed the "shank" to enter the bore and created a really effective fouling trap.

Dan



OOPPS forgot photo.

[Dphariss]

Good stuff here, lots of opinions. I have rebuilt lots of old wrought iron barrels over the years and never had one that was sealed at the front. This should tell us something. Next rifle I build will be breached this way.                 

But back in the day the breech was usually tapped with a tap made in the screw plate that made the breechplug. So the threads were a very tight fit.  Some were even tapered from photos I have seen. With modern tolerance not sealing the bore will result in fouling back 3-4 threads of the plug. This from pulling actual breeches. I prefer a seal.
Having these "leaks" also makes a great oil trap until filled with fouling.
While with real BP this may not produce any significant corrosion  we do have to consider much more corrosive substitute powders which have been known to eat "crawdad holes" parts  and people DO use it in flintlocks with a BP booster.
Due to this and various other considerations I try to build to a higher standard and not take as many shortcuts as possible as was done by a barrel maker in the breech shown the photo in my previous post.
Dan

[Acer Saccharum]

......we do have to consider much more corrosive substitute powders which have been known to eat "crawdad holes" parts  and people DO use it in flintlocks with a BP booster.

Dan, your concerns are spot-on. This is personal safety we are talking about. Yours or someone else's. It's serious.

I like your admonition to take the breeching to a higher standard. Most excellent advice.

Tom

[Dphariss]

When I think about breeching I often remember the short article sent to the old Buckskin Report by a doctor. He had found the account in an old medical journal.
Seems a man came to a doctor complaining of chronic headaches.
On examination he found an anomaly at the right eye. Further investigation revealed a piece of iron hidden in the eye socket on the left side of the right eye. It turns out that some years before the mans rifle had burst, the breech plug had entered the corner of his eye and lodged there. IIRC The doctor was able to remove it and the patient made a full recovery.
I take considerable care in fitting breeches, vent liners and nipple seats.

Dan

[Acer Saccharum]

I have examined some antique breechings where the plug actually rattles around in the barrel. Easy cleaning, I suppose. Not so much fun to get the plug in your eye, tho.

Same thing with drums, ugh. They loosen up from frequent pounding of the hammer, shooters taking them out to clear mis-fires, for cleaning, etc. Did I tell you I hate drums? I won't stand to the right of a gun with a drum. I don't want an iron drum in my ear.

[shortbarrel]

Safety is the main thing in breaching a barrel. I thread the barrel with high quality taps then grind a threading tool to exactly match the thread form or V. on the taps. The breach plug is then threaded on a lathe for 100% thread and the barrel is drilled to tap 100% threads. Screw the plug in the barrel one or two turns if it will go, if not, take a little deeper cut on the breach plug threads.Then you will have to run the breach plug in and out of the  the barrel until it bottoms on the barrel shoulder. I use a mixture of STP and graphite on the threads.
See no safety factor in this method, even if the front of the plug is not sealed and fowling on the front of the threads is  minimum. BTW, I don't use bought barrels and breech plug, I make them.

[Scota4570]

The only way I can see to get threads that do not interfere with the plug nose is to put a relief cut at the bottom of the barrel thread.  The thread chips need somewhere fall off.  One could do it in a lathe.  Could lathe bore the threads too.  It seems like a lot of trouble for little gain.  This would be required to have a true 90-degree face on the end of the plug. 

If one were to use a big drill then a tap, the end of the hole would be 60-degrees.  I can not see anything wrong with that as long as the plug was also 60-degrees.  The it would be  spotted to 100% contact on the shoulder.  The conical part of the end of the plug would be filed perpendicular to the bore and bore diameter.  The inside of the barrel would end up the same flat end contour.  This way the tap be able to get a hair further in.   

Seems like a lot of head scratching over a simple job though.  I just run a tap with no taper until it stops.  I then spot the plug threads until I get 100% contact on the end of the plug.  The last one I did had an impression of the rifling in the Dye-chem blue.  That seems adequate to me. 

[Acer Saccharum]

I can't agree with cutting a groove at the bottom of the hole. This creates a stress point, a natural place for the barrel to fail.

Taper the nose of the plug to match the taper of the tap. Do a little hand fitting, and you're done.

[Dphariss]

I can't agree with cutting a groove at the bottom of the hole. This creates a stress point, a natural place for the barrel to fail.

Taper the nose of the plug to match the taper of the tap. Do a little hand fitting, and you're done.

Right.
If the cut is made at the front of the plug recess were a little too deep it will weaken the breech. Its easier to do a little file work on the plug.
This is a critical safety issue and people treat it  like it were some HS shop project and many of the breechings are about that quality.
Dan

[ShutEyeHunter]

Considerable wisdom here! 

I can't agree with cutting a groove at the bottom of the hole. This creates a stress point, a natural place for the barrel to fail.

Taper the nose of the plug to match the taper of the tap. Do a little hand fitting, and you're done.

Right.
If the cut is made at the front of the plug recess were a little too deep it will weaken the breech. Its easier to do a little file work on the plug.
This is a critical safety issue and people treat it  like it were some HS shop project and many of the breechings are about that quality.
Dan

Thanks to all!

I ended up with something about like Dan's picture, minus the engraving...

Got a good line seal at the shoulder, so I feel pretty good about it.  Will feel even better after a Greener style proof test.

[jerrywh]

 I have taken out  a couple of breech  plugs that were two hundred years old and about a dozen that were 150 years old. None of them were sealed as some suggest today and none of them were rusted out or even near that. A breech plug is normally at least 4 times less likely to fail than the barrel. The idea that one has to be seated perfectly may be ideal but it is certainly not mandatory and history proves it. Sealed is sealed whether it be sealed with plumbers dope or whatever.  Often times these answers turn into a matter of who is the most  correct rather than what is sufficient. None or the originals I un breeched even had sealer on them.  I have never found an original gun that was seated as described. Besides a lot of them were dished out on the touch hole side with a round file.  Does anybody really believe that a breech plug is going to turn sideways and blow out.
  Go get a piece of 12L14 wit ha .200 wall thickness and try to blow it up. You will get a big surprise. I put 600 GRNs of ffg and three ounces of shot in one at least four times with no sign of failure. I seriously doubt if the so called seated ones are really sealed anyway.  Commence firing.