Author Topic: Threading a breechplug to fit a barrel (Read 6590 times)

rickw · « **on:** February 10, 2017, 06:43:05 AM »

I'm having a problem fitting a breechplug to a barrel. The one I have fits too loosely, and I'm not sure how to determine the proper size.
The barrel is for a smoothbore trade gun, and the bore measures .625. The outside dimensions are just over 1 inch across the flats at the breech, and it tapers to about .820 at the muzzle. It had a 3/4-16 threaded breechplug in it when I got it. I pulled it out to check the fit, and found that it was about 1/8" too short in length. It fit loosely too; I could wiggle it around while screwing it in to the breech.
I tried another 3/4-16 breechplug that was the proper length, but it fit just as loosely.
My plan was to turn and thread a piece of steel to fit the existing threads, and then weld on a tang. I've tried the turning and threading, but it usually ends up too loose also.
The inside diameter of the threads in the breech are .690. From that, I need to determine the outside diameter of the breechplug.How do I go about doing that? I have a copy of Machinery's Handbook, which gives the standard dimensions of a 3/4-16 thread, but I'm not sure how to calculate it.
Any help would be greatly appreciated.
Rick W

Mark Elliott · « **Reply #1 on:** February 10, 2017, 07:02:28 AM »

I am not a machinist, but it seems that it was the barrel that was improperly threaded and breeched. I would return it if I could. I would not use it. I have never encountered a barrel with a loose threaded breech plug.

Clark Badgett · « **Reply #2 on:** February 10, 2017, 07:11:11 AM »

If you measured correctly, it sounds as if the breech was over bored or possibly the tap drill could have been sharpened wrong which caused it to drill large, and the threads are shallow. A little bit of wiggle is pretty normal with items that are power tapped. Most taps are made for about 70% thread engagement. Fit the second plug as normal and see if it tightens up, or contact the barrel maker if not.

David Rase · « **Reply #3 on:** February 10, 2017, 07:36:33 AM »

My Starrett tap drill chart calls out for an 11/16" (.687") diameter drill. If you are measuring .690" minor diameter your breech does not sound that far off.
David

coutios · « **Reply #4 on:** February 10, 2017, 07:46:56 AM »

I would not mess with it.. Call the maker. Tell him you want it replaced. You cut on it....You bought it..

Regards
Dave

Clark Badgett · « **Reply #5 on:** February 10, 2017, 07:47:53 AM »

David is correct. If one doesn't have a tap drill chart, the formula for finding the correct drill size is MajDia-(1/TPI), example in this case .750"- (1/16)=.6875".

The extra .003" combined with the very real possibility that the tap was probably a Class 1 or 2 fit tap would cause some wiggle.

bob in the woods · « **Reply #6 on:** February 10, 2017, 03:44:48 PM »

if you purchased the barrel with the breach plug installed, I would send it back, based on the plug being too short.

P.W.Berkuta · « **Reply #7 on:** February 10, 2017, 09:00:09 PM »

If you are using a commercially made breach plug then they are machined to a standard diameter tolerance for a perticular thread size. The corresponding matching part should be made to fit the standard breach plugs. This is NORMALLY 75% full threads. It sounds like your barrel is not bored to standard 75% thread engagement. I'd send the barrel back OR if you have a mill, a lathe & TIME make your own plug to match the barrel.

Hudnut · « **Reply #8 on:** February 10, 2017, 09:59:25 PM »

If the threads in the barrel are good, I would just machine the breechplug to fit those threads - and not worry about the exact diameter.

T*O*F · « **Reply #9 on:** February 10, 2017, 11:00:31 PM »

Quote

I need to determine the outside diameter of the breechplug.How do I go about doing that?

Make a Cerrosafe casting of the hole and then copy it/

rickw · « **Reply #10 on:** February 11, 2017, 05:44:41 AM »

Thanks for all the responses.
I am trying to make a breech plug to fit the existing threads.My question was about how to figure the right diameter of the breech plug given the interior diameter of the barrel threads.
It does seem that the barrel was threaded incorrectly. Unfortunately, I can't send it back, since I bought it from an individual about 5 years ago.
Making a cast with Cerrosafe sounds like a good idea. Unfortunately, I don't have any. I'll look into getting some.
Thanks,
Rick

jerrywh · « **Reply #11 on:** February 11, 2017, 06:06:02 AM »

I sometimes make my own breach plugs and use a 90% thread . There are different classes of fit. For a full rundown on clearances read this. It will solve your problem. You machinist manual should have the specs on these fits.
https://img.fastenal.com/content/feds/pdf/Article%20-%20Screw%20Threads%20Design.pdf

Pennsylvania Dutchman · « **Reply #12 on:** February 11, 2017, 06:38:24 AM »

If you are threading on the lathe, I would turn the plug to .750 diameter. The tap drill size for 3/4-16 is 11/16 (.6875) for 75% threads, so .690 is fine. When threading the plug, I normally stop when the threads look close to finished and check the fit. You can take light cuts, threading and checking until you get the fit you want. You may have to remove the tailstock to check the thread fit. These threads do not seal on the crest and root of the threads, so you don't need to worry about making the diameter oversize.
Mark

coutios · « **Reply #13 on:** February 11, 2017, 06:59:12 AM »

Hey, Me again.. Lots of good info in above comments... Question... When you screw in the plug. Does it have the same amount of slop all the way in or does it tighten up somewhat towards the bottom? The tap could have been slightly off center during the threading operation..

You know my opinion... Call the supplier. They will fix you up...

Good Luck
Regards
Dave

Clark Badgett · « **Reply #14 on:** February 11, 2017, 07:08:44 AM »

Quote from: rickw on February 11, 2017, 05:44:41 AM

Thanks for all the responses.
I am trying to make a breech plug to fit the existing threads.My question was about how to figure the right diameter of the breech plug given the interior diameter of the barrel threads.
It does seem that the barrel was threaded incorrectly. Unfortunately, I can't send it back, since I bought it from an individual about 5 years ago.
Making a cast with Cerrosafe sounds like a good idea. Unfortunately, I don't have any. I'll look into getting some.
Thanks,
Rick

When you do a casting, don't be surprised if your major diameter is right at .75". If your current breech is measuring .69" that is in all reality only .0025" off nominal. Did you measure with dial calipers or a more accurate metrology tool?

FDR · « **Reply #15 on:** February 11, 2017, 10:24:31 PM »

Link to cerrosafe:
http://www.brownells.com/gunsmith-tools-supplies/index.htm?k=cerrosafe&ksubmit=y

David R. Pennington · « **Reply #16 on:** February 12, 2017, 03:07:10 AM »

I had a barrel that I wasn't comfortable with breech threads. I forged a new plug. It had 9/16 - 14 threads. I found an antique adjustable die that size and fixed it.

photobucket

<a href="http://s1239.photobucket.com/user/DavidPflint/media/IMG_2885_zpsebp3wk18.jpg.html" target="_blank"><img src="http://i1239.photobucket.com/albums/ff510/DavidPflint/IMG_2885_zpsebp3wk18.jpg" border="0" alt=" photo IMG_2885_zpsebp3wk18.jpg"/>[/url]

rich pierce · « **Reply #17 on:** February 12, 2017, 05:44:23 AM »

Dang. Nice work.

P.W.Berkuta · « **Reply #18 on:** February 12, 2017, 06:32:46 PM »

Quote from: David R. on February 12, 2017, 03:07:10 AM

I had a barrel that I wasn't comfortable with breech threads. I forged a new plug. It had 9/16 - 14 threads. I found an antique adjustable die that size and fixed it.
photobucket <a href="http://s1239.photobucket.com/user/DavidPflint/media/IMG_2885_zpsebp3wk18.jpg.html" target="_blank"><img src="http://i1239.photobucket.com/albums/ff510/DavidPflint/IMG_2885_zpsebp3wk18.jpg" border="0" alt=" photo IMG_2885_zpsebp3wk18.jpg"/>[/url]

YUP -- very nice work but it does not help the OP if he can't do the forge work. Maybe you want to make him a breach plug

David R. Pennington · « **Reply #19 on:** February 12, 2017, 10:22:28 PM »

Thanks for the positive comments about the BP pictures I posted.
PW, Perhaps my post might help the OP. I see a lot of work here that is way above my skill level. Most of it inspires me to improve my work. I did not know how to forge a breech plug until I tried. My first two attempts were failures. (I did not post them.) My intention was just to show that it could be done, without a machine shop, and the OP is certainly able to message me if he chooses and I would be happy to be of any assistance that I can.
Please accept my apology if my post offended anyone in any way.
David R.

Steve-In · « **Reply #20 on:** February 13, 2017, 02:04:09 AM »

Do you have a machine shop near you? If so call them and ask if they have a 3/4-16 go/no-go gage. Ask them if you bring the barrel in if they will gage it.
Otherwise when you have the plug in your lathe remove the tail stock when you can start but not turn on a standard 3/4-16 Nut. Carefully try to screw your barrel on. Ease the compound in and cut .0002 deeper. continue doing this until it fits like you want.
Minor diameter over does not affect thread fit, just engagement.

rickw · « **Reply #21 on:** February 13, 2017, 06:27:18 AM »

Thanks again for all the help.
The original breech plug measures .750, and the other one measures .748 in diameter. Both are loose.They both screw in loosely, and tighten up just a bit before bottoming out.
I measured the inside diameter of the barrel threads with a dial caliper. I know that isn't the most accurate method, but I didn't want to use my hole gauges because their rounded ends would register in the grooves of the threads. That would give a measurement that was larger than the actual distance.
There is indeed a lot of work shown on this forum that is way above my skill level too. However, it serves to inspire me, and to show me that these things CAN be done.

Rick W

Clark Badgett · « **Reply #22 on:** February 13, 2017, 07:25:59 AM »

Quote from: rickw on February 13, 2017, 06:27:18 AM

Thanks again for all the help.
The original breech plug measures .750, and the other one measures .748 in diameter. Both are loose.They both screw in loosely, and tighten up just a bit before bottoming out.
I measured the inside diameter of the barrel threads with a dial caliper. I know that isn't the most accurate method, but I didn't want to use my hole gauges because their rounded ends would register in the grooves of the threads. That would give a measurement that was larger than the actual distance.
There is indeed a lot of work shown on this forum that is way above my skill level too. However, it serves to inspire me, and to show me that these things CAN be done.

Rick W

Sounds about normal to me. I've cut a lot of threads over the years and tapped a lot of holes. Bolts usually have a little play until they are tightened, especially when the holes are tapped. My cut threads are usually class 3 fits, but they are made that way.

P.W.Berkuta · « **Reply #23 on:** February 13, 2017, 08:53:13 PM »

Quote from: David R. on February 12, 2017, 10:22:28 PM

Thanks for the positive comments about the BP pictures I posted.
PW, Perhaps my post might help the OP. I see a lot of work here that is way above my skill level. Most of it inspires me to improve my work. I did not know how to forge a breech plug until I tried. My first two attempts were failures. (I did not post them.) My intention was just to show that it could be done, without a machine shop, and the OP is certainly able to message me if he chooses and I would be happy to be of any assistance that I can.
Please accept my apology if my post offended anyone in any way.
David R.

Sorry if I sounded condescending it was not what I ment it to sound like

-- what I should have said was that you could give him instructions as how he could have made one. Most people (me included) don't have the equipment or skill to forge something like that. For me I would rather machine it out of a solid block of steel - again sorry for my ignorance

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Author Topic: Threading a breechplug to fit a barrel (Read 6590 times)

coutios

coutios

Steve-In