Breeching question

[Habu]

I've got a piece of cut-off rifle barrel (Douglass 1"x.54 cal) that will eventually be a pistol barrel.  Breechplug thread size will be 3/4-16.  Book says 11/16" tap drill size for 75% thread engagement; I know I need to go undersize from that but can't recall how much. 

So when I take this barrel to the machinist to get it bored, what diameter should I tell him to bore it to? 

Thanks,
Jim

[Acer Saccharum]

The drill size 11/16 is OK. For tight thread engagement, the tap needs to be undersize from std. But I don't think you need tight thread fit. Look for the plug face shutting off against the bore dia. This will give you the required tightening as you screw the plug home. Simultaneously, the standing plug shoulders should fit against the barrel end.

You'll need 1/2" long plug length or longer.

Don't make the tap drill diam 11/16 hole much smaller, or the tap teeth will be in danger of breaking during tapping. 75% thread is plenty strong.

Tom

[Habu]

Thanks! 

It's been a while since I did one of these, and for some reason I was thinking last time we bored the hole maybe .010-.020" under 11/16".  Once the barrel is tapped (and fortunately, I still have my taps from the last time I did this), it is just like fitting any other breechplug.  No worries there. 

[Acer Saccharum]

It's OK to be a little under. I don't like OVER sized tap holes. Pretty soon, the thread engagement is down to 50%, which is Baaaad.

[Dphariss]

I would look for an H1 tolerance tap if I were worried about engagement. This is the best answer.
But its not cheap.
http://www1.mscdirect.com/cgi/NNSRIT2?PMAKA=74477217&PMPXNO=2058791&cm_re=ItemDetail-_-ResultListing-_-SearchResults
 But 3/4-16 is strong enough its a non-issue with standard thread engagement.
The weak point is the barrel at the end for the threaded hole where the wall is thinnest and takes all the strain of the pressure load. But with a 1" barrel this is also a non-issue with 3/4 threads.

Dan

[Habu]

I'm in full agreement about the problems of oversized tap holes when it comes to breeching.  I'm pretty cautious (hence the original question). 

Thanks Dan, I'll keep the H1 tolerances in mind for future purchases of taps for breeching.  Would you suggest the same preference for taps for nipple on percussion guns?

Jim

[coutios]

  Also just another thought... What class is the plug made to??? Makes a difference. I'd like to know before I bought a special tap... Sometimes we are so concerned about the threads in the barrel that we forget to discuss whats going into it...

Just some food for thought...

Dave

[Dphariss]

  Also just another thought... What class is the plug made to??? Makes a difference. I'd like to know before I bought a special tap... Sometimes we are so concerned about the threads in the barrel that we forget to discuss whats going into it...

Just some food for thought...

Dave

Most of the modern stuff has ample slop built in so I would not expect a problem with a H1 which is still over sized. We need to remember that all modern stuff is made so that a part made in California will screw into a part made in Maine with neither maker having ever heard of the other. "Tolerance" is the bane of good gun work and it rears its ugly head in places you would not suspect.
H1 is .0005 over the base line size. Unless the plug is made oversize the H1 should simply remove some of the slop.

See
http://www.newmantools.com/taps/taptech.htm
 for thread tolerance explanation.
You can use standard hardware store taps and sill get a good fit if you make your own plugs using a split die that can be expanded to make a larger plug. Its possible to get very nice fits on plugs and vent liners in this way.
I like to use H1 taps for parts subject to pressure. But many parts like barrels and breeches are already threaded for plugs and nipples.
It will also help in some cases in locks if you use screws that come with the kit. It just gives a nicer fit.
I will be making a plug, starting the threads in the lathe then finishing with a split die, for a heavy McLemore barrel, a very nice 1.25" x 44 X 50 caliber, tomorrow. Very smooth, patches slide down it like they were buttered and will just barely accept a .500 minus plug gage.
And its made of 4150.

Dan

[coutios]

   Dan,I'm familiar with threads and engagements.. Those that have the skillset and understanding to bring a task to its proper closure are not the ones I have concern for.. In my short 25 years of muzzleloading I have seen enough hack jobs to last me two lifetimes.. Nipples, liners and drums blown out. Barrels bulged... There is not alot I'm strick about. But breeching, liners and drums must be installed right.. I support the prior comment about having a Machinist do the work.. Good idea if you are unsure of your skills or do not wish to invest in the tooling.. Sometime I think we take for granted what skills are needed to do some of the metal work...

Sorry if I babbled on it's late
Regards
Dave

[Rolf]

I will be breeching a pair barrels for the Le Page pistols I'm building. I checked with a professional firm that sells threading tools to the offshore industry.  I was told that Class1 threads have the loosest fit and that class 3 is the tightest. They said the main difference between a expensive high quality tap and a cheap one is not the tolerances, but how long they last. If you are going to thread only a few barrels, the cheap stuff is good enough. I was also told that professional barrel makers use class 2 for breech plugs.

I've bought a split die(8$) and bottom tap(5$) for 9/16-18 from Tracy tools, England. These are class2 and made of carbon steel.

Here is a link http://www.tracytools.com/

Best regards

Rolf

[Rasch Chronicles]

Dan,

Once again thank you for taking the time to ellucidate upon and include reference material on a subject many of us may be unfamiliar with. I for one am very greatful for your time and and tuteledge!

Best regards,
Albert “El Matamoro” Rasch
The Rasch Outdoor Chronicles
Hunting Merriam’s Turkey: Tips and Technique
ΜOΛΩΝ ΛΑΒΕ!