Gunsmithing Labor: install breech plug, fitted inside

Gunsmithing Labor:
install breech plug,
fitted inside

Part Number:	LABOR-BP
Availability:
Price:	$30.00

Our gunsmith will install a breech plug we offer, in a barrel we offer, with the front face of the breech plug tightly sealed against the inside shoulder of the breech thread, and the barrel maker's name (if any) indexed to the bottom flat. Threads are lubricated with Birchwood Casey's Choke Tube Lube anti-seize, so the plug may be removed, even after decades of use.

Never remove a breech plug, unless you have a compelling reason. The plug is never removed for cleaning, and should only be removed by a skilled gunsmith, who has the correct tools to avoid marking your polished and finished barrel and plug.

Install it yourself: installing a breech plug, at home, is easy, but it does require close attention to several points. It can be a tedious job.

Print this page, to save these do-it-yourself instructions.

Your safety, and the safety of the future users of your gun, depend on your correct fitting of the plug. Take time, use care, and follow these steps:

Prevent galling: any threaded steel plug can "seize" in a threaded steel hole. The problem is especially severe when installing a wax cast steel plug, due to the microscopic nature of the mating surfaces. Apply a tiny amount of Birchwood Casey's Choke Tube Lube, or a good anti-seize grease, or graphite grease, before testing any threaded plug in any barrel. We have seen "seized" plugs that were inserted by finger pressure only. Seized threads can "gall" (tear out chunks of thread), when you attempt to remove them.

Thread fit: several classes of thread fit (tightness of fit) exist for any thread size. Common nuts and bolts assemble easily, with fingers. Breech plugs have straight threads, never the tapered threads used for pipe fitting.

Barrel makers and plug makers do not all agree on which class of thread fit they use. Thus, the barrel makers tend to make the threaded holes slightly small, and some plug makers (especially makers of cast steel plugs) thread their plugs slightly large. You must make the threads fit together. The ideal fit will start with finger pressure, and tighten fully with one hand on a small wrench, with no side-to-side rattle when inserted about 2/3 depth.

If your plug rattles side-to-side, when inserted 2/3 depth, thread fit is too loose. If so, we recommend exchanging plugs, to find one that fits correctly. Loose threads are rare, in today's "over-tight" market, but it happens.

If your plug must be installed and removed with a long handled wrench, both hands, and great effort, thread fit is too tight. If so, the following two steps, bottom tapping and thread relief checking, will nearly always provide correct fit.

Zero clearance bottom tap: nearly all muzzle loading barrels are factory threaded for the plug. None are completely finished inside. Only the machine work is done. The final bottoming tap step must be completed by you, by hand.

A factory finished bottoming tap has one (1) thread relieved (chamfered) to allow the tap to be easily started. Make a special zero clearance bottom tap, by grinding the end of the tap perfectly square, with no chamfer. This special tap will be a bit more difficult to start, and may "chip" the front tooth, when you hit bottom. This is normal.

Apply thread cutting oil (lard oil or re-sulphurized cutting oil) to your tap. Insert the tap and run it up to the inside shoulder. Turn it snug, and back off a half turn, and repeat a few times, to extend the thread until it touches the face of the inside shoulder. A few large caliber barrels don't have much of an inside shoulder, but this procedure works equally well for those barrels. Remove the tap, inspect the tap's front, and carefully remove the loose tooth of the tap, from inside the breech threads, if it chipped. Clean the breech threads of oil and chips.

Thread relief: a few breech plugs have thread relief, a recessed area at the face of the plug, where the thread journal joins the plug. We dislike any undercut in this area, because it limits the torque that the plug will withstand. Despite that, these plugs work fine.

Many plugs, especially wax cast Hawken style plugs, have no thread relief. The threads simply stop short of the end of the thread journal.

Apply thread cutting oil to your plug threads. Start your threading die, backwards, on your plug threads. Snug the backwards die against the shoulder of the plug, withdraw one half turn, and repeat a few times. You will cut the last half turn of the thread, on many plugs. Check the fit of your plug to the barrel. If too tight, many dies have a split-ring adjustment screw, or a different die with a different class of thread fit, may be used to bring the threads to the desired size for easy fit. Excessively tight threads add very little strength, and can cause thread galling.

Measure plug thread length: using your dial or digital calipers, measure your plug journal's thread length. For example, perhaps your plug threads measure .625" length.

Measure barrel thread depth: using your dial or digital calipers, measure your barrel's breech thread depth. For example, perhaps your barrel threads measure .750" depth. Plugs are generally shorter than barrel threads.

Trim plug, or barrel, to matching length: in our example, above, the plug thread is .625" length, and the barrel thread is .750" depth. These must match exactly. So, you must trim off the difference. In our example, trim .125" from the breech face of the barrel. A gunsmith will use a metal lathe to simplify this work. A milling machine will easily trim a straight octagon or round barrel (or a tapered barrel if your vise is set to compensate for taper). At home, you may trim off the barrel with a large mill file, and a bench vise. Paint the end of the barrel with ink, scribe the desired line, and file slowly to maintain control and keep the file flat. Rotate the barrel after a few strokes, to keep the end flat. Regardless of the tool used, cut less than needed, measure your progress, and sneak up to your desired depth in several small cuts. When the measurements match exactly, remove the tiny burr from the end of the threads.

Test your plug's fit: wipe away any cutting oil, and paint the face of your breech plug with Magic Marker ink, or Dykem blue. Apply anti-seize to the threads, and insert the plug into your barrel. Using a small adjustable wrench or gunsmith's plug wrench (shop made to fit the plug's wrench lug), turn the plug until it is seated fully. If both hands plus great force is required, your plug is too tight. Notice where the plug stops. If the plug aligns exactly with the top of your barrel, you are very lucky, and you are done.

Inspect your plug's face: remove the plug, and inspect the ink on the plug face. If the plug face touched the inside shoulder of the barrel, the ink will be smudged and marked, proving a good seal. If not, your barrel is still a tiny bit too long. Adjust the length, and repeat the test.

Measure the index angle error: if your unmarked barrel is octagon, any flat can be the top. If your unmarked barrel is round, any position can be top.

In the real world, your octagon or round barrel will probably have a name or caliber mark stamped on a side or flat, near the breech.

In our example, we assume that our plug fits inside, with no gap outside, but the plug needs to turn in three (3) more flats, to align with the desired top flat. Thus, we need to cut a bit more off the barrel length, and the same amount from the plug face, so the plug will fit exactly, when the top flat aligns with the seated plug.

Calculate the amount of error: the amount to be trimmed from the barrel and plug will depend on the thread pitch (number of threads per inch):

Your 9/16-18 plug, or 5/8-18 plug, has 18 threads per inch. Each full turn of the plug enters 1/18" or .056". Thus, each octagon flat is 1/8 turn, or .007" forward distance, per flat turned.

Your 3/4-16 plug has 16 threads per inch. Each full turn of the plug enters 1/16" or .062". Thus, each octagon flat is 1/8 turn, nearly .008" forward distance, per flat turned.

Your 7/8-14 plug has 14 threads per inch. Each full turn of the plug enters 1/14" or .071". Thus, each octagon flat is 1/8 turn, nearly .009" forward distance, per flat turned.

Trim both plug and barrel, to a new matching length: in our example, if our 5/8-18 plug stops three (3) flats short of the desired alignment, we cut 3 x .007" = .021" from the face of the plug. And, we cut the same .021" from the breech of the barrel. Plug and barrel thread lengths must exactly match.

Install the plug: wipe away chips and oil, apply anti-seize, and snug the plug into correct alignment. You may wish to withdraw the plug, and check for marked on the inked on the plug face, to insure a good seal.

If you have cut too much, you must re-calculate the number of flats in error (perhaps seven), and repeat the fitting process. We consider .500" thread engagement (counting only the length of good threads), to be the minimum acceptable amount for optimum strength, for a small plug. Cut less than you estimate, check your work, and approach a perfect fit slowly.

Draw-file and polish your plug and barrel: after you have achieved a perfect fit, draw-file the plug to match the barrel. Hold the file in both hands, and slowly drag it at right angles to the barrel and plug, keeping it perfectly flat. Clean your file frequently, brushing away all chips. We wrap abrasive cloth around the file, to finish the work. Abrasive coated denim disks, with adhesive gum back, can be cut into strips and wrapped around your file. These aggressive disks are used in auto body repair, sold at auto parts stores. When we install your plug, our gunsmith does not draw-file or polish.

Match-mark your plug and barrel: if you will ever remove the plug, after draw-filing and polishing, strike a match-mark line across the joint, on the bottom flat, to insure perfect re-alignment. We use a sharpened tiny cold chisel to make this small mark, only when it is requested.

Let our gunsmith install your plug: we can do this work, typically within 7 to 10 days shop time, at low cost. When ordering other labor, such as dovetails for sights, or rib installation, we MUST install the plug, first.