Here is a list of all common screwthreads listed in size order in .pdf format for you to download
I find the UNF and UNC are the most useful, followed by the B.A. threads – the metric seldom fit any old British guns.
Percussion Nipples are often 1/4 BSF (26 t.p.i), which DOESN’T correspond to 1/4 UNF (28 t.p.i) – You will often find that the thread diameter in holes is annoyingly just a bit oversize – you can sometimes get away with opening up the die to the maximum extent the die and holder will allow, but sometimes you need to enlarge the hole in the dieholder – its easier to do that with a tailstock dieholder in a lathe. Sometimes you crack the die in opening it up – you can sometimes still use it if it sits in the holder snugly.
Smith’s Imperial caps seem to correspond more or less to an oversize 1/4 UNF 28 t.p.i. thread.
Nipple holes that are nominally 1/4 inch get worn, rusted and oversize – 1/4 BSF 26 t.p.i. threads can be opened out with a 9/32 BSF tap which is also 26 t.p.i. and larger nipples bought or made. You need a plug tap to reach the bottom of the hole and even then may need to make a special tap by grinding off the end of the plug tap.
Old gun threads are almost always NOT the same threadform as modern threads even if they fit – the threads are much more rounded and therefore the thread is usually rather shallow – if you leave new made screws soft and screw them into hardened parts they will usually swage to fit fairly well. It is vitally important to make sure nipples are a good fit on percussion guns that might possibly be fires as it is extremely dangerous to shoot a gun if the nipples are not a good fit in a clean thread in the false breech. If you can still wiggle the nipple around just before it is finally screwed down, it is NOT good enough and should NOT be fired.
click here to download 3 landscape pages of the table as a pdf;- thread sheet 2020
Or use the table here ( it’s an older version so the pdf is a better bet!) ;-
|USEFUL GUN THREADS FOR RESTORATION WORK Copyright Tim Owen 2016|
|DIM 3/32nd >>>>>>>>>>>>>>>>>>>>>>>>>>>>||0.094||2.383|
|DIM 1/8th >>>>>>>>>>>>>>>>>>>>>>>>>>>||0.125||0.000||3.175|
|DIM 5/32 nd>>>>>>>>>>>>>>>>>>>>>>>>||0.156||3.967|
|DIM 3/16th >>>>>>>>>||0.188||4.763|
|DIM 1/4th >>>>>>>>>>>||0.250||6.350|
|AS str. Pipe||1/16th||0.281||27||0.000||7.137||0.941|
|DIM 5/16th >>>>>>>>>>>||0.313||7.938|
|AS str. Pipe||1/8th||0.378||27||9.606|
|Brass threads are all 26 t.p.i. as are British Standard Cycle threads. Model Engineer threads are available in 40 or 32 t.p.i from 1/8 to ½ dia.|
|Early threads were made from hand made die plates and taps that were derived from earlier generations of handmade dies and taps so that the diameters tended to vary considerably and the pitch was not standard.|
|A degree of standardisation was beginning to be implemented from late 18th early 19th century and percussion guns are likely to have threads similar to modern BSF or UNF ( but 1/4 BSF (used for many nipples) 26 t.p.i is NOT the same as 1/4 UNF 28 t.p.i – the other sizes are the same?) and Whitworth or UNC threads – you’ll find those threads are quite common on British guns, but often the diameter is a bit out
I’ve found that 26 t.p.i is quite common in side nails as well as nipples, and both 1/4 an 9/32 BSF have 26 t.p.i. This is handy as it means that if a 1/4 BSF nipple hole is badly eroded, it can be retapped 9/32 on the same thread. You can often get smaller diameters by closing down the die, or larger by opening them up, sometimes by cracking the die, or spot softening it with a short burst from the TIG welder..
|The thread profiles was almost always different from modern threads – thread angles were shallower and the tops and bottoms of the threads were almost always much more rounded than modern threads, but provided the part you have made is not hardened it will usually form itself to fit.|
Sizes of old woodscrews – e.g. Nettlefold countersunk steel and brass.
No.2 = 2mm thread diameter; 4mm head diameter.
No.3 = 2.3mm thread diameter; 4.5mm head diameter.
No.4 = 2.6mm thread diameter; 5.2mm head diameter.
No.6 = 3.5mm thread diameter; 6.6mm head diameter.
No.8 = 4mm thread diameter; 8mm head diameter.
No.10 = 4.8mm thread diameter; 9.4mm head diameter.
No.12 = 5.5mm thread diameter; 10.9mm head diameter.
Screw lengths are measured overall including the head.