I have a Mauser 98/29. The rear angled locking screw does not go all the way into the trigger guard. I would like to chase the threads to clear the obstruction but can not tell the correct tap thread pitch needed. Any help is appreciated.
Mauser screw threads were initially all variants of Imperial Withworth threads (Barrel, 12 TPI, 55 degree W form, King screws (receiver) 1/4x22 55 degree W form, ( in the M1871 rifle.)
This continued in every Mauser made till 1945, except for such as the Siamese (RS121/T45/46 Rifle)
The Lock screws were a later addition, as was the Bolt stop hinge screw, and the cleaning rod screw and plate, and the Handguard retaining screw(into rear sight base) of the Export Mausers. These small diameter screws were all standard metric. The Lock screw and Handguard retaining screw were (IIRC) 3,0 x0,5, and the cleaning rod/plate was 4,5 x 0,7 or 0,75.
Measuring a screw to get the diameter will only give you an undersized approximation of the "true" screw diameter...a 3mm screw will show about 2,7-8 over the threads, and so on. The Pitch can be measured with a proper toothed Pitch gauge (metric of course.)
Every Gun enthusiast should have a good Micrometer or Dial Caliper, preferably "double reading" (Inch and metric concurrently)...and a multiple blade set of Imperial and Metric Pitch gauges ( a complete double set is about half the size of a Pack of Cigarettes).
Occasionally the screws correspond closely to both Metric and USF etc. series of threads, and that's when extraneous knowledge ( such as what type the makers originally used) will decide what reading is correct.
Here are some guides to who used what: (Mausers and other rifles)
Germans ,Czechs and FN etc...: Withworth Big screws, Metric Fine screws
French: French Metric ( some small differences from "ISO" Metric).
Austria: Sellars system (pre UNF American) and Metric
Italy: Metric, some Europeanised USF/Sellars
Russia: Withworth Imperial (Berdan II, Mosin Nagants, Maxims); Metric ( all semi and Full autos after mid 30s)
Japan: Metric and some early USF ( Imperial rifles are Metric in Pitch, but US in diameters, T99 is all Metric, Siamese T66 Arisaka is US Pitch based on 1/64" measurements of metal stock.
British: "Enfield Inch" ( some 0,0004" out from Proper Imperial Inch,) and thread pitches all "odd" requiring special gearing for screw machines, and specially cut (Enfield Factory) Tap and die cutting tools: they sensibly moved to BSF and BA systems of threads(and Imperial Inch, with the Rifle No.4 (late 1920s/early 30s)
US systems: in 1795, Congress adopted the French Metric system of measurment for fine tooling at Springfield Armory ( as well as the Charleville Musket( improved by the(French) Republic)
Whilst theUS "Inch" was used in decimals for measurements, the Pitch was in metric equivalents, with a 60 degree Form (same as in metric).
Up to the end of M1873/84 Springfield trapdoor rifle production ( 1890s) this system prevailed. Some US made Springfield gunscrews are interchangeable with old French Metric
The Sellars rational system of screw threads, first proposed by Sellars of Philadelphia at a conference of engineers in 1860-61, was finally widely adopted by US Engineering by the 1890s, and finally standardised as the UNC-UNF and UNFS systems. (FS, Fine-Special), all based on a metric style , 60 degree thread. Most of the Pitches per inch mirrorred the British Withworth system, except for 1/4 and 1/2 inch, and of course the "number series" of US screw threads were not found in the British system, which used the BA system, based on a metric Swiss threading system (for clock and instrument making) for the smaller sizes and pitches.
Whilst some of the diameters for US (gun)screw threads may be special, the Pitches are usually UNF or UNFS...Smith & Wesson has its "Own" thread pitches and diameters for all its handguns...no freely available commercial hardware equivalent.
That about covers most of the "Gun Threads" in Modern guns ( from 1890s onwards.)
Many a time I assumed a thread would be metric, it turned out to be Imperial or otherwise Non-standard, depending on the Country which made the Gun originally.
For more information about threads, a book such as Kent's "Machinery's handbook" ( an Engineers' Bible) and other similar texts on Screw thread systems in standardised common use, will give some insight into the arcane and "non-compatible" systems used in Gunmaking.
Armourers Charts ( common in British Military usage) have detailed descriptions of all the screw threads used in British Military rifles and accessories ( even "British Cycle thread" for tubing, as found on Bipods and Tripods, etc is essential for some British Military equipment, besides Bicycles.
regards, Doc AV
BTW, all these comments refer to Military type guns, and not commercial sporting guns...many unstandardised screw systems survived up to the First World War, in small Gunmaking...using such things as French Number series screws ( very common in French and Belgian Flintlocks and Fowling Pieces,) and similar German Clockmakers Threads...I have a set (incomplete ) of these, with die-plates, and taps, probably made in the early 1800s, the original property of a 1920s German -Australian Gunsmith, who died in 1941 (Black Powder flash) but his shop lives on today...he brought the set out from Germany after WW I, and it was probably well used then...