UNUSUAL SCREW THREADS

[Bigbjorn]

I have had a couple of odd ball request for taps and/or dies in the last two weeks.

One was a brass mixture needle from a Ronaldson-Tippett stationary engine. Measured at 0.388" x 27 tpi. The owner and "experts" in the old engine fraternity had no idea. It is Unified Thin Wall Tubing Thread, a 27 tpi constant pitch system, rarely seen today and pretty much unavailable commercially. Interestingly all other sizes in the series are expressed in fractions. This one is expressed in decimal as 0.390 x 27 tpi. If required, the tools will have to be made to order.

Another was for a tap and die in 25mm x 1.25mm pitch. This was required by a guy who makes "no longer available" auto parts. The thread is used on sheet metal nuts on early Commodore steering racks made in Australia by TRW. These are no longer made by the OEM. My enquirer will make replacements out of hex. bar, not sheet metal. Once again, the tools are not commercially available and will have to be made to order. Pitches available in 25mm are 1.00, 1.5, 2.00, and 3.00.

Other recent enquiries were for 3/4" x 32 tpi, 5/16" x 30 tpi, 7/32" x 32 tpi, 9/32" x 32 tpi, all used on pre-war vehicles.

[mox]

One unusual thread I heard about was one a cousin of mine found somewhere on an old Alfa Romeo. 12mm British Standard Brass, which all have 26 threads per inch and all imperial diameters except for this one. Enquired about getting a tap. Engineers suppliers thought no such thing existed. However it did according to a thread chart the family engineering business had. However I don't think think he obtained one. which would have been added to a collection of several rare sizes they had accumulated which were presumably obtained for special jobs.

[Bigbjorn]

BSB still has a British Standard and some manufacturers still list the common sizes as available. Whether they have any stock is a moot point. BSB has several diameters which are expressed in decimal, 0.320", 0.4724", 0.5512". All others are expressed in fractions. BSB is classed as an obsolete thread system and it use no longer recommended. Sizes ranged from 1/8" to 2".

A form of this thread was adopted in Germany for use with brass and this may be where your 12 mm. size came from. The German series is no longer in use.

[bblaze]

Just out of interest, whats the rough cost of getting a custom tap and die made, say for example, something in the 16mm size
cheers
blaze

[Blknight.aus]

depends on how well you know the guy with the thread cutting lathe and the mill.

[Bigbjorn]

Just out of interest, whats the rough cost of getting a custom tap and die made, say for example, something in the 16mm size
cheers
blaze

PM sent.