On 29 Jun 2004, at 16:50, Doug McNutt <[EMAIL PROTECTED]> wrote:
4-40 NC or 6-32 NC are US designations for machined screws which have flat topped threads and require a pre-threaded hole or a nut. The first number is a size designation. larger numbers are for bigger screws. The second is the number of threads per inch. There are NC (national coarse) and NF (national fine series). A 6-40 is a NF screw.
Incidentally, those hard disk screws are 6-32 and not 4-40 as I once said.
I'll put on the old Mech Eng hat and try to remember those lectures from twenty years ago... NC and NF are American screw thread standards. Around about 1940 when the US was supplying war machinery to the UK, NC and NF were adopted as an ad hoc UK standard; there are subtle differences between US NC/NF and UK UNC/UNF (thread root angle?) but the intention is that the two are interchangeable in general use. UNC and UNF finally did get a BS standard in the late 1960's, just about the time that UK manufacturers were encouraged to switch to metric...
The biggest problem is that UK manufacturers only used UNC/UNF fixings in larger sizes -- the smallest diameter I can ever recall seeing is 3/16" but that size would be quite rare. Even British car manufacturers who used loads of UNC/UNF fittings for mechanical parts (1946-1980) continued to use the archaic and completely incomprehensible BA standard for electrical components and the even more obscure BSP standard for brake pipes. If you were really unlucky, you may have bought a 1950's MG with bolts using an obscure French metric thread and a Whitworth standard head.
I'm not 100% sure what the 6-32 NC nomenclature means in the UK but my guess is that it equates to 6/32" diameter (~= 3/16" UNC). If so then a replacement screw should be obtainable from a classic car fastenings specialist or from a decent engineering fastenings shop -- those wonderful relics from a distant era where blokes (and the rare woman) wandered round in tan "shop coats" with thread gauges poking from the top pocket, more than happy to spend twenty minutes rooting around in boxes before proudly presenting you with some exquisitely preserved piece of old stock and announcing "I'm not sure of the the price but does 35p sound all right". If you live in a rural area, it would be worth asking in an old established hardware shop (3/16" UNC would have been used on agricultural machinery) but forget about the DIY sheds and places like Screwfix. Your best bet is the olde engineering shop where they know how to measure a thread.
Phil --- http://www.mandrake.demon.co.uk/Apple/
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