Unidentified Thread in Bathroom Fitting

Hi all
My mother collared me the other day with a request to fix a toilet roll holder. At first I couldn't see anything wrong, but eventually found that a grubscrew is missing. This grubscrew centralises a chrome flap which half covers the toilet roll and, I think, provides the user with a flat edge to tear the serated sheets off against IYSWIM. The strange thing is, the thread does not conform to UNC, UNF or metric pitching. Fortunately, the grubscrew that retains the holder onto its wall bracket is the same thread, so this can be examined closely.
Are these fittings renowned for using odd-ball thread types? The toilet roll holder was part of a refurb job, so unfortunately maker's details are not known.
TIA
Phil
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Not an uncommon problem. Does it have to be a grub screw? Is it into an alloy casting, or steel? For alloy, I'd probably be looking in my box of assorted self tappers. Unless it's very fancy or there's a quick fix you're probably better to thow it away.

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Since it's small, BA?
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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What is the approximate diameter?
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Matty F wrote:

Probably BSP thn..
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When I was sorting out a ton of old bolts I made up a table so that I could work out what they were. It's too big to post all of it as it covers 1/16 inch to 6 inch diameter threads. But it has only BA BSF BSW M MF UNC UNEF UNF as I don't use pipes :)
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"Matty F" wrote

Thanks to all
Took the fitting to the town's iron mongers on saturday. Using a drill gauge type-thing, the guy reckoned it was 5/32" dia. The only nut that would come anywhere near it was M4, but an M4 screw would not fit into the threaded hole. Also when visually comparing the threads, metric wasn't close, nor were UNC, UNF, or BA (all of these were finer than the grub screw). Looks like it will have to be drilled and tapped out to a suitable "standard" size :(.
Phil
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Here's part of the table, sorted in diameter order:
Type    Size    Dia Inch    TPI    Dia mm    Pitch mm BSW    1/16    0.0625    60    1.59    0.42 M    1.6     0.063    72.57    1.6    0.35 M    2     0.079    63.50    2    0.4 BSW    3/32    0.09375    48    2.38    0.53 M    2.5     0.098    56.44    2.5    0.45 MF    3    0.118    72.57    3    0.35 M    3     0.118    50.80    3    0.5 BSW    1/8    0.125    40    3.18    0.64 M    3.5     0.138    42.33    3.5    0.6 BSW    5/32    0.15625    32    3.97    0.79 MF    4    0.157    50.80    4    0.5 M    4    0.157    36.29    4    0.7 BA    3    0.1614    34.84    4.10    0.73 M    4.5    0.177    33.87    4.5    0.75 BA    2    0.185    31.35    4.70    0.81 BSF    3/16    0.1875    32    4.76    0.79 BSW    3/16    0.1875    24    4.76    1.06 MF    5    0.197    50.80    5    0.5 M    5    0.197    31.75    5    0.8
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Only common coarse thread you've missed out is Whitworth.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Remember that metric has both fine and coarse pitch. For most sizes the coarse variant is pretty rare, but they might have used it in castings?
I'd be looking at the next larger self-tapper I had handy, ideally a Taptite (slightly different thread profile) as the least-abusive way to bludgeon a thread into a cheap diecasting.
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