M bolt sizes to Allen/hex key.

[RMutt]

I wonder if someone can help me find the correct size replacement bolt/screws. I am using my Allen keys to try to find the correct size bolts but the conversion charts are either contradictory or at least confusing. I wonder if someone can point me to a chart or give me the right sizes. Bolt one is a socket head and the Allen key that fits is a 2.5 mm. Bolt two is countersunk and a 2 mm key fits. I get them both to be M3. It is confusing that the M size changes with the head type. Am I correct? Thanks in advance. Andy.

[Firebottle]

It is confusing, best to measure the thread diameter of the bolt, it will be close to 3mm or 2.5mm.

[RMutt]

Thanks Alan. I'm not sure my eyesight is up to that! and I don't have a micrometer. I'll have a bash.

[Stratmangler]

http://mdmetric.com/tech/bondpage.htm

[RMutt]

As an example of the confusion. I have bolts that I know are M2 and according to the charts should take a 1.5mm Allen key but actually only take 1.3 mm. It's a good job I'm not building something serious like a plane or a nuclear reactor!

[RMutt]

Brilliant, thanks Chris. I think that chart confirms what I thought. Andy.

[Pegwill]

Hi

One of the easiest ways I ‘ve Is if possible go take the bolts into somewhere like B&Q and try to find a nut that fits.

Hope it helps

[RMutt]

Thanks William. I'ts only a couple of quids worth so I'm taking a punt on ebay. Andy.

[Lawrence001]

As an example of the confusion. I have bolts that I know are M2 and according to the charts should take a 1.5mm Allen key but actually only take 1.3 mm. It's a good job I'm not building something serious like a plane or a nuclear reactor!Even then they mix up metric and imperial and still mess them up [emoji38] can't remember the famous case maybe someone here knows

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[RMutt]

Even then they mix up metric and imperial and still mess them up [emoji38] can't remember the famous case maybe someone here knows

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A couple here Lawrence.
https://www.simscale.com/blog/2017/12/nasa-mars-climate-orbiter-metric/
https://www.bbc.co.uk/news/magazine-27509559

[Barry]

Not sure if this helps: https://en.wikipedia.org/wiki/Hex_key, but as far as I'm aware the metric designation for 'Allen' keys, is the distance measured in mm across the flats of the hex key.

[RMutt]

Not sure if this helps: https://en.wikipedia.org/wiki/Hex_key, but as far as I'm aware the metric designation for 'Allen' keys, is the distance measured in mm across the flats of the hex key.

Yes, that seems to be correct Barry. The difficulty comes when using a key to determine the M size for a bolt/screw. The same key will fit two different M sizes dependant on whether they are countersunk or socket head.

[Barry]

Yes, that seems to be correct Barry. The difficulty comes when using a key to determine the M size for a bolt/screw. The same key will fit two different M sizes dependant on whether they are countersunk or socket head.

Yes, the 'M-' designation for the hex key has nothing to do with the M size of the bolt/screw, save the designation of the key can never be numerically larger than that of the bolt/screw, as the hole would be larger than the diameter of the shank.

The knobs on my preamp are secured by grub screws that have a 'Torx' shaped hole. One of them (the imput selector) came loose, so I bought a set of Torx keys to tighten it. I had to buy a set as it was not all obvious what the correct size was. Even though I now know which key to use, the relation between the designation to some measured dimension still escapes me; and the Wikipedia article I cited does not explain it either.

[Pigmy Pony]

When I've needed any bolts I go into William Moore of Preston, which sells a lot of tools and every type of fastener. (Every town will have a place like that).

I go in, and say "have you got bolts that will fit this nut?" They take me round the back, where everything is sorted on shelves, and after a quick search he finds what I'm after. "How many do you need?" he asks. I reply "Two". And he doesn't pull a face. We go back to the till, he presses a few keys, then says to me "36 pence please", and cheerfully prints out a receipt. I feel so guilty I vow that one day I'll buy something really expensive, but so far I haven't.

Much better than faffing with tinternet imo :)

[Barry]

You are very lucky to have a traditional 'ironmonger' near you. We have a B&Q where they do sell fixings individually, but the range is limited.

I can't imagine finding a nut or bolt with, say, a 3/16 inch Whitworth thread.

[Pigmy Pony]

Honestly Barry, I'd be very surprised if they didn't have even that, and Whitworth isn't a word I've heard since school. B&Q are ok for a lot of stuff, but their expertise is usually limited to "If it's not over there, we don't have it" :(