7mm thread...why so unusual?

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Matata

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I have a six mm hole on the side of my honda....doing the thermostat...long storey. for a new bolt I was going to simply tap 7mm . Why is this size so uncommon. 8mm is too big, not enough meat around it. Ta Nik
 
Matata said:
I was going to simply tap 7mm . Why is this size so uncommon.
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There's a large range of metric threads. Industrial standards define ist, 2nd and even 3rd. choice diameters in order to rationalise and reduce necessary stocks as far as reasonably possible.
M7 is a 2nd choice size, so not common, but threading tackle and bolts etc are available if you insist.
 
7 mm is a "2nd choice" in the ISO system with 6 mm and 8 mm being "preferred."
In all my engineering i have never used 7 mm.

Could be simply that a) 6 mm corresponds roughly with 1/4 inch and 8 mm with 5/16 inch and for 99% of the time that gradation of 2 mm steps is as fine as it needs to be.
Same reason you never see a speed limit of 52 km/hr. No technical reason why you couldn't have a 52 km/hr limit, but 5 km/hr gradations seem to meet the need most of the time.
Good luck with the 7 mm tap
cheers
John
 
If the thread is stripped it might be cheaper to Helicoil it. The metric kits are very inexpensive now and you will have some spares for any future shenanigans.
 
Citroën used M7 as the standard "small" thread size on the 2CV, DS and other cars of that sort of age. All us DS owners have boxes full of M7 threaded stuff, M7 taps and dies and 11mm (or 7/16") spanners and sockets.
 
Matata said:
I have a six mm hole on the side of my honda....doing the thermostat...long storey. for a new bolt I was going to simply tap 7mm . Why is this size so uncommon. 8mm is too big, not enough meat around it. Ta Nik
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Its part of the "round numbers" mentality that we all have inc design engineers. If you look at a set of design drawings you will see most dimensions given in what the designer sees as standard sizes. You will see things made symetrical when function doesnt require them to be so. Ask yourself why you are thinking of 7mm and not ( say) 7.18mm - the reason is the same one.

Can you not manage with 1/4 in? ie 6.4mm?
 
Problem with a 6mm helicoil insert, as suggested, is that the hole will have to be tapped to nearly 8mm. major dia. OP says that there isn't enough metal for that.
 
The only 7mm screws I've come across were on a Peugeot car, I can't remember which bit, I only remember it took ages to find some replacements
 
earlybird said:
Problem with a 6mm helicoil insert, as suggested, is that the hole will have to be tapped to nearly 8mm. major dia. OP says that there isn't enough metal for that.
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If there really is that little metal, you might consider aralditing in a stud?
If that's convenient.
I wonder if the lack of metal is in the component that bolts on though?
Or, if it's lightly loaded, can you helicoil down to M5, or use some other insert?

Or is it time to start talking nicely to the welding man?
 
savageseadog said:
The only 7mm screws I've come across were on a Peugeot car, I can't remember which bit, I only remember it took ages to find some replacements
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Probably a hangover from the Citroën side of the company.

Mind you, Citroën did even worse than 7mm. They also used M5 ... or rather, they used 5mm diameter metric threads, but started before the Ms were standardised. M5 coarse is 0.8mm pitch, M5 fine is 0.5mm pitch and Citroën M5 is 0.75mm pitch. Fasteners in that size are well nigh unobtainable, though the Tap and Die Company can supply taps and die (there's a surprise) to suit.
 
birdseye said:
Its part of the "round numbers" mentality that we all have inc design engineers. If you look at a set of design drawings you will see most dimensions given in what the designer sees as standard sizes. You will see things made symetrical when function doesnt require them to be so. Ask yourself why you are thinking of 7mm and not ( say) 7.18mm - the reason is the same one.
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I find it mildly interesting that mechanical engineers traditionally use the even numbered BA threads but electrical engineers use the odd numbered ones. Civil engineers just hit things with hammers.
 
cliff said:
Well bugger me sideways with a barge pole if The Tap and Die Company http://www.tap-die.com/index.html based in London, does not have the 7mm tap and die requested, guess the OP does not need kiwi luck after all.
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Well i was only trying to explain why 7 mm was unusual, not that it was unobtainable.

(Its all about standards you see: Everyone loves standards and that is why we have so many of the damned things -often directly conflicting!)
cheers
 
JumbleDuck said:
I find it mildly interesting that mechanical engineers traditionally use the even numbered BA threads but electrical engineers use the odd numbered ones. Civil engineers just hit things with hammers.
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I've never really come across odd numbered BA sizes anywhere except model engineering, where 5BA and 7BA closely approximate to 1/8" and 3/32", hence scale up nicely for old imperial fasteners.
BA is really commercially obsolete.
Also, TBF, I've known some Civils who have learnt where to hit things!
 
earlybird said:
I've never really come across odd numbered BA sizes anywhere except model engineering, where 5BA and 7BA closely approximate to 1/8" and 3/32", hence scale up nicely for old imperial fasteners.
BA is really commercially obsolete.
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The clamp screws in 13A plugs are (traditionally) 5BA and the screws holding the back on are 3BA. As you say, odd numbered BA is rare, but the electricians love it.
 
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