Metric sizes in machine screws and bolts. M7 seems very rare.

I'm getting ready to fix a TV bracket to a vertical wood panel in the caravan.
My (fairly) trusty electronic callipers give an internal diameter of 6.93 for the 6 fixing holes in the metal plate which fixes to the wall. This should make them M7.
What I need is 6 countersunk machine screws 25mm (or possibly 30mm) long, with suitable nuts and washers. Stainless if at all possible.
I thought a domed nut would probably be the smartest.
For some reason the online stores I have looked at so far don't seem to stock M7. After M6 they seem to go up in even numbers only.
Is this because once you get to a certain size the machine screws are man enough that you just go down a size, or do I have the usual case where I have assumed that fixings will be available but they aren't?
M7, where are you?
Cheers
Dave R
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On 18/03/2017 14:28, David wrote:

A few on Ebay, though whether the pitch of the Thread is the same...
Seems M7 is used in split rim wheels, didn't know that ;)
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Sorry, scratch that, not in countersunk style there isn't :(
Bolts though :)
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On Sat, 18 Mar 2017 14:28:48 +0000, David wrote:

If they are holes, it doesn't really matter what screw you put through them. 1/4" is 6.35, they should work.
If they are threaded, then the internal diameter of 6.93 suggests (a loose but tolerable fit for) 8mmx1.
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On Saturday, 18 March 2017 15:12:04 UTC, Judith wrote:

For once she's right. Any woodscrew upto 6.9mm dia should fit.
7mm threaded holes does not mean M7, it means M8. 8mm is the outer diameter of an M8 bolt, not the inner, which is what your calipers are measuring.
NT
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On Sat, 18 Mar 2017 15:12:01 +0000, Judith wrote:

Thanks - not threaded just a metal plate with counter sunk holes for bolts or screws.
Cheers
Dave R
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On Saturday, 18 March 2017 17:39:33 UTC, David WE Roberts (Google) wrote:

then I don't understand why you'd want M7. Something's not right with this picture.
NT
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On Sat, 18 Mar 2017 10:50:39 -0700, tabbypurr wrote:

I measured the hole to see what the size of bolt would be to match.
The dimensions suggest that you can fit an M7 bolt through the hole.
Biggest is usually best, especially when fixing heavy stuff with leverage (bracket folds out from wall).
So I was opting for the chunkiest bolts I could fit through the hole.
Cheers
Dave R
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On 3/19/2017 12:03 PM, David wrote:

Yes but these brackets are usually fixed to a masonry wall. This implies wall plugs of some sort, and you can't fit these with perfect accuracy. So having holes in the bracket which are slightly oversize to the screws you use makes life very much easier. In any case, if fitted correctly what stops the bracket moving around is not the lack of diametral clearance between the bracket and the screws, but the friction between the back of the bracket and the wall. You get the friction from the tension in the screws. The bracket may be "resting" on one or two of the screws, it's not likely to be resting on all of them (unless they are countersunk).
ICBA to do the sums at the moment, but the shear strength of a single M6 bolt is something like a ton.
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On Sun, 19 Mar 2017 12:25:48 +0000, newshound wrote:

Just to add to other responses, the holes in the bracket are counter sunk.
It isn't about the shear strength but about spreading the load around the area of the nut on the back of the wood panel.
M6 looks O.K., but M7 would be nicer. Just not a lot of it about.
Cheers
Dave R
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On 3/20/2017 8:47 AM, David wrote:

Penny washer!
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On 20/03/2017 08:47, David wrote:

Drill it out to 8mm?
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M7 is going to be difficult to find. But M6 will be more than up to the job. CSK SS with an allen socket may look the best.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On Sat, 18 Mar 2017 15:19:19 +0000, Dave Plowman (News) wrote:

Ta.
There seem to be plenty of M6 so I will go for some of those.
Still not sure why nobody seems to use M7.
Cheers
Dave R
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On 18/03/2017 17:41, David wrote:

There's no demand for it because it isn't available. It isn't available because there's no demand for it.
Bill
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Bill Wright wrote:

It is quite popular in the motor trade M7 x 1.0 (same pitch as M6) I got asked to make quite few car bits over the years and so bought a set of taps and a split die for M7 (Iso-coarse).
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Bob Minchin wrote:

Hex head is normally M11 for these.
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On 3/18/2017 6:14 PM, Bob Minchin wrote:

It's also a 7 mm af Allen key, isn't it? I recall having to buy one for some VW brake callipers, iirc.
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The important thing is how they sit in the countersink.

Use two M3.5 electrical screws? ;-)
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While M7 is standardised at three different pitches it's not a preferred size, M3 M4 M5 M6 M8 M10 M12 etc are preferred sizes under ISO 262
The only place I've seen an M7 in widespread use is on the first motion shaft extraction thread on a GM 1980's gearbox where you could remove the end cover from the gearbox, remove a sheet metal cover plate underneath the clutch, press the clutch pedal, clamp the clutch and cover plate to the flywheel with u-clips, withdraw the first motion shaft, and unbolt the flywheel and clutch as one, drop it through the hole in the bellhousing and thus change the clutch without removing the gearbox / driveshafts etc. An easy 30 mins for a complete clutch change vs three or four times that after they changed the design to a solid bellhousing.
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