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

[David]

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

[Lee]

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 ;)

[Lee]

Sorry, scratch that, not in countersunk style there isn't :(

Bolts though :)

[Judith]

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.

[Dave Plowman (News)]

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.

[tabbypurr]

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

[David]

Thanks - not threaded just a metal plate with counter sunk holes for bolts or screws.

Cheers

Dave R

[David]

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

[tabbypurr]

then I don't understand why you'd want M7. Something's not right with this picture.

NT

[Bill Wright]

Use M6. It's a telly not the QE2.

Bill

[Bill Wright]

There's no demand for it because it isn't available. It isn't available because there's no demand for it.

Bill

[Bob Minchin]

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).

[Bob Minchin]

Hex head is normally M11 for these.

[Dave Plowman (News)]

The important thing is how they sit in the countersink.

Use two M3.5 electrical screws? ;-)

[F Murtz]

They are clearance holes for m6 screws or 1/4 screws

[David]

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

[David]

The bracket is built like the QE2.

My main concern is the leverage when the bracket is fully extended with a TV on the end. It folds out quite a long way.

Cheers

Dave R

[newshound]

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.

[Dave Plowman (News)]

It is almost certainly designed for wall mounting using screws and wall plugs. With these, the larger the better. So the plug doesn't pull out of the bricks. But the chances of shearing an M6 nut and bolt, remote. The fixing of the bracket to the 24" TV back here is 4 x M4.

I'd say your main problem in a caravan is finding something strong enough to fix the bracket to - rather than the strength of the bolts.

[newshound]

It's also a 7 mm af Allen key, isn't it? I recall having to buy one for some VW brake callipers, iirc.