Identify a screw (bolt?) please

[StephenOak]

I am trying to repair one of a pair of chairs, where one of the screws has come out and been lost.
The underside of the chair that is okay looks like this

and the screw looks like this

I thought it looked a bit like a roofing screw, close enough that I could use them. However the thickest roofing screw I can find is 6.3 mm (needing an 8mm driver) and the one I need is c. 10mm in diameter (needing a 13mm driver).

Any idea what this is and where I can get hold of some of them?

 

[JohnD]

coach screw

(not a coach bolt)

aka carriage screw

 

[conny]

Some timber yards have accessory shops attached to the yard or you could try a builders merchant. Alternatively try Amazon/Ebay.

 

[StephenOak]

@JohnD @conny Thanks very much.

I have found someone via Amazon (who I have used before as it happens) who sells these in small quantities. However I am a bit confused as to the size I need. The M10 says 'Spanner/ Wrench Size- 17mm' and the M8 says 'Spanner/ Wrench Size- 13mm'.

The ones I have need a 13mm socket, however the bolt diameter to the top of the threads (which is what I think is where it is measured) is 9mm. For completeness, the diameter on the bolt body is 7.8mm. Any idea whether this is M8 or M10?

As it is a screw going into wood it does not have to be spot on, but I want it close.

BTW, I wondered if this might be an imperial screw (the chairs are old) but the head is clearly 13mm and is just too big for a 1/2" (12.7mm) spanner.

 

[JohnD]

M8 is a thread size of a bolt that fits a nut and goes through an 8mm hole.
The head size of a metric nut or bolt is always bigger than the nominal size of the bolt, and it doesn't matter to your job what size or shape the spanner is.

I don't know how you define metric wood screw sizes. I think it's the size of the unthreaded shank.

The holes may have worn loose so a slightly bigger screw may fit tighter.

 

[Burnerman]

Its an M8, Stephen....its the same sizing that's applied to metric screws and studs.
Your tapping size, if you ever use these screws from scratch, is equal to the diameter of the root of the thread - somewhere in the region of 6.5mm.
John

 

[JohnD]

might be this

edit
root, eh.

 

[StephenOak]

Cheers both, I'll order some M8 screws.

 

[conny]

Definitely M8 size required.
As you say, the hole may be slightly worn but that is no problem. There are a number of ways to correct this type of problem.
You can cut a suitable length of dowel to fit the hole snugly, glue it into position, cut it flush and then drill a pilot hole for the new bolt. (6.5mm in this case should be suitable)
Or you can simply pack the hole with matchsticks, (make sure you strike them first or cut the match heads off!), and then screw into these. I'm not keen on this method as the wood is very soft and will compress on itself rather than give a good grip on the screw.

 

[Chud]

Definitely M8 size required.
As you say, the hole may be slightly worn but that is no problem. There are a number of ways to correct this type of problem.
You can cut a suitable length of dowel to fit the hole snugly, glue it into position, cut it flush and then drill a pilot hole for the new bolt. (6.5mm in this case should be suitable)
Or you can simply pack the hole with matchsticks, (make sure you strike them first or cut the match heads off!), and then screw into these. I'm not keen on this method as the wood is very soft and will compress on itself rather than give a good grip on the screw.

I tend to go for wooden cocktail/tooth picks instead but dip them in some pva wood glue first.

 

[foxhole]

Definitely M8 size required.
As you say, the hole may be slightly worn but that is no problem. There are a number of ways to correct this type of problem.
You can cut a suitable length of dowel to fit the hole snugly, glue it into position, cut it flush and then drill a pilot hole for the new bolt. (6.5mm in this case should be suitable)
Or you can simply pack the hole with matchsticks, (make sure you strike them first or cut the match heads off!), and then screw into these. I'm not keen on this method as the wood is very soft and will compress on itself rather than give a good grip on the screw.

Cocktail sticks better for packing as hardwood .

 

[FrodoOne]

Matches, cocktail sticks, tooth picks and PVA glue etc are all well and good but quite "old school" - and we have moved into the 21st century.

If you really want a decent renewed start up "hole" into which a new "coach screw" may be inserted, coat the inside of any existing "stripped" hole in the woodwork concerned with one or two coats of a (two part) Epoxy Resin, to which you may wish to add a suitable filler (such as fiberglass, sawdust or talcum powder) in the second coat.
(Yes, this takes time for the Epoxy resin to soak in and "set".)

When the resin has soaked into the timber and "set", you will have a much stronger base of a slightly smaller diameter than that provided by the original stripped timber.
The new resulting "Epoxy Resin" reinforced timber joint will be much stronger than the original raw timber/coach screw joint.

 

[StephenOak]

Thanks for all the help. Well, despite the confident replies (from usually reliable people) that this was an M8, it wasn't!


M8 below the original; Original in existing hole; M8 much deeper in existing hole.

So I bought some M10 screws (mail order) but other things got in the way and I only fixed it the other day. To reinforce the holes I used some two part wood filler and used a scrap of wood to make a conical hole in that, so the threads had something the right shape to bite into.


M8, original, M10. Sorry this is fuzzy, it looked okay on my phone but on a large screen it clearly isn't.

The head is the same size as the M8, but the body seems just a bit smaller than the M10.

 

[JohnD]

They look quite modern, I wonder if they were made for the American or Liberian market?

Or, perhaps, somewhere like India that might have some old stock?

I wonder if the head size is 13.34mm which is 1/4" Whitworth spanner (and, amusingly, nowhere near 1/4", but a bit bigger than 1/2")

9mm is almost 3/8"

 

[Burnerman]

Maybe the coach screw was 11/32" imperial, Stephen......who knows? Odd ball sizes do show up from time to time.
I'm finding Citroen are increasingly using 7mm screws in the crankcase which is almost unheard of - but there we are.
John