Do you get "extra-coarse" M6 screws?

[Coryton]

I have a chair which has lost a couple of screws.

 

The existing ones measure 6 mm outer diameter, but an M6 screw won't go in.

 

Putting an M6 and one of the mystery screws side by side, the pitch seems to be a little coarser than M6.

Any ideas?

 

Do you get extra-coarse M6's?

 

Is there another "standard" screw with 6 mm OD but a coarser pitch?

 

0BA is also 6 mm OD, but it seems that the pitch is also the same as an M6 so it's not that.

 

[Gilbert]

Are you in Whitworth territory?

[Dick Turpin]

Sounds like 1/4 UNC

[kevinlms]

If it's a plastic chair, perhaps it's a screw designed for plastics.

 

https://engineeringproductdesign.com/knowledge-base/self-tapping-screws-for-plastics/

[DLT]

2 minutes ago, Gilbert said:

Are you in Whitworth territory?

Sounds like it!  Could well be 1/4 inch BSW (British Standard Whitworth)  Not sure where you would get them nowadays, Engineering suppliers?  Furniture restorers?

[didcot]

1/4 BSW is 20 TPI ( Teeth per inch) which equates to a 1.27mm pitch.

A standard M6 pitch it 1mm, whilst fine is 0.75mm from memory. 

[Jeremy Cumberland]

1/4 UNC (Unified) and 1/4 BSW (Whitworth) are both 20 TPI, and nuts and bolts are basically interchangeable in non-critical situations. Most likely what you have is one of these.

 

1/4 BSF is 26 TPI, which is a little finer than M6, and 0BA has a 1 mm pitch, the same as M6, as you say.

 

There are a large number of other thread forms, and perhaps furniture makers used to use something else (just like bicycle makers used to), but if they did they must have kept it fairly quiet since I have never encountered anything of the sort.

Edited January 8, 2023 by Jeremy C

[Coryton]

 Thanks for all the comments.

 

It's a modern chair (but no plastic). The screws go into metal inserts in a wooden leg - surprisingly robust but not much use when you don't have the screws.

 

I'm pretty sure it's not 1/4 inch anything - I measured the OD with digital calipers (not a ruler) and it came out very close to 6 mm.

 

 

[spamcan61]

I've used Stig Fasteners in the past for buying small quantities of nuts and bolts with old school threads.

 

https://a2stainless.co.uk/shop/

Edited January 9, 2023 by spamcan61

[Dbr1295]

There are metric coarse threads, M6 x 1.0 pitch if that's any help

[Coryton]

10 hours ago, Dbr1295 said:

There are metric coarse threads, M6 x 1.0 pitch if that's any help

 

There are indeed, but it seems to be even coarser than that.

 

(And while I've never thought about thread pitch when ordering metric screws I think coarse threads are what you get when you don't ask for anything special.)

[tomparryharry]

The Chinese sometimes make old-style nuts & bolts. Some of their best solar collectors still use BSW thread forms.  3/16" Whit, perchance?

 

Or, a BA thread?

[Kickstart]

On 09/01/2023 at 09:51, Coryton said:

 

There are indeed, but it seems to be even coarser than that.

 

(And while I've never thought about thread pitch when ordering metric screws I think coarse threads are what you get when you don't ask for anything special.)

 

Sometimes varies, or even the wrong item is packed (had that with some titanium replacement nuts!). Do you have a thread gauge?

 

All the best

 

Katy

[Oldddudders]

Sorry, but Extra-Coarse M6 Screw sounds like a lady you might not want to meet at Keele Services. 

[Gilbert]

7 minutes ago, Oldddudders said:

Sorry, but Extra-Coarse M6 Screw sounds like a lady you might not want to meet at Keele Services. 

But you can get a Big Mac meal....