Imperial thread identification

[hotponyshoes]

Bought some cheap chinese crap but it looks like it's got imperial bolts in it?
Thread o/d is 0.245" (so 1/4" I presume)
Pitch is pretty close to a 1.25mm gauge.
What bolt will it be? Trying to replace with stainless button head or csk if I must

 

[Shox Dr]

UNC. Might be BSW

 

[hotponyshoes]

How do I tell?

 

[optima21]

1/4" Whitworth

 

[Hood]

Most likely to be UNC if it is Chinese (USA is a big market).
Main difference between UNC and Whit is the included angle of the thread, UNC is 60 degree (like metric) and Whit is 55 degree.

 

[Dr.Al]

Most likely to be UNC if it is Chinese (USA is a big market).
Main difference between UNC and Whit is the included angle of the thread, UNC is 60 degree (like metric) and Whit is 55 degree.

Definitely agree with all this. UNC and Whitworth have the same pitch (but UNF and BSF differ). Telling the difference between the angles is difficult but UNC is much, much more likely if it's come from anywhere other than the UK.

Having got frustrated trying to figure out threads, I made a big table of them so you measure the pitch (in mm or TPI) and the diameter (in millimetres) and look it up in the table to work out what a particular thread is. Doesn't help you tell the difference between UNC and Whitworth though - the best option for that is probably a thread pitch gauge and some very good lighting.

Table is here in case you're interested: https://www.cgtk.co.uk/metalwork/data/threadtable

 

[R Kraft]

https://en.wikipedia.org/wiki/Unified_Thread_Standard

 

[Hood]

... UNC and Whitworth have the same pitch .....

Mostly 1/2" being one of the differences, Whit 12TPI, UNC 13.

 

[Bullet2012]

1/4 UNC

 

[hotponyshoes]

Looks pretty close to a 1.25mm metric gauge?
Easier to see on the photo than with my actual eyes

 

[gordon stephenson]

!/4" whit or 1/4"uni, 20 TPI= 50thou= approx 1.25mm

 

[Hood]

View attachment 219426

Looks pretty close to a 1.25mm metric gauge?
Easier to see on the photo than with my actual eyes

20tpi, which Whit or UNC would be, is 1.27mm pitch and thus a 1.25mm pitch thread gauge will almost fit.

 

[pressbrake1]

Chinesium stuff tends to be based on yank threads cos they(the septics) can’t grasp metric

 

[Pete.]

If you're not sure run a 1/4" BSW tap through it then they'll both fit

 

[zx9]

Mostly 1/2" being one of the differences, Whit 12TPI, UNC 13.

I thought 1/4" was the odd one out in that it is the same pitch, most of the rest are different.

Edit: you can include BA2 (?) as well IIRC

 

[Hood]

I thought 1/4" was the odd one out in that it is the same pitch, most of the rest are different....

Nope, 1/2" is the one that is different pitch between UNC and Whit. There is another, at least, but it is up int the big sizes and afraid I can't recall which.

 

[hotponyshoes]

To be honest the bolt is so slack in the hole I think any 1/4" bolts might work. I was just going to tap out to m8 but the heads of the bolts will be too big.

 

[zx9]

Nope, 1/2" is the one that is different pitch between UNC and Whit. There is another, at least, but it is up int the big sizes and afraid I can't recall which.

I do apologise @Hood you are correct, now trying to remember what I was dealing with, the comparison between BSF and UNF possibly but that one does not have an exact pitch match anywhere that I can see at a glance.

 

[Hood]

Possibly NPT and BSP, a few the same but most different.

 

[Morrisman]

The mounting brackets on my TomTom bike GPS had 1/4 UNC screws in, which surprised me somewhat.