Hex key sizes and loose/tight fit FAQ

Or is it an Imperial size?

The worn end of my 4mm hex key measures 3.98 mm...

Thomas Prufer

Pretty close or a few 0.0n mm under.

Could it be a 3/16" imperial socket head?

Brand spanking new bicycle so I sincerely hope that it isn't imperial. "Designed in California, built in Taiwan" so I suppose there is a chance, but the threads on tappings seem to be metric, mainly M5.

Cheers

Dave R

I bought a towel rail recently, made in China. I needed to shorten the bars which were held in place by imperial hex grub screws. Bit of a pain but I did manage to find an old set of Allen keys.

I imagine that the manufacturer had a stock going back years, or bought someone's old stock cheaply.

We don't want any of that communist metric rubbish here in the land of the free?

We don't want any of that communist metric rubbish here in the land of the free?

I meant to add this

Is it #10 UNF? OD 0.192in, or 4.87mm, 32tpi is pitch of 0.79mm This is close to M5 if you are not expecting something else

Going back to O level Metalwork, I seem to recall there was a British Standard Cycle thread but I cannot recall any details about it. Would the yanks have used that especially for their fondness of using imperial units?

or they're making them for the American market.

Yep. 32 & 40 TPI But that's not relevant to this thread

Relevant in so far as if an imperial thread is used then presumably it would require imperial hex keys and might explain the OPs “ loose fit”

Having sourced a 4.5 mm hex key, I can confirm that the socket in question is a sloppy 4 mm, not a 4.5 mm.

Cheers

Dave R

But have you tried some imperial hex keys? Americans are weird. Expect the unexpected.

Tim

But its not a readily available size.

I measured my Allen keys with dial callipers & they all measure the stated size. In my experience if an Allen key isn't a snug fit, then it's the wrong size. HSS drill bits however are 0.1mm (or is that 0.01mm) less than their stated sizes.

Point of information. The republic of Taiwan (officially ROC) is not communist, has never been communist, and doesn't want to be communist.

The mainland Communist regime (officially PRC) thinks it owns Taiwan.

The people of Taiwan are justifiably nervous - especially given Ukraine.

However... back on topic... is it possible that particular socket is a US size, rather than Imperial?

Andy

I know, I was commenting on the "Designed in California,"

Or as I said,

"Is it #10 UNF? OD 0.192in, or 4.87mm, 32tpi is pitch of 0.79mm This is close to M5 if you are not expecting something else "

It could be a "security bit"? I know they use imperial hex screws on some temporary fencing, to discourage casual entry...

Thomas Prufer