Equivalent threads imperial/metric

[donkmeister]

I have a vague recollection that there are certain pairings where a metric thread has a very close imperial equivalent. However, I may have confused that with the head drive sizes.

Does anyone know if there's a metric thread that is effectively the same as 1/4" BSPP, 19 TPI? I have a nut that fits onto such a die, the threads get cleaned but no metal removed, but it's from a modern Toyota so... I am perplexed.

I've not got my thread gauges out to verify yet but thought the hive mind might know! It's probably about M12 1.0, but not in quite.

[GreenV8S]

Obviously you know about the metric/imperial hex sizes that are close enough to share spanners.

I'm not aware of any threads that are close enough to be interchangeable. There's a big difference between a nut that is loose enough and close enough on pitch to thread on, and one that can actually fit well enough with the correct thread profile to tighten properly.

What are you actually trying to do? It seems unlikely you are trying to fit a nut to your tap.

[donkmeister]

Ignore me, mystery solved... I got out the calipers and thread gauge and can confirm t's an M14x1.5. And I now need to buy new fasteners as my friend has seemingly run a BSP die through my metric nuts

[donkmeister]

So, looking at it a bit online, the major diameter of a 1/4bsp male thread is 11.2mm, and the minor diameter of an M14 is 11.8mm. understanding that the die will be slightly oversize to allow for fitment, presumably the die is just about gripping the nut threads rather than engaging with them.

I'm trying to convince myself the threads weren't buggered up enough to damage the male thread when he put them back on after "cleaning" the threads. The nuts are cheap and easy, the bits with male thread are expensive and have a long lead time. My mate was adamant that no metal was removed.

[gazza285]

Taps are for female threads, dies are for male threads.

Sounds like there is enough difference in diameter for the difference in pitch to not bind up. The drill size for an M14 x 1.5 tap is 12.5mm.

[donkmeister]

Sorry, in my surprise at being told my modern Japanese car apparently had imperial threads I inverted taps and dies as frankly, my world was upside down.

[GreenV8S]

If all he's done is run a small tap through the nuts without any significant metal removal and then fitted and tightened the nuts, I wouldn't expect there's any risk of damage to the bolts/studs unless the nuts were torqued to the point of stripping the thread. I'd be more cautious if these studs were very highly stressed and had been done up to their yield point, but this doesn't feel like a dangerous scenario to me.

[E-bmw]

^^^^ Wot 'e said.

It shouldn't have done any damage if no metal was removed.

[Flibble]

donkmeister said:

I have a vague recollection that there are certain pairings where a metric thread has a very close imperial equivalent. However, I may have confused that with the head drive sizes.

Does anyone know if there's a metric thread that is effectively the same as 1/4" BSPP, 19 TPI? I have a nut that fits onto such a die, the threads get cleaned but no metal removed, but it's from a modern Toyota so... I am perplexed.

I've not got my thread gauges out to verify yet but thought the hive mind might know! It's probably about M12 1.0, but not in quite.

Just as a FYI, BSPP (and Whitworth) are 55° while metric (and US) threads are 60°, this alone is going to cause problems even if you found some that are close enough in diameter and pitch.