Because the eye does not see what the mind does not know…

jaymenon

Would someone knowledgeable please take a look at this picture, and tell me if this is likely to be a metric M9 thread please?

BillDL

With reference to the expressions used in the image above, the Major diameter of a closely toleranced M9 threaded bolt will be pretty close to 9mm, while the Minor diameter is usually around 7.65mm.  Obviously a nut is just reversed with the thread on the inside rather than on the outside like a bolt, so if you were to measure the inside diameter of a nut you will be measuring the Minor diameter and the threads in the nut are the Major diameter. The inside diameter shown for your nut (assumed to be the screw-on button part of a combined endpin socket for an electro-acoustic guitar) is shown to be 8.7mm, so somewhere between the Major and Minor of an M9, so at a guess it is most likely to be an M9 thread that somebody just measured the Minor diameter of the internal thread with a Vernier caliper and got 8.7mm.

0.34 inches (8.73mm) is exactly 17/50 of an inch as a simplified standard fraction.  As a standardised linear measurement that is closest to 11/32 of an inch.  As far as I know there are no Imperial UNC or UNF threads with a Minor diameter of exactly 0.34 inches, but there ARE some that come pretty close (UNC = Unified Coarse thread and UNF = Fine threads):

3/8-24 UNF with a thread pitch of 24 threads per inch and a Minor diameter of 0.33 inches
7/16-14 UNC with a thread pitch of 14 threads per inch and a Minor diameter of 0.36 inches

You also have to bear in mind that some metric threads can come as different Thread Pitches, although this is far less common than Imperial threads.  The thread pitch equates to how far in mm a nut will travel along a bolt or a bolt into a threaded socket for each complete turn. The common metric threads like M3, M4, M5, M6, and M8 have standardised thread pitches of 0.5, 0.7, 0.8. 1.0, and 1.25 respectively.  M9 isn't the most common thread in use, and I am sure it can come with a fine thread pitch of 1.0mm or standard 1.25mm or coarse 1.5mm, but I may be wrong with this.  I've forgotten a lot since I worked in an engineering factory years ago.

There looks to be a lot more threads inside that screw on end cap that doubles as a strap button than I recall seeing inside ones I've used, so if I'm correct about there perhaps being a Fine M9 thread pitch as well as a standard, then that could be a fine pitched thread.

Clearly this hasn't answered your question, but it's probable that you are more likely to have 3/8" or 7/16" old bolts or plumbing fittings to try in the thread than you will be to have an M9 bolt kicking around.  Take it to B&Q and burst open a pack of M9 bolts to test the thread

ICBM

It’s not M9. It’s the same thread as a Switchcraft jack or CTS pot, which are 3/8”x32 UNEF imperial threads. M9x0.75 is used on a lot of far-east jacks, and they are not compatible either way round - the M9 nut won’t fit on the 3/8”, and the 3/8” nut will go on the M9 but not tighten without the threads jumping.

BillDL

@jaymenon.   I presume you would be wanting a gold one if this is for the guitar you mentioned elsewhere that has gold tuners?

https://allparts.uk.com/products/end-pin-jack-stereo-can-be-tightened-from-outside-the-guitar-switchcraft?variant=660377665

jaymenon

ICBM said:

It’s not M9. It’s the same thread as a Switchcraft jack or CTS pot, which are 3/8”x32 UNEF imperial threads. M9x0.75 is used on a lot of far-east jacks, and they are not compatible either way round - the M9 nut won’t fit on the 3/8”, and the 3/8” nut will go on the M9 but not tighten without the threads jumping.

Very impressive and spot-on - thanks John (I found out the hard way

jaymenon

BillDL said:

@jaymenon.   I presume you would be wanting a gold one if this is for the guitar you mentioned elsewhere that has gold tuners?

https://allparts.uk.com/products/end-pin-jack-stereo-can-be-tightened-from-outside-the-guitar-switchcraft?variant=660377665

Thanks Bill - I need a Metric one, my Guitar has an LR Baggs strapjack plus which is strangely metric

ICBM

jaymenon said:

Very impressive and spot-on - thanks John

I did have to look up the imperial thread size - I just know them as US and far-east sizes . I've known that they don't fit each other since shortly after I started doing repairs forty years ago though.

Worse, since I posted that I realised that there are also far-east-made endpin jacks - I've got one in my box of spares, so I checked... and of course it has an M9 thread, actually very obviously smaller than the Switchcraft ones, and of course the nuts are not compatible either! I'm almost certain that the one in your pic is imperial, but it would probably be wise to check.

jaymenon

It's really impressive that you could deduce that from the photograph - indeed, I was expecting that you could do it.

...and therefore my title in the first place.