Identifying a thread
This article is a straight cut and paste of a very useful description posted by Dave Baker on identifying threads - and in this particular case a 14mm thread. The original question:
"Need to find out what this thread is - it's on an air temperature sensor for a car. Its dimensions (male) are:- Thread outside diameter Threads per thingie metric 13.8mm 1.5 imperial 0.54in 16 tpi"
After 30 years of engineering and accumulating hundreds of tools, thread gauges are something I've never felt a pressing need for. Provided you have an accurate steel rule or preferably a digital vernier and know a few basic facts it's pretty easy to identify most threads. Even if diameter and pitch are similar you always have thread angle to help distinguish types. Occam's Razor says that most threads you'll come across in anything modern are metric so it's best to check for those first.
O/d will always be a tad under nominal so the bolt doesn't bind in the nut or female thread. For pitch I measure crest to crest over 10 threads and divide by 10. Hopefully it'll be a nice round number of mm or at worst a half mm. You can use a digital vernier by eye across thread crests to at least 0.25mm (10 thou) or better so you can work to an accuracy of 1 thou per thread which is ample. Easiest way is to preset the vernier to the expected measurement i.e. 15mm if you think it's a 1.5mm pitch rather than trying to adjust it manually to line up with the crests. It's actually even quicker with a steel rule marked in 0.5mm increments. If that lines up cock on over 10 pitches and the o/d is just under an integral number of mm then usually you're done.
A very useful set of things to have handy is one each of nice new non-rusty bolts in 6mm, 8mm, 10mm and 12mm sizes. The thread pitches on these will be 1mm, 1.25mm, 1.5mm and 1.75mm respectively. That covers most of the normal pitches you'll ever encounter in metric threads apart from the very small sizes of 5mm and under and the big stuff. If you can mesh the appropriate bolt with the teeth of the one you're trying to identify and you can't see any gaps against a good light then job's a good un. The o/d of the bolt is immaterial - for a given pitch the thread form will be exactly the same on any bolt.
While I'm on the subject of metric thread pitches I guess I could cover how these really work. The standard pitch for any size metric bolt is always a coarse thread. In fact the coarse pitches were taken pretty much from similar sized UNC bolts which were designed for threads in coarse grained weak brittle materials like cast iron and cast aluminium. Finer pitches like UNF are used in finer grained stronger, or more ductile, materials like steel, forged aluminium, brass, bronze etc and there are metric fine pitches for similar use. However you never see anything coarser than the standard coarse metric pitch even though there may be many finer variants for each size. You'd only need an even coarser pitch for even weaker materials like wood and then either you'd actually use a wood screw or drill right through and use a bolt and nut.
Finer pitches create a stronger bolt because the core diameter of the bolt is larger i.e. the pitch depth of the thread is smaller in exact proportion to its length. Bolts that take very high loads such as conrod big end bolts, flywheel bolts etc will always be fine pitch with as big a core as possible. Common sizes for these are 8mm, 9mm and 10mm all of which use a 1mm pitch.
The 60 degree tooth angle on metric (and also UNC and UNF) bolts was very cunningly designed for easy use. The top and bottom of the threads are rounded to avoid stress raisers and in such a way that if you deduct the thread pitch from the nominal o/d you always get the tapping drill size which therefore, near as dammit, will also be the core diameter of the bolt. For example the tapping drill for an 8mm x 1.25mm standard coarse bolt is 8-1.25 = 6.75mm so it's an easy calculation to do in your head and no need to carry a Zeus book around in your pocket all the time. This applies equally to UNC and UNF except that you have to do more calculations because the pitch is shown in tpi not length.
For fine pitches there can be many variants but the smallest fine pitch you're ever likely to see is 1mm. There are in fact fine pitch variants even for bolts of 5mm and smaller but you'll never come across them except in very specialised equipment.
Your 14mm bolt is quite a good example. The standard coarse pitch is 2mm but by far the most common thread in that size is actually 1.25mm which is the standard spark plug size fitted to nearly every petrol car engine ever made. Only the 18mm x 1.5mm Ford Pinto engine plug and the more recent 12mm, 10mm and even 8mm plugs fitted to motorbike engines and the like will be different. There are also 1.5mm and to a lesser extent 1mm pitches in common use on the 14mm diameter.
Now here's a thing and to be honest I've only just thought of it. I said above that coarse pitches are best used in coarse grained and weak materials but all spark plugs are fine pitch and yet they screw straight into the cast iron or cast aluminium of the cylinder head. However when they seize they strip the threads right out of the head which probably wouldn't happen if they were standard coarse pitch better suited to the material they screw into. Of course it's far too late to change that now because it's an industry standard but probably not a very clever one. I digress.
Non metric threads
Once you've ruled out metric then the job gets easier. Imperial threads don't generally have variants. UNC is the standard imperial coarse and UNF the standard imperial fine. On old machinery you might come across Whitworth which is an early coarse or its fine equivalent BSF. Both of these have 55 degree thread angles not 60 degree ones. BA can also be found on electrical gear but it's easy to spot because of the very pointy 47.5 degree thread angle. It won't come close to matching a 60 degree metric bolt even though the diameter and pitch can be quite similar. You can actually screw many BA bolts into metric female threads but not vice versa.
For hydraulic fittings it's usually NPT or BSP. These are quite distinctive and often tapered.
Since Dave's original article above was published, some addition contributions have been added:
For more information on American thread forms see here