Are standard "M4" nuts & bolts normally fine or course pitch?

Hello

When one buys some "M" system of metric nuts and bolts, by default are they normally fine or course pitch?

e.g. If you purchase some standard "M4" bolts, the diameter of the outside of the thread should be about 4.0mm... but will the pitch normally be 0.7mm ("course" ) or 0.5mm ("fine" ) per revolution?

Background
I need to standardise all my nuts & bolts to the metric system. And I want them all to fit each other. In particular I also want to buy some (internal) metric taps that will work well with my nuts & bolts, so for example I can drill a hole and put a thread into it.

With thanks

J

Edited By John Smith 47 on 12/01/2022 23:10:55

Edited By John Smith 47 on 12/01/2022 23:11:09

Any metric thread is the coarse pitch unless you specify otherwise. Or it should be: cars for example are littered with M12x1.5 threads instead of 1.75 which would be 'normal'

Good ol’ Wikipedia puts it thus:

[quote] A metric ISO screw thread is designated by the letter M followed by the value of the nominal diameter D (the maximum thread diameter) and the pitch P, both expressed in millimetres and separated by the multiplication sign, × (e.g., M8×1.25). If the pitch is the normally used "coarse" pitch listed in ISO 261 or ISO 262, it can be omitted (e.g., M8).^[4]: 17 [/quote]

Which is a slightly convoluted way of confirming that if you just buy M4 it will [or at least should] always be coarse thread.

MichaelG.

Ref. https://en.wikipedia.org/wiki/ISO_metric_screw_thread#Preferred_sizes : **LINK**

OK now I'm rather bemused. I have a collection of nuts and bolts that I've bought over the years. I have always tried to buy metric, but now that I measure some of them at c. 4mm diameter, the actual diameter seems measure c.3.86 to 3.89mm, but I can't get the nuts to fit onto my "4 x 0.7" metric tap.

Which seems more likely: Have I accidentally bought Imperial nuts & bolts (i.e. "5/32 inch"?), or are are they fine not coarse pitch?

J

EDIT: As a rule of thumb, how much smaller should the hole that I drill be if I am going to tap it? At these sizes would 0.5mm smaller suffice?
e.g. If I am going to fit a M4 bolt should I use a 3.5mm drill?
And if I am going to fit a M5 bolt should I use a 4.5mm drill?

Edited By John Smith 47 on 12/01/2022 23:46:36

Use a cocktail stick or similar piece of wood that can be put into the nuts to get an imprint of the thread and measure what you have got. The last part is a but tricky but should give some idea of the pitch.

Martin C

With the metric range of threads if you deduct the thread pitch from the thread diameter you will get the tapping drill size.

So for your M4 drill will be 3.3mm and the M5 tapping drill will be 4.2mm.

However you may be happy with less engagement and go slightly larger on drill size but I always keep to the calculated size on threads up to 8mm.

Emgee

Hi John,

Harold Hall has a very good page on tapping drill sizes, and he gives various depths. You can get data for both Metric, BA and ME threads. I use about 75% for softer materials and somewhat less for harder materials.

Thor

With metric threads I have always subtracted the pitch from the diameter to give the tapping size rounded up to the nearest 0.1. Works fine for most materials with the accuracy of small drills generally giving, at least me, a slightly bigger hole, with harder materials I do increase the diameter slightly.

Most metric threads come in both coarse and fine, and sometimes extra-fine as well. They're all standard.

In ordinary life, what you happen to buy or salvage depends on the purpose for which the fastener was made. Purpose drives choice. Coarse threads are quick and easy to assemble, but weaker, so most of the time they are preferred for ordinary work Fine threads are stronger and less likely to vibrate loose, but are more likely to cross-thread on assembly. As fine threads slow the job down they're avoided unless specifically called for.

Fine threads tend to be found on precision gear, such as instruments and whenever coarse threads aren't 'good-enough'.

DIY store threads are almost always coarse because they're sold as general-purpose fasteners. Ironmongers, convenience shops, and such are in the same class. An exception is box sets, which sometimes contain a mix because they're sold as spares. Otherwise, threads from these places are probably coarse.

Engineering sellers carry both coarse and fine threads, and expect the buyer to order what he needs. They too sell spares as mixed box sets. Buying online, I always check the pitch.

Properties like strength and corrosion resistance etc. are often important in engineering applications. DIY store nuts and bolts are made to the lowest standard and zinc-plated because there's no reason for general-purpose fasteners to be wonderful. They're plenty good enough for most jobs. But beware! They're no good for brakes, aircraft, load bearing structures, and anything else safety critical. For these purposes it's vital not to substitute ordinary fasteners for the proper item. Likewise, not smart to mix brass, steel, stainless or nylon just because they happen to fit together!

Dave

John,

Don't forget the other variable may be the tap, speaking as someone who has bought a new tap from an English supplier at a show, that looks like a tap, but fails to make any kind of thread at all.

For tapping hole sizes buy a Zeus book. Nuts and Bolts from the like of Big Box DIY stores tend to be loose fit, whereas automotive stuff is a much tighter spec.

Misread initial post

Edited By Mick Berrisford on 13/01/2022 11:25:12

Coarse

regards Martin

I just used a Zeus book throughout my working life & still do.

Tony

Edited By Frances IoM on 13/01/2022 14:55:06

A simple rule of thumb for tapping sizes of Metric threads is:

Tapping size = (nominal - pitch )

so M4 x 0.7 Tapping size = 3.3 mm

M5 x 0.8 = 4.2 mm

M6 x 1 = 5 mm

and so on.

Metric is a 60 degree thread so the maths will confirm what most people do AND Zeus charts for sizes above

1.6 mm coarse threads..

The same logic is applicable to any 60 degree thread, such as UNF, UNC or Cycle.

You REALLY should buy a set of Zeus charts, it will contain the answer to a lot of the questions that you are going to ask, before you ask them!

You will consult them from time for the rest of your life. (You should see the state of mine, bought in 1958 and used ever since! )

Howard

You could have BA mixed up with them.

A word of caution. One of the members of the railway society was making some parts for a university project and he asked me for a couple of extra nuts to fit the bolts he had purchased from one of the major DIY stores (B&Q). I got him a couple of nuts from the back of my van (M5 x 0.8, metric coarse), only to discover that they wouldn't fit. Checking the packet it turns out that the items he had purchased were M5 x 0.9. Try finding them anywhere else.

Martin

While not a substitute for Zeus, an android app called 'Thread Data' is pretty handy if you have a tablet computer kicking around your workshop; I also use 'Fasteners 1.0' which gives dimensions for bolt heads, nuts, etc, and 'Triangle Calculator', which does what you'd think and saves all that business with Some Old Hag//Cracked All Her//Teeth On Asparagus

No Andy!

"Some People Have, Curly Black Hair, 'Til Painted Black"

Howard

That's a new one on me! I had to google it to find out what P and B stood for, and of course found many alternative mnemonics. I feel I should have known this one -

“Some Old Hippy Caught Another Hippy Tripping On Acid.”