# metric thread questions

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#### werowance

##### Well-Known Member
HMEM Supporter
Hi, I normally work in standard threads not metric, but I am working with a set of metric plans that call for "tap m2" and cut "m4" threads.

well, my metric tap and die set are like this m2x1.5 or m4x1.75 or something like that. which i understand to be similar to 1/4x20 which means 20 threads per inch.

but when plans call for just m2 or m4 threads without the number of threads what does that mean? would it be m2x1.0 ? would the 1.0 just be a given?

sorry to be so ignorant.

#### werowance

##### Well-Known Member
HMEM Supporter
Thanks, after i just posted this i thought why not email the person i purchased the plans from and ask. hopefully ill be able to confirm what you just said after while.

thanks again.

#### Charles Lamont

##### Well-Known Member
HMEM Supporter
If the pitch is not given they will be the normal coarse series as in the link given above. The great majority of metric threads are coarse series. (In most of the world, metric is 'standard'.)

#### werowance

##### Well-Known Member
HMEM Supporter
(In most of the world, metric is 'standard'.)
you got me there, except in the USA, try getting metric drills here. or at least getting them at a "normal" price anyway.

would a better word have been "imperial" ?

#### xpylonracer

##### Well-Known Member
HMEM Supporter
If an M4 thread is called for it will be from the coarse series so M4x0.7mm, usually if metric fine or extra fine is needed the rate of pitch would be shown. The link above shows a good table of metric thread sizes.

Your example of 1/4x20 for number of threads per inch does not apply to the metric threads, the second number is the pitch of 1 thread so for the M4x0.7mm is 0.7mm.

xpylonracer

#### Cogsy

##### Well-Known Member
Staff member
Global Moderator
you got me there, except in the USA, try getting metric drills here. or at least getting them at a "normal" price anyway.

would a better word have been "imperial" ?
If you're new to metric threads you're probably not aware of drill sizing either, luckily it's very simple. You simply subtract the thread pitch from the major diameter and you get drill size. For example, M4x0.7 results in 4-0.7 = 3.3mm drill.

Everything I hear is that metric isn't easy to get in the USA, but in most of the rest of the world it is standard (and the world is much bigger than the USA ).

#### XD351

##### Well-Known Member
Depending on what you are making you could substitute the metric fasteners for equivalent inch sizes and use the taps you have or can get readily
With inch sizes the constant for distance travelled is 1 inch and the number of threads or turns per inch changes , with metric the constant is 1 turn of the thread and the distance travelled changes .
Basically opposite to each other .
I guess we are blessed here in Australia as we can get both readily although finding BSF / BSF is getting harder .
By the way what are you building ?

#### goldstar31

##### Well-Known Member
And BA- British Association threads are Metric and were in constant use by model engineers.------ and other mortals

N

#### django

##### Active Member
BA Threads are NOT Metric!!

#### rodw

##### Well-Known Member
I buy a lot of Metric bolts, around \$500 a month...

There are two common or standard metric threads:
Metric Coarse - the standard thread used on most bolts
Metric Fine - a finer pitch often used in motor vehicles
When you see M2 or M4 it refers to the coarse thread so M2 x 0.4 and M4 x 0.7 as per this table

Having said that, there are many other metric thread pitches in various specialist applications and some vehicles pick crazy thread pitches. One even has a M13 bolt which does not exist anywhere! I'm not looking forward to cutting a M10 x 0.9 to repair a coffee machine at the moment.

As said, selecting a drill size is easy, just deduct the pitch (which is in mm per thread) from the bolt size regardless of the pitch.

#### rodw

##### Well-Known Member
you got me there, except in the USA, try getting metric drills here. or at least getting them at a "normal" price anyway.

would a better word have been "imperial" ?
If you have a good to imperial drill conversion chart, you can usually get away with using a number drill for drilling holes to be tapped metric.

A good set of metric drills from 1mm to 10mm in 0.1mm increments also costs an insane amount even in metric countries.....

#### dalem9

##### Well-Known Member
HMEM Supporter
What ever you decide on make sure the screw are available . And yes I would use the taps and dies you have .

#### RM-MN

##### Well-Known Member
If you have a good to imperial drill conversion chart, you can usually get away with using a number drill for drilling holes to be tapped metric.

A good set of metric drills from 1mm to 10mm in 0.1mm increments also costs an insane amount even in metric countries.....
And besides being quite expensive, except in some very specific conditions the small increments are not necessary as the strength of the thread can be nearly the same with less engagement. That is, a hole drilled a small amount bigger than optimal may yield nearly the same strength but will be a great amount easier to tap. After breaking a few of my smaller taps using the correct drill size, I've started using a slightly bigger one and quit breaing taps yet get acceptable strength for the application. Note that I am not making car engines where the head is held on with bolts that are torqued to yield and require optimal thread engagement to have the required strength.

#### darco2

##### New Member
I like this debate.
I am from europe, I work in metric.

For your expensive sets of drills and screwdrivers at metric levels, because it's "exotic"
For ours, expensive of course, inch threads.

If you need to tap or die for occasional work, usually is enough and some from ebay.
Price is acceptable

peace
Richard from Slovakia

#### werowance

##### Well-Known Member
HMEM Supporter
Thank you all for the help. learned a lot. hope I didn't offend anyone with the "standard" comment. that's just what Ive always heard imperial referred to around where I live. I may just switch it to imperial threads anyway now that I know its coarse threads and my selection of screws is much larger in imperial than metric.

now if I can just find where to purchase a metric adjustable wrench at a decent price around here lol. :hDe:.

#### goldstar31

##### Well-Known Member
Thank you all for the help. learned a lot. hope I didn't offend anyone with the "standard" comment. that's just what Ive always heard imperial referred to around where I live. I may just switch it to imperial threads anyway now that I know its coarse threads and my selection of screws is much larger in imperial than metric.

now if I can just find where to purchase a metric adjustable wrench at a decent price around here lol. :hDe:.
As I passed my driving tests on a 1935 Morris 8 and a LE Velocetter, I am far more aware of the changes in fastenings rather than the actual 'design' which got all sorts of threads-- as well as these funny things called British Association which was developed from the Thury threads for watchmaking and general instrument work.

It get a bit confusing as that corner of Switzerland, the Jura and Franche Comte seemed to be mixed up with Pierre Vernier living in Ornans in the Doubs and was French but the place was Spanish rather like dear old Geordie Stephenson of Rocket locomotive fame who copied the Roman Imperial Chariot Gauge of four foot eight and a half inches wide.

That was before he could read and write!

It's not always what is imparted on poor unsuspecting apprentices
N

#### xpylonracer

##### Well-Known Member
HMEM Supporter
BA threads are not the same as Metric sizes but the original BA sizing was based on the metric system.
Starting at 0BA which is 6mm diameter x 1.0mm pitch as is M6 but BA threads are 47.5 deg and not 60 deg as Metric sizes.

xpylonracer

#### fcheslop

##### Well-Known Member
I seem to remember either on this site or on the web there is a comparison chart covering metric BA unf bsf and those funny american numbered threads and many more long lost froms that gives the nearest size and the various equivalents
Despite many years of metrication they still dont seem to have a standard and only confirms my belief its a French plot to make small things seem bigger
A tongue in cheek reply for the fun of it

#### goldstar31

##### Well-Known Member
Frazer, I have my 1941 copy of Machinery Handbook- where like the Folies Bergere- All will be revealed!

Bonnes Fetes

N