Can someone explain M8 vs M10 vs M12 to me?

[BaltimoreBimmer]

From my understanding an M8 bolt means that when you measure the flat of the hexagon head you get 8mm. Thus, and M8 is a hexagon top with 6 sides each measuring 8mm in length.

But now I am questioning myself. Does M8 actually mean that it's 8mm from the middle of one of those flat sides across the bolt head to middle of the opposite side?

I'm thinking it would be the later no? B/c how else would it make sense that there are no M9s or M11s for example when we clearly have 9mm and 11mm sockets....

I found this diagram in a textbook I am reading and still seem to have questions (b/c this one makes it look like you measure point to point across the head as opposed to flats)

Can anyone chime in and explain to me what is the correct way? I feel pretty stupid right now....



(http://imgur.com/FKUJVVN)

[alistairolsen]

Ok,

M = Metric

8 = 8mm

The 8mm is the wire diameter before the thread is cut. So it is the outer diameter of the threaded section

Thereafter there are a series of standards (ISO and DIN) defining other things such as:

Head size - The 'across flats' or AF dimension which is how some imperial bolts were defined is generally 13mm for M8, 15 or 17mm for M10 and 19mm for M12. Beware however as the germans and japanese like to reduce these to 12, 14, 16 and 18mm.

Thread depth and pitch - These should be quoted, like M8x1.25 but generally they aren't. There is a coarse and fine pitch for each size

https://www.boltdepot.com/fastener-information/measuring/metric-thread-pitch.aspx

The thread depth is related to the pitch by the thread form



This Wiki link gives a lot of info on standard head sizes and so on

https://en.wikipedia.org/wiki/ISO_metric_screw_thread

Some of it may seem a little funny and backward compared to imperial, but the major advantage is that in Europe you now only have one system rather than AF, Whitworth, UNC, UNF, BSF, BSP......

Hope that helps

[BaltimoreBimmer]

thanks. Just realized I missed a key sentence in my book lol:

The size of a metric bolt is specified by the letter M followed by the diameter in millimeters across the outside (crest) of the threads

this makes sense, b/c it applies to all metric bolts regardless of the head style.

[alistairolsen]

Yeah exactly. On the Wiki link you'll see a table showing exactly that.

One standard defines the threaded portion and then separate linked standard define the head geometry for cap heads, hex heads etc.

[BaltimoreBimmer]

Yeah exactly. On the Wiki link you'll see a table showing exactly that.

One standard defines the threaded portion and then separate linked standard define the head geometry for cap heads, hex heads etc.

yeah, I'm learning about the various aspects of fasteners and how there are coarse vs fine threads and so forth. interesting stuff. especially how everything is separated between metric and imperial (eg: pitch being either threads per inch or mm between crests and so forth.)

[JGood]

While this wouldn't specifically answer your previous question, it is a great free resource on the topic of hardware specifications, torque, stresses, etc...

http://www.pirate4x4.com/tech/billavista/NutsandBolts/Nuts&Bolts_signed.pdf

And this book is well worth the money:

https://www.amazon.com/Fasteners-Plumbing-Handbook-Motorbooks-Workshop/dp/0879384069

[alistairolsen]

yeah, I'm learning about the various aspects of fasteners and how there are coarse vs fine threads and so forth. interesting stuff. especially how everything is separated between metric and imperial (eg: pitch being either threads per inch or mm between crests and so forth.)

Basically what happened when standardisation begain was Britain had a whole heap of different fasteners for different uses

https://en.wikipedia.org/wiki/Britis...dard_Whitworth (1841)
https://en.wikipedia.org/wiki/British_Standard_Fine (1908)

https://en.wikipedia.org/wiki/British_Standard_Pipe

These were used on everything, all over the world due to the empire

The USA then defined the United States Standard thread which lead on to UNF, UNC, NPT, and moved from 55 degree threads to 60 degrees and a flat tip because they were easier to machine.

The british assosciation screw threads and SAE threads were developed too.

Europe developed the metric system using the same 60 degree angle and standardised it in 1898

The first world war lead to lots of study and development into fasteners and the second highlighted a lack of compatability and resulted in Britain adopting UNF/UNC and so on.

In 1947 ISO was founded and in 1960 SI units were defined and the UK adopted the metric system (sort of) in 1972

The US still uses a lot of imperial threads even on new stuff, whereas in the UK, unless you own a pre 70s classic, you dont even see them anymore.

[TobyB]

The US still uses a lot of imperial threads even on new stuff,

Yeah, and frustratingly, it's STILL a hodgepodge, 50 years into 'Metrification'.
I have a 1968 Datsun- Metric engine fasteners, BS (that's British Standard)
body fasteners, and a little Whitworth just to make life truly hell.

I had a 1980 Chebby Monza- Metric BODY, SAE engine and trans, and a few places
where neither would work.

Almost all domestic cars are metric now, but seatbelts continue to have SAE head sizes,
and every industry's different. Construction is still firmly in the "Feet and Inches" camp.

Ironically, the best threadform to date seems to have been Whitworth, tho the dimensioning of it wasn't the most straightforward.
The radiussed roots are very good for strength, and the 55 degree pressure angle's good, too.

t
rambling, I guess.

[alistairolsen]

Yeah, and frustratingly, it's STILL a hodgepodge, 50 years into 'Metrification'.
I have a 1968 Datsun- Metric engine fasteners, BS (that's British Standard)
body fasteners, and a little Whitworth just to make life truly hell.

I had a 1980 Chebby Monza- Metric BODY, SAE engine and trans, and a few places
where neither would work.

Almost all domestic cars are metric now, but seatbelts continue to have SAE head sizes,
and every industry's different. Construction is still firmly in the "Feet and Inches" camp.

Ironically, the best threadform to date seems to have been Whitworth, tho the dimensioning of it wasn't the most straightforward.
The radiussed roots are very good for strength, and the 55 degree pressure angle's good, too.

t
rambling, I guess.

We're not quite that bad, most stuff is metric now, they just pick a new head style every 5 mins (hex, torx, spline, Ribe....)

Seatbelts are an odd one, theyre always 7/16s UNF. I assume someone mandated it once upon a time after some testing.

Agree on whitworth, but tremendously expensive to manufacture. So for normal purposes you just use a larger fastener and where you need increased strength in a small size (fastener replacement, or weight critical applications) you wind up with stuff like rolled thread forming and heat treatment.

Things like landrovers are a hodge podge here with fasteners.