Can someone explain metric bolt designations?

[autobonrun]

I understand how to convert say 10 mm to inches but the M designations have me confused.

I have a table that gives torques for each M designation bolt. Does this mean that an M8 bolt regardless of the use should be torqued to that amount? I seem to recall seeing different torques for the same M designation bolt in different applications. I can't believe an M8 bolt inside an engine, bathed in oil, would require the same torque as one in a dry application.

Also, is the M series bolt only available in one thread pitch? I did find some discussion of thread pitch but it is unclear to me.

In trying to find some of these answers, I stumbled across an excellent website that contains lots of good info on everything from bolts to types of stainless steel. I thought you may find it a good resource.

http://mdmetric.com/fastindx/inxtst.htm

[Zeke]

Metric fasteners are designated as XX mm x X.X thread pitch. Example: 10Mx1.5 or 12Mx1.75. The grade is stamped on the top, 8.8 being the approximante equivalent of hardware grade bolts, 10.8 the prox = to grade 5 and 12.8 prox = to grade 8. All the socket head bolts I have seen for sale even at The Home Defect are 12.8. That grade is harder to find in a hex head bolt. Don't use less than 10.8 in any steering or brake componet application. Same for motor mounts, etc.

[TimT]

The markings 10.9 8.8 etc are markings that signify which grade or strength the fastener is. The different grades tell you the tensile strength of the piece..

The M wakes you up and tells you its metric.

Regarding torque to apply to fasteners. The people that publish the spec usually have a coefficient of friction they use to make the standard..... i.e. lightly oiled threads..

The torque to tension a same diameter fastener is the same if it is an inch long, or a foot long..

lookhere

[ChrisBennet]

Originally posted by Zeke
Metric fasteners are designated as XX mm x X.X thread pitch. Example: 10Mx1.5 or 12Mx1.75. The grade is stamped on the top, 8.8 being the approximante equivalent of hardware grade bolts, 10.8 the prox = to grade 5 and 12.8 prox = to grade 8. All the socket head bolts I have seen for sale even at The Home Defect are 12.8. That grade is harder to find in a hex head bolt. Don't use less than 10.8 in any steering or brake componet application. Same for motor mounts, etc.

I thought 8.8 approx Grade 5?

[cstreit]

To carry it further... I believe (and I'm SURE someone will correct me if I'm wrong) The pitch is the amount the bolt will move in mm during one full turn of the bolt. So a pitch of 1.5 will move 1.5 mm during one turn

[autobonrun]

Now it makes sense. I knew you guys would know. Thanks for the information and the links. Sounds like I should use specific torque settings rather than a general one since my bolts typically are less than 1". Perhaps a bolt like the one that holds the bottom of the shock would be an example of where the general torque setting could apply.

[jim72911t]

Or, to put it another way, the pitch is the distance in mm (not Motor Meister!) between threads. i.e: M5 x .8 is a 5mm dia. bolt with .8mm between threads. Thus, one turn moves the bolt .8mm.

Jim

[howie944]

O.K. I understand the metric thread sizing but the torque value is something I am a bit confused on. On a bolt is the torque value the value of maiximum tensile strength before the bolt either strips or deforms/breaks (just the bolt itself) thus the maximum application a bolt can be used for? Whereas the torque value given say for engine assembly is the max value given to hold the fastner correctly and apply the correct force to the parts being joined?

[ChrisBennet]

Raceware includes some interesting info with their fasteners:

From Raceware instructions
Torque Friction & Fasteners
(2nd page)

When you use a torque wrench to install a threaded fastener, the wrench is
actually used to apply and measure a force. The force is designed to
PROPERLY tension the fastener so it can perform it's job properly. If the
fastner is under or over tightened, it will prematurly loosen or fail.
Torque wrenches do not measure TENSION, they measure TORQUE. In the case of
a threaded fastener, torque is the radial force applied to overcome
friction in the threads and to streach or tension the fastener.

In quality threaded fasteners as much as 80% of the torque applied to
tighten them is used to overcome the friction in the threads. Only about 20%
of the force is used to actually tension the fastener. That means that any
change ins the FRICTION has DRAMATIC effect on tension. All quality
fasteners have specific installation instructions that dictate what
lubricant if any should be used when tightening the fastener. Lubricants
such as motor oil, anti-seize, moly lube, cam lube, STP(tm), ect, ALL have
different friction characteristics.

USING THE WRONG LUBRICANT CAN CHANGE THE THREAD FRICTION BY AS MUCH AS
30%, CAUSING A FASTENER TO BE IMPROPERLY TENSIONED.

Thread design and pitch also have a great effect on Torque and Tension.
Fine threads exert a greater tension on a fastener for the same amount of
Torque applied. This is because of the angle on the threads is smaller and
therefore the 'wedge" effect is greater. Thus if you tighten a fine thread
fastener to the same torque as a coarse thread fastener, the fine thread
fastener will have a higher tension, and clamping force. (clamping force is
the force created by a fastener that holds two or more parts together.) Due
to the difference in friction and "wedge" effect, fine threaded fasteners
are often tightened to a LOWER torque the coarse threaded fasteners, to
prevent over-tightening and failure.

Friction in a threaded fastener assembly is effected by many variables
including but not limited to: thread design, thread pitch, lubricant,
smoothness of surfaces, if a washer is used or not, LENGTH of thread
engagement, and fastener design, (i.e. a bold verses a nut), just to name a
few. When a bolt is tightened into an existing hold like a block or head
installation, friction is present at BOTH the threads and the bolt head. The
length of the thread engagement effects friction and torque. A nut has
considerably shorter, (in most cases), thread engagement than a bolt.
Therefore it is not necessary or desirable to tighten a nut to as high a
torque as a bolt, because the nut has less friction to overcome to
properly tension the fastener.

[JWest]

Here's some more info on the size:

Metric fastener size designation nomenclature. As fully explained in ISO 965-1, Sect. 5, metric fastener size designations always begin with capital M or MJ followed by fastener nominal diameter and thread pitch, both in units of millimeters (mm), separated by the symbol "x", as follows. M10 x 1.5-6g-S means metric fastener thread profile M, fastener nominal size (nominal major diameter) 10 mm, thread pitch 1.5 mm, external thread tolerance class 6g, and thread engagement length group S ("short"). If referring to internal thread tolerance, "g" would be capitalized. A fit between threaded parts is indicated by internal thread tolerance class followed by external thread tolerance class separated by a slash; e.g., M10 x 1.5-6H/6g.

Default metric fastener thread pitch and engagement length. If metric thread pitch designation (e.g., " x 1.5") is omitted, it specifies coarse pitch threads. For example, M10 or M10-6g, by default, specifies M10 x 1.5. The standard metric fastener thread series for general purpose threaded components is the M thread profile and the coarse pitch thread series. If thread engagement length group designation (e.g., "-S") is omitted, it specifies thread engagement length group N meaning "normal."

Default metric fastener thread tolerance class. If thread tolerance class designation (e.g., "-6g") is omitted (e.g., M10 x 1.5), it specifies "medium" thread tolerance, which is 6H/6g. The 6H/6g fit is the standard ISO tolerance class for general use.

Equivalent imperial thread tolerance classes. Imperial internal and external thread tolerance class 2B/2A is essentially equivalent to ISO thread tolerance class and fit 6H/6g. Imperial tolerance class 3A is approximately equivalent to ISO tolerance class 4g6g, though class fit 3B/3A is approximately equivalent to ISO class fit 4H5H/4h6h. For full details, see ISO 965-1, Sects. 5.2, 7, and 12.

Metric fastener thread profile compatibility. Metric fastener thread profile M is the normal, commercially-available thread profile. Thread profile MJ designates the external thread has an increased root radius (shallower root relative to external M thread profile), thereby having higher fatigue strength (due to reduced stress concentrations), but requires the truncated crest height of the MJ internal thread to prevent interference at the external MJ thread root (just as the UNJ external thread profile requires the UNJ internal thread). However, M external threads are compatible with M and MJ internal threads (just as UN and UNR external threads are compatible with UN and UNJ internal threads).

[JWest]

And on the strength:

ISO metric fastener material strength property classes (grades). As given in ISO 898-1, ISO metric fastener material property classes (grades) should be used. For example, fastener material ISO property class 5.8 means nominal (minimum) tensile ultimate strength 500 MPa and nominal (minimum) tensile yield strength 0.8 times tensile ultimate strength or 0.8(500) = 400 MPa. (In a few cases, the actual tensile ultimate strength may be approximately 20 MPa higher than nominal tensile ultimate strength indicated via the nominal property class code.