Bolts Have Identities Too

Did
you know bolts have their own identity? If you’ve looked at any bolt
before you have maybe noticed a stamp on the head.  If you have wondered
what it is this short article will give you an explanation on why many
bolts have stamps on them.

BOLT HEAD NOMENCLATURE

Why should you care about the stamp on the head of the bolt?

By Mark Zimmerman Photography by Mark Zimmerman October 5, 2016


determining the meaning of bolt head numbers

Many, but not all bolts,
are stamped with either a number (metric bolts) or a series of lines
(fractional or “American” bolts) indicating their minimum strength or,
more technically, their property class. Besides, the head markings’ most
reputable manufacturers also stamp their trademark in the bolt head so
that it is readily identifiable and traceable. Why aren’t all bolts
stamped? The laws regarding the manufacture of hardware are pretty
vague, and there are lots of ways to get around them. For example,
hardware manufactured to OEM specification isn’t required to be marked.
That’s why there are no indicators stamped into the head of your
swingarm bolt. In another example, if a hardware manufacturer claims his
high zoot chrome plated stuff meets or exceeds a particular OEM
standard and says so in writing, he doesn’t need to actually stamp the
head of a bolt.

So
why should you care what’s stamped on the head of a bolt? Two reasons;
first if you need to replace a bolt you certainly don’t want to
substitute one that’s weaker than the one you’re replacing. Second, if
you know the property class of a bolt and its diameter, you can
determine the suggested basic torque setting,
even if you don’t have a shop manual handy. Most industrial supply
stores or specialty hardware suppliers will gladly provide a chart that
lists torque settings based on property class, thread pitch and diameter
for a wide range of bolts. For instance, 8mm bolts with a 1.25 thread
pitch are normally available in property class ranges from 8.8 to 12.9.
The higher the number, the stronger the bolt. Depending on their
property class, 8mm bolts can be safely torqued from 19 foot-pounds to
37 foot-pounds. Torque an 8.8 bolt to 37 foot-pounds though, and you’ll
end up with two halves of a bolt, or one that’s so overstressed it may
as well be broken.

By
the same token, replacing a bolt marked 12.9 with an 8.8 bolt is a
recipe for disaster. The softer 8.8 bolt just isn’t strong enough to
cope with the loads born by the 12.9. Going the other way, replacing a
low grade bolt with a stronger one is generally acceptable although
there are times when a soft bolt is used to cope with certain types of
loads that would shear a harder one.

Now
that you have a basic understanding of bolt head markings there is no
excuse for installing the wrong bolt just “because it fits.” If the bolt
you’d like to replace isn’t marked, or you can’t find one identical to
it, you’ll need a service or parts manual to determine the grade. But if
you do know the property class, replacement is only as far away as your
nearest hardware store.

Read the full article here: http://www.motorcyclecruiser.com/bolt-head-nomenclature

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