I know what you’re thinking, import or domestic, what is he
talking about? Is it going to be an article about cars, beer or something else?
You’re right! It is going to be about something else. We work with questions
about these every day in our shops and they are a little more exciting than
plain old import-or-domestic cars and beer.
I am just going to dive into the nuts and bolts of this month’s topic. This month I am talking about threaded fasteners, primarily nut and bolts. You ask, what does title, “Import and Domestic,” have to do with nuts and bolts. Well it has a lot to do with nut and bolts. The nuts and bolts used to assemble turf equipment in general, will be either SAE (domestic) thread or Metric (import) thread and definitely not the same threads used to repair our clothes.
If you are like me, I always have an interest in the history
of anything mechanical and how it originated and developed into what we see and
use today. The origin of the modern nuts and bolts can be found dating back to
the seventh century BCE. The Assyrian
King Sennacherib used wooden screws as a part of the water pumping system to
provide water to the gardens of Babylon. By the first century BCE, wooden screws
were commonly used in oil and wine presses. These screws had a permanent handle
attached as part of the mechanism. The wooden screw and handles were turned
together to crush the olives and grapes for the production of olive oil and
Moving forward into the 1400s, metal screws and bolts began
to be used in Europe. During the 18th century both British and the
US used patented screw thread cutting lathes. These lathes produced custom
screws with square heads in limited quantity.
There was no standard sizing at this time. The Square heads on the
screws and bolts were easy to make and the wrenches to turn them did not need to
be exacting in size. Square head bolts were rather cumbersome and required more
room for them to turn. Some standardization
began in the mid 1800s; at the same time hex head bolts were developed to allow
bolts to be more compact. During the same era, steel was starting to be produced
with an accurate thickness. This allowed for mass production of nuts that lead
into the development of the humble hex-head bolts and nuts we know today.
In 1864 the standard USS thread was developed by Willian
Sellers. The details for this standard were written by Sellers in a paper
titled, “A System of Screw Thread and Nuts”.
The standard for metric threaded fasteners was developed in 1898 and soon
was followed in 1906 by the SAE thread which is based on the USS standard, but
with a finer thread pitch.
During World Wars I and II the use of different threads between
the Allied Forces became a huge obstacle in their war winning efforts. During
the late 1940s, Britain, the USA and Canada agreed on a unified thread to be
the standard for all of these countries. The quality of nuts and bolts continues
to improve even today, as engines run hotter and produce more horsepower than
ever before. These engines will demand
bolts made from aluminum, magnesium, titanium.
Before getting into the different sizes and threads, I want
to clarify the difference between a screw and bolt. A screw cuts its own threads
as it is turned into material such as wood or mild steel. Bolts are turned into preformed threads that
match the bolts’ threads.
With so many different bolts how do we identify them? There
are three main parts that, together, reveal the size of a bolt: the overall
length not including the head of the bolt, bolt diameter and thread pitch.
Bolt length can
be measured with ruler, tape measure or caliper. Bolt lengths vary but are in
common increments such as 1/2, ¾, 1 inch and so on. Metric lengths are measured
in millimeters 10, 15, 25 and so on. One inch is almost equal to 25
Bolt diameter is
the size of the threaded hole in which the bolt will fit. This is the
measurement across the diameter of the bolt. This measurement can be found with
a caliper, bolt size gauge or even the open-end American or Metric wrench fits the
Thread pitch is
the number of threads per inch or millimeter. This is most easily determined
with thread pitch gauge that will measure American and metric threads. For
American thread they will be ether fine or course thread. Metric gets tricky
with a multitude of thread pitch measurement from 0.5, 0.7, 0.8, 1.0, 1.25, 1.50,
1.75, 2.0 thread. Thread pitch is the number of threads per inch or millimeter
in length. For example, a 5/16-18 course
thread bolt has 18 threads per inch, while a 5/16-24 is fine thread bolt with 24
threads per inch. Similar sized metric bolts are 8mm x 1.25mm course thread or
8mm x 1.0mm fine thread. Metric thread pitch gauge is very handy when trying to
determine metric thread pitch.
Another way to identify bolts are by the grade marks or numbers on the head
of the bolts. SAE will have a blank bolt head for grade 2, three marks on the
head for grade 5 and six marks for grade 8. Grades 5 and 8 are the most common.
Metric will have a number designation on the bolt head such as 5.8, 8.8, 9.8
and 10.9, with 8.8 and 10.9 being the most common.
The grade mark is the tensile strength of a bolt. Tensile strength is the maximum amount of stress a bolt can
endure from being stretched or pulled until it fails. Tensile strength
determines the amount of tightening torque that can be applied to the bolt.
Proper bolt tightening torque is critical for components like a cylinder head
or connecting rods and main bearing and even wheel lug nuts.
Use the included
charts and picture as an aid in identifying the Domestic or Import bolts.
Happy wrenching and feel free to submit your comments, questions and
ideas to Brian Duffy at firstname.lastname@example.org.