by James Wonder
I figured I would start some conversations and talks on
various subjects and the first one (and most likely the most important) would
be bolts.
Bolt grades:
As with everything else in life, bolts have grades with various hardnesses. Following is a list of the hardnesses
of the bolts and their grades.
Grade 2 bolt: No head
markings, and has no equivalent metric rating, stay away from them!
Grade 5/Metric 8.8: US has three lines on the
head and metric has 8.8 on the head. The
Grade 8/Metric 10.9: US has 6 lines on the head
and metric has 10.9 on the head. You can opt to use these in place of the 8.8
bolts, but the cost will be higher and the benefits are questionable based on
bolt flex vs. brittleness.
Note: There are better bolts, and aircraft quality bolts, but those do
not concern us in karting.
When to use which grade:
Most of the bolts on karts are 8.8 bolts, but bolts for the front uprights are
definitely 10.9 or better. Most of these come from the manufacturer, but I know
they are very good quality bolts.
So, why use 10.9 or 8.8? 8.8 bolts are very common and allow for a good amount
of flex when installed properly. Think about the wings of an airplane when you
look out the window. They are constantly flexing, if they are made too stiff
they break. Bolts are the same, and you actually want some flex in them to get
the best life and most strength without breaking.
In current F1 racing, as well as modern aircraft, most of the bolts are very
small 10/32 bolts. Those are much smaller than the ones that hold your bodywork
on! They use many of these smaller bolts to lessen weight, and to allow for
flex points in the material between the bolts. Believe it or not, it is MUCH
stronger using three (3) 10/32 bolts in the right pattern than it is using a
single 5/16 bolt which has a much higher tensile rating.
I use 10.9 bolts in my bearing carriers because the bolts are in single shear
and you require a stronger bolt in that application. I will get into single
shear vs. double shear in a later post, but these bolts do take a great deal of
stress in our application. There is an argument that you want those bolts on
the rear carriers to flex under extreme (accident, off-road, etc) conditions
and that they should be 8.8 instead of 10.9. Most everyone runs 8.8 and they
are fine, but I still opt for 10.9.
Fine thread vs. Course thread:
Most automotive stores now stock almost only course thread bolts, and even
hardware stores do at this point. For our application they should not be used.
You use a course thread bolt when you are threading the bolt into a soft
material such as aluminum or wood. The larger pitch of the thread holds the
softer material better. A fine thread bolt in wood would pull out very easily,
the same is for aluminum. Most steel threads are still fine thread. So, stay
away form course thread bolts unless you need to use them to thread into an
aluminum part.
Conclusion:
Hopefully now when you go to the hardware store you will be able to read the
head markings on these bolts and know which bolts to buy and for which
application.