Preface: The purpose of this article is to familiarize you with a few of the concepts that go into fastener design, selection, and their inherent properties. There is a lot more science behind it, but the general concepts can provide a lot of good information without being too complicated.
Fastner Size & Grade
Most of the hardware provided in a parts bag with any HMF product will be metric, therefore metric hardware will be the main focus here.
Note: There is technically a difference between a bolt and a screw. This depends on the application. A bolt is used to assemble parts that are not threaded, and will normally feature a nut on the bolt to provide clamping force. A screw is used to assemble parts that already have threads, or where the screw will cut threads into the parent material. Therefore a particular fastener can be a bolt or a screw, it just depends on how it is used. For this article the term ‘bolt’ will be used for the sake of simplicity.
Metric hardware is typically referenced by the diameter of the threads with an “M” in front of it. For example an M8 bolt very simply is a bolt with 8mm thread diameter. There are other identifying numbers to the fastener that expand upon this.
An M8-1.25x30 bolt has the same 8mm thread diameter like the previous example, but now the thread pitch and length are also known. Thread pitch in metric bolts refers to the length in mm between each thread. This can also be understood as the length a screw will thread in for each full rotation. In this example that is 1.25mm. The final number in the description refers to the length of the fastener under the head. Below is a picture showing this example with all the same measurements described above.
There is also a grade stamped into the head of each bolt. In the example photo above you’ll notice the bolt has a marking of “8.8”. HMF will typically use 8.8 grade bolts for most hex head bolts, and most bolts that have an allen head will be grade 12.9. There is a grade 10.9 in between that is used from time to time as well. The higher the number, the higher the strength. A grade 8.8 metric bolt is roughly equivalent to an imperial grade 5 bolt, 10.9 metric grade is equivalent to imperial grade 8, and 12.9 metric bolts exceed grade 8 imperial bolts.
The metric grade is measured in megapascals (MPa) which is a unit for pressure, much like psi. Using the example bolt from before that is graded at 8.8, this indicates both the tensile and yield strength of the bolt. The first part of the number is 8 which means that the approximate tensile strength is 800 MPa (8 x 100). The second part, the decimal, denotes the yield strength as a percentage of the tensile strength. So here it is 80% (8 x 10) of the original 800 MPa tensile strength. This means the approximate yield strength of a grade 8.8 bolt is 640 MPa (800 x 80%).
When a bolt is torqued the bolt stretches and deforms slightly from its original size. This stretching is what creates a clamping force holding everything together. Torque measurements are an approximate way to achieve the correct stretch. Many different factors will affect the final outcome including use of a lubricant, thread locker, dirt on the threads, the material itself, etc. Higher grade bolts require a higher torque to achieve the same amount of stretch and clamping force because they are more resistant to deformation.
Most of the bolts used with HMF products will be M6, M8, and M10, with an occasional M12. For the sake of simplicity our instructions will not specify a torque number for each fastener. Rather it is much easier to use the list below as a general guide for fastener torque. There are some special instances where torque is very important like where a bumper ties into a suspension pivot point, these fasteners must be torqued to the manufacturer’s specifications for safety reasons.
M6: no more than 12 ft-lbs (~16 Nm)
M8: 20-25 ft-lbs (~27-34 Nm)
M10: 40-60 ft-lbs (~54-81 Nm)
M12: 65-100 ft-lbs (~88-136 Nm)
For higher grade fasteners in more “critical” areas that require more strength it is good practice to properly torque all fasteners. “Ugga-duggas” are not a unit of torque despite this being a somewhat common joke amongst both backyard mechanics and certified technicians Most people end up over-torquing fasteners without knowing. This leads to metal fatigue, broken bolts, and stripped threads.
If you have any questions please contact our staff at (216) 631-6980 or through our message system on the right side of this webpage.
Posted Thursday, August 26, 2021
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