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Heavy Hardware Means Stronger Bolts

Posted in Features on August 16, 2016
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Having a suspension link bolt snap under throttle is simply not a good thing. Actually, there are a lot of places we have hardware where a failure can really ruin our day.

If you've been around the automotive world much you know a bit about quality hardware. You know not to use the Grade 2 bolts you get from the big DIY home stores, and you should probably skip the respectable Grade 5 hardware to confidently use hardened Grade 8 material in most cases. Yet, what if you've experienced bolt failures using Grade 8 hardware?

There are several things to consider. First, ensure you're purchasing hardware from known suppliers that give you confidence you're not getting counterfeit import junk. Poor quality look-alikes are out there yet they are weaker than their class spec requires. Second, ensure you're properly torquing bolts to the correct spec. That way you'll get the full-rated clamping force and strength. In fact, many people are surprised to learn that the torque rating on fine thread (UNF) bolts is greater than that of coarse thread (UNC) bolts. Actually, fine thread bolts are stronger due to their shallower thread-cut depth and they resist loosening better than coarse thread bolts. Of course, fine threads are more easily damaged during transport or assembly but offer a greater clamping force than coarse threads for a given tightening torque.

Lastly, there are stronger options over Grade 8 bolts if you need more strength and can lay out more cash. AN aircraft bolts are probably the cream of the crop in bolt products. They are designed for superior strength and longevity (fatigue strength), plus they can be purchased in more detailed incremental lengths to perfectly suit your application dimensions. AN bolt sizes are specified by ANx number where 'x' refers to increments of 1/16th inch. For example, an AN6 bolt has a 3/8-inch diameter shank.

Bolts with no line markings can be assumed to be Grade 2. Three markings designate Grade 5 and six markings designate Grade 8. F9.11 bolts are about 20% stronger than Grade 8 pieces and have their own unique marking as shown here. AN bolts may have varied mill markings. Beware that counterfeits exist for many of the higher grade bolts.
When purchasing a bolt, we typically request the overall bolt (or shank) length that includes both the grip length (unthreaded portion of shank) and the threaded length combined. On a standard bolt these dimensions are fixed for a certain length bolt. However, custom grip length can be specified on an AN aircraft bolt. Generally, the threaded portion on an AN bolt is shorter than that of a standard bolt and is constant based on the bolt size. It is just long enough to accommodate flat washers and a nut with just a little bit extra. AN bolts also use a tighter thread tolerance spec so the male-to-female thread engagement with a nut provides a superior fit as compared to commercial hardware. AN bolts can optionally be purchased with the head drilled for safety wire or the threaded shank drilled to accept a cotter pin.
You can clearly see here that using a standard bolt in this link mount leaves the weaker, threaded portion of the bolt in contact with one mount tab. For some applications this is certainly fine, but if ultimate strength is required here, this is not optimal. If an AN bolt is used, you can specify the grip length such that no threaded area is inside or touching the mount tabs. For a given diameter of AN bolt, all bolts will have the same length of thread, regardless of overall bolt length.
Here’s an example of recommended torque specifications (dry threads) for some SAE bolts. You can see that torque specifications rise with bolt diameter or increase in grade, and ratings are a bit higher for fine-thread versus coarse-thread bolts.

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