Nuts & bolts 101

[CompClassics]

I was going to call this thread "Nut & Bolt Etiquette" but it is really a little more serious than that.
I recently purchased a new high end Cobra for a client that needed to have the drivetrain and a few personal touches that the client thought he would like to have added to his vehicle. I will keep all brands and businesses out of this thread as it is about the proper way to assemble components using the correct assembly procedures. Upon receiving the Cobra we started in on installing the drivetrain when we noticed a component that had an issue, no problem it is covered under warranty. What we didn't expect to find was that during the removal and replacement procedure was the nuts and bolts that were used to assemble the halfshafts to their respective components were very questionable in terms of safety and performance. As anyone building one of these vehicles knows there is an abundance of power and torque applied to the drivetrain components of these vehicles and they need to be assembled to withstand the power that is being put through them. As someone once said, " a chain is only as strong as its weakest link". This is where we get into when is a bolt not really any bolt. A short story on the chain of events that produced this particular Cobra, this Cobra is assembled to a "roller" by a company that the delivers it to a subcontractor that installs the components needed to transform the Cobra into the Cobra that that Cobra manufacturer specs their Cobra to before it ships to the buyer or drivetrain installer (me). I hope everyone was able to follow that last bit. In any event the subcontractor that installed those special components to make the Cobra something else is the reason for this technical mumbo-jumbo. I found a couple of reasons for concern this being the biggest. With all of the power and torque output created by these engines one would think that the subcontractor would have used the strongest hardware and assembly practices that you could, I know I would. Could you imagine causing the carnage that would happen when these components come unglued?! Here's what I found vs the way something this critical should be put together, the subcontractor had used to assemble the halfshaft flanges to each of the outer componts flanges, fully threaded (coarse) "grade 8" bolt, a lock washer positioned at the head of the bolt and a nylock nut at the other side of the flanges. There is no end to the many ways that this kind of assembly procedure could fail. I do give the subcontractor credit for using a "grade 8" bolt but that's it.
Here's the reason for not using fully threaded fasteners in a shear type assembly:
1) The threads are already a predetermined point of failure, the root of the thread (actually a fully threaded bolts largest outside dimension, .380") is already a built in stress fracture point. Even a fine thread bolt would have been stronger than what was used.
2) The outside thread diameter (.431") of the bolt is less than the outside diameter (.461") of the hole that it is going through in the flanges this creates a cutting motion each time that torque is applied. You can think of it like scissors cutting paper.
I would have been a little more at ease with this subcontractors assembly practices if they would have used a "grade 8" bolt that had the proper load bearing width (called the grip length) and cut down the thread legth to fit the application, fine thread as it is stronger. As for the use of a lock washer and nylock nut, neither is acceptable in this application:
1) The lock lock washer causes a stress riser as it bites into the metal surfaces, in this case in the root of the bolt head and in the flange surface.
2) With the heat generated in this area the nylock function is questionable at best, it should have had at least a flat washer to seat against instead of the flange surface.
What should have been installed in this critical area is a superior strength bolt with the proper grip length to support the width of both flanges a flat washer, either thick or thin to adjust the grip length if needed, and a fully metallic locking nut.
I use nothing but aircraft specification hardware when assembling these types of components, you can never be to careful when assembling someone else's vehicle, or your own!
As a closer to this article I'll leave you with this. The subcontractor in their infinite wisdom decided to use a nylock nuts to secure the differential hanger bolts, only problem was when he welded them to the chassis bracket! I'll let you decide what happened from there...