Copied from this ClubCobra thread
Dan Case
    1960's Ford Enthusiast

Registered: Apr 2004
Location: Hartselle, AL
Posts: 117
Cobra Make & Engine: Unrestored CSX2310 & Modified CSX2551

Bolt Grades: My 2 cents Worth Of Comments

I will not claim to be a fasteners expert but I do have to make choices of fasteners where I work (Fortune 500 company), I started working on cars with my Dad at age 5, and Dad was the lead metallurgist for toys like the SR71 Black Bird and I learned a lot from him, and I have been designing and building equipment to manufacture durable consumer goods where I work since 1986. That said, bolt grades are performance standards and not metal alloys per se.

I see so many “Grade 8” and higher specification bolts fail, they break off, I would never use one in anything my life depended on like holding a car suspension anything. All of the texts I have read say the same thing on these higher grade bolts, use them only where very high to extremely high clamping forces are required and never in any application where the fastener will be subject to bending moments or shear, this includes large thermal cycles. Grade 8 and similar bolts are intended for applications where two pieces of metal need to be clamped together very tightly. The natural application is holding the steels together in a stamping die. Grade 8 bolts are subject to early breakage if you put them in a bend, in side load or impact. A true Grade 5 bolt, not an Asian who knows what, will bend, twist, and stretch a long way and almost always (hydrogen embitterment is always a concern for zinc, cadmium, or chrome plated carbon steel bolts and screws) stay intact.

It has been my experience the last 46 years that Grade 8 and up specification bolts fail by breaking and that Grade 5 fail by bending or stretching. If I have a choice I’ll take bend and stretch. I have worked on a variety of cars and motorcycles originally produced between 1929 and 1995. Except for special application high clamping force bolts (rod bolts, head studs, etc) I don’t recall ever finding an OEM chassis (Ford has their own specification system for critical applications) with something that would fit the Grade 8 performance standard. I have found worn, rusted, bent, twisted, and stretched bolts but they were still in place holding whatever they were suppose to versus breaking and letting the vehicle come into pieces. (I have found original British fasteners in original Cobras bent and or stretched severely but still doing the job of at least holding the car together. The bolts that hold the leaf springs are usually bent badly on a car that was ever wrecked or raced. Bent and stretched make for a loose spring, but the car is still in one piece. When I brought CSX2551 home it had a lot of bent, some stretched, and some bent and stretched original fasteners, it’s been wrecked and run hard, but none were broken. The bolts I have tested were all made of medium carbon steel and were not heat treated to a Grade 8 type condition, most were in an annealed condition.)

We had a double Grade 8 bolt failure this past week at work that cost our company about $100,000 in down time and repairs. I did the failure analysis for the plant manager. The device had two 9/16 socket head cap screws on one end. One appears to have been a manufacturing defect and it failed, snapped off in the threads at an apparent inclusion in the steel. With one bolt missing the assembly flexes. The flexing put the neighboring bolt in a bending moment and its head snapped off and shot across the room. Now we had a catastrophic machine failure. Microscopic examination indicated that three bending cycles (just three) from crack initiation to head separation was all it took. The device was ok at 07:00 in the morning and by 07:45 it was a big mess. These bolts were brand new when installed. This was their first use. Unfortunately this device needs high clamping forces so Grade 8 is the right choice, otherwise I would have Grade 5 in there. That bolt that snapped its head off in bending would have been fine in all likelihood if its neighbor had not failed. Another approach would, except there is not enough room in this device, design in a third bolt such that any two would still work.

SPS Technologies has a very thorough section on their website describing how fasteners are classified.

Dan Case
1964 Cobra owner since 1983, Cobra crazy since I saw my first one in the mid 1960s in Huntsville, AL.