New hardware

[Ian S.]

Hello,

Are there any safety issues to worry about with repro bolts and fastners? I was looking to get a sway bar link kit and there are several out there including GM, repros with correct head marks and repros without correct headmarks. Just wondering if all of the hardware is made to the same specs as original GM. Not just this application but any fastners where strength is a concern.

Thanks,

Ian

[Joe L.]

Re: New hardware

Ian-----

I think that the reproduction fastners are made to the SAE specs shown on the bolt head. To be honest, though, I do worry about this myself. Personally, I wouldn't use these fastners for any application requiring a Grade 8.

One more thing: I have observed some reproduction bolts with CUT threads (this is the thread type in which threads are cut into a bolt blank with a die). I would NEVER, EVER use a bolt with CUT threads for ANY high strength application, whatsoever. ALL high strength bolts that I would use have ROLLED threads (this is the type of thread in which the threads are "rolled" onto the bolt blanks with special high force machines).

[Chuck S.]

Re: New hardware

Ian, unless someone actually in the repro parts business gives us an honest, objective answer, I doubt many of us will know the answer to this question since few of these cars are driven like they were meant to be driven.

I have some opinions that I will share with you. In the late seventies or early eighties, alarms were sounded in the engineering business about imported fasteners from some countries not meeting strength specifications of domestic fasteners. As I recall, these fasteners were not European or Japanese, but were from third world countries in Asia.

This situation may have improved over the decades, but until I heard that specific imported fasteners met US standards, I would exercise caution. I expect some repro Corvette parts are also being supplied by low-cost suppliers in the same region. For that reason, I would use no repro fasteners in critical areas where safety can be an issue. This is probably irrelevant anyway; there aren't that many repro fasteners available for critical applications. If you drive your car hard, a sway bar link bolt may be considered a critical part, but for cruising around town, it's probably not.

Anything supplied to GM (Not GM Restoration Parts) may not have the proper headstamp, but functionally, I expect that GM still requires the fasteners to meet the original specs, and initially tests (and may periodically test) to insure that suppliers meet the spec. GM has a lot to lose from faulty fasteners where a small offshore manufacturer many not.

I would either find original used fasteners (if replating make sure they are post heat treated), or use the new GM part knowing I could see deducts for slightly different configuration.

[Chuck S.]

Rolled Threads...For Those That Don't Know

Joe didn't go into the details, but rolled threads are used in high strength fasteners to allow the "root" or the bottom of the thread to have a formed rounded bottom (microscopically) rather than the sharp acute angle provided by cutting the thread with a sharp tool. Sharp corners in machinery parts are "stress risers" which cause higher stress and rapid initiation of cracks. Once a crack starts, the fatigue strength and service life of the part is significantly reduced.

Rolling the thread also causes the root of the thread to be in extreme compression, which also reduces the tendency for crack initiation in the bottom of the thread.

[Ian S.]

Re: Rolled Threads...For Those That Don't Know

What is the best way to tell the two types apart when looking at them?

[Chuck S.]

To Tell The Truth...

Ian, I don't know that I have made that determination on small automotive thread sizes, and I would expect it to be difficult to ascertain. Generally, I would expect cut threads to have sharp, rougher? edges on the peaks of the thread, and rolled threads to have a smoother feel to them because they will actually have a minute radius on the peak as well as the root of the thread. Maybe Joe has some additional thoughts.

I believe it's going to be the kind of thing you have to learn by experience, like thumping a watermelon to see if it's ripe ; e.g. someone identifies one of each type for you, or you are given the specifications for each fastener, and you feel them, look at them, etc.

My experience with rolled threads has been with specialty fasteners (AISI 4340 steel, 2 inch OD )...in that case a detailed spec confirmation was done by materials engineers.

[Clem Z.]

fasteners are like women

the more money you spend on them the better off you are.

[Eugene B.]

Re: Rolled versus cut threads

Gents,
This post has raised some questions about my own restoration.

I reuse every factory fastener that I can. I do however, like to chase every nut and every bolt with a tap or die after cleaning and wire brushing to make reassembly smooth.

My concern now is, perhaps I have been "converting" some rolled threads to cut threads by chasing them with a die. If there is really a difference in strength, perhaps my procedure is not a good one. What do the experts say?

Thanks,
Gene

[Clem Z.]

Re: Rolled versus cut threads

if when you used a die and it removed metal you could have changed the strength of the fastener. i would just use a wire wheel to clean the bolts. they make a special thread cleaning taps that do not remove any metal.

[Chuck S.]

Re: Rolled versus cut threads

Gene, it will depend on the actual thread profile on the fastener versus that on the die, but generally, if you are removing steel, you are probably reducing the strength of the fastener, either by reduced cross section (insignificant) or by introducing stress risers in the root of the thread.

I am not that familiar with the differences in rolled thread profiles versus cut thread profiles, but die thread profiles are probably designed with some extremely short radius on the root cutting edge. Therefore, we are talking about degrees here; the fastener strength will be reduced, but remember that a significant safety factor is designed into these fasteners from the beginning. In spite of that, I personally never "chase" fastener threads except in rare cases where the threads have been "buggered" in an area and I have no choice.

I wouldn't lose any sleep about this issue. First of all, I doubt that you removed much material, because frankly, if you begin cutting steel, this method of cleaning threads becomes drudgery in a hurry. That is another reason I don't do it; it's not time effective...too much time and work...better ways to clean threads.

The other thing is, you can't go back and change it now. Worry solves no problems; just periodically inspect your suspension and other critical fasteners for missing heads (all of us should probably do that on these old cars anyway). I say "missing heads" because this type fastener failure will typically occur in the first fastener thread outside of the tapped hole.

The primary failure mode will be fatigue (slow progression of a crack with load cycles until the remaining cross section is insufficient to carry the load). Even if you did introduce stress risers by chasing the fasteners, it is highly unlikey that all of the critical fasteners on a safety component will fail at the same time resulting in a catastrophic failure...just keep an eye out for failed fasteners.

[Terry M.]

Re: Rolled versus cut threads

Gene,
Chuck has covered the area of fastener strength well. I only wish to add that there are dies that have a split on one side and a small screw run in perpendicular to the slot. Not sure what they are called, but I imagine there is a name for them. That screw can be adjusted to widen the slot and allow the die to take less of a cut on the threads. This type of die can work well in a "clean-up" or thread chasing type of operation, if it is adjusted so that the die barley takes a cut, or takes no cut at all.
Not to confuse the thread issue, but bolts can be manufactured with a variety of thread depths. There is a standard for fasteners called Class of Fit that is determined by the depth of thread cut, and of course is dependent on the depth of thread cut in the mating part. I am not sure if this degree of quality is used in the automotive industry, but there are standards for it - or at least there were when I was learning engineering drafting in the early '70s, before that business turned into a world of computers. Not a whole lot has changed in the fastener world, except where they are made, so I am guessing that class of fit is still a viable standard.
Chuck's advise to check fasteners often is sage words for all of us - even on newer cars and trucks. More often than I would like, while judging a Corvette, I find a lose fastener. When doing a major restoration it can be very difficult to be sure that all the fasteners are properly secured. If multiple individuals work on the car there is always the possibility of thinking that someone else tightened up a particular bolt. --- Let's be careful out there.

[Chuck S.]

Class Of Fit Is...

A good point that I had forgotten, Terry. From the "Standard Handbook for Mechanical Engineers" for those that are interested,:

"Classes 1A and 1B: Provided liberal allowance for ease of assembly even when threads are dirty or slightly damaged." (Classes A for external threads only; Classes B for internal threads only.)

"Classes 2A and 2B: For production of bolts, screws, nuts, and other commercial fasteners. Permits external threads to be plated.

"Classes 3A and 3B: For close tolerance work where no allowance (read slop for assembly) is required."

Comments inside parenthesis are mine. Most automotive fasteners (in fact most manufacturing fasteners) probably fall into Class 2, with some in Class 1. I suspect Class 3 is for high tech stuff like specialty aerospace, ultra high strength applications, or hand-made laboratory projects