All you want to know about bolts...Grade 5 or 8 or aircraft or whatever....

[Dan Case]

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.

[lovehamr]

Dan, that was an excellent dissertation. You mentioned that your company's machine bolts broke in only three cycles, that's incredible. At a job I worked a few years ago one of my many duties was firearms accident investigation. One of the problems that we had were slides on our pistols cracking and sometimes failing completely. During the finite element analysis (FEA) on one such failure it was determined that a crack on one side had cycled over 600 times before its eventual catastrophic failure. The metal in the slide of this pistol, though not soft by any means, withstood considerable bending moments before its failure. I wish I could remember the Rockwell hardness rating that we found in that slide but alas I'm afflicted with CRS (can't remember s**t). The point is that, as you pointed out, there are jobs for hard, brittle metals and there are jobs for softer, more pliable metals. The two are not the same. If used in the wrong application, either way, the results can be most unpleasant.

Steve

[GeorgiaSnake]

Dan thanks for the brief class on fastners. Many of the participants on this site don't understand the dynamics of fastners and seem to think more (harder) is better.

I would encourage anyone swapping bolts to read and understand the differences in shear, bending, compression and stretching.


This is a great topic - hopefully it will be used to educate rather than seek revenge.

Randy

[Tim Brewer]

Right-on Dan!
I've always considered every nut and bolt on my own car. My Lonestar came with a nut and bolt package, and they were all grade 5. I replaced every nut and bolt NECESSARY with fine thread grade8. I said necessary because of each nut and bolt's application, (like Dan said so well) determines weither it needs a grade 5 or 8. I've been building and wrenching on everything from bicycles to motorcycles to cars for over 30 years, and will tell you first hand that it is not necessary to have all grade 8 hardware throughout. My .02

Tim

[Rick Parker]

Dan:

One could say you "Hit the nail on the head". Your brief and knowlegable summary is excellent. Those with the desire to increse their understanding on this complex subject would do well to read one of the many books available on 'High Performance Fasteners". As you have briefly described, different applications require different grades or types of fasteners The quality control used for a typical grade 8 fastener leaves a lot to be desired, at best they are very brittle. Factor in the fact that many of the "off shore" fasteners on many shelves are made from metal of unknown origin in countries using questionable manufacturing standards and you have a bad situation waiting to happen. This entire subject is not one to be taken lightly. Hardware store bolts have a very limited application in automotive construction.

Rick

[Michael C Henry]

My experince isn't so intense. I worked for a local marine deisel repair for a short time many years ago.. I came away with an attitude adjustment ,If the manufacture doesn't put their name or mark on the head of a bolt, I won't use it.If Catapilar built it it has the best bolts available ,they even have better than normal sheet metal screws.
No name bolts that are bought by the pound are not worth carring out of the store. the only plus is they are easy to drill out.
OEM car manufactures use mostly grade 5 bolts in less critical areas..A grade 8 bolt under stress in a part with out a good washer is not relieable either. The part can fail and the fastener can still be in place undamged.Then it becomes a design or assembly issue. I chose to replace many of the bolts when in the process with ARP bolts.In critical areas it was black steel, in lesser areas like intake manifold and valve covers it was their SS bolts. For hood and dash I used SS fasteners supplied by local store. I'm sure that grade 8 national fine 3/8" bolts in the windsheild supports are overkill. The 2 legs of the windsheild braces will give up first.
I'd be more concerned with OEM and salvaged parts being used that are not up to the application.Also the best bolt in place and not tight is not much better than a lesser bolt that is correctly tightened.
All thing being consideced we could form a heck of a pannel of experts ? I work on Army heavey mobile equipment.

[blykins]

Being a Mechanical Engineer, I've had lots of courses on materials, machine design, and finite element analysis.

A material has a yield strength. That's the stress limit at which the material deforms....this could either happen plastically....or it could happen like Dan was explaining....in a "brittle" fashion.

In most cases, the ultimate shear strength is within a certain percentage of the ultimate yield and tensile strengths...if I remember correctly from school, it's in the upper 80% range. However the material's metallurgical properties can be altered by different methods of working the metal. This would backup Dan's argument that a G8 bolt would be less likely to plastically deform....but it would just fail at stresses equal to its tensile/shear strength.

A good example of this would be a high strength steel and a nodular iron. Both would have extremely high strengths, but a nodular iron is more of a "brittle" material....which would be less likely to plastically deform...

What you have to analyze though in this situation, is whether the strength difference is enough to warrant the grade difference.

30000 psi over a 1/2 bolt would allow around 6000 lbs more force in a tensile strength situation.

So even if the G5 bolt plastically deforms, it would still start deforming before the G8 bolt would....even though the G8 bolt would yield in a much more violent manner.

One thing I don't quite understand is the explanation on how the G8 bolts fail with different modes of loading. I'd like to hear more on that Dan, if you could sometime.

It seems to me, when you put a fastener in a high load situation such as the high clamping mode you were talking about in your post, that would be in an axial tensile strength situation.

However, you say that a G8 bolt is more likely to fail under a bending moment, side load, or impact.

A bending moment is a function of tensile strength. When you bend a piece of metal, you are essentially stressing it at the top and bottom of the metal. You're pulling it on the top, and compressing it on the bottom.

The side load or impact would be a complete shear load....not a function of tensile strength, but of shear strength.

I don't have a list in front of me of the fastener shear strengths.....only tensile strengths. This difference that you were referring to could be a function of shear strength only......not tensile strength or axial loading.

[Todd Hedges]

Dan,
I was taught the same way.
Grade 5 bolts yield and stretch prior to failure.
Grade 8 snap like a crisp cracker.

[trularin]

Well, I used grade 8 on almost all of my car and it wasn't as much a matter of PSI as the bolts cost the same as the grade five.

I build cars for a living and what it says it is and what it is are two totally different things.

The next time you think you are getting grade 5 or grade 8, you might want to think about getting a machine to verify those claims.

I hjave a nice set of ARP, Certified Flywheel bolts that are all stretched and broken and these were supposed to be some super 180000 PSI bolt that would never fail. PUCKY!!!

One more thing now that I am so freeking tired of this crap; The car is not going to kill me! It has no power unless I say it does...period.

A suggestion to Jerry; "Jerry do ya think you could ever just start a thread, conversation or dialog with, 'hey I was wondering' or 'just thought I'd drop in' instead or telling us your legal woows.

And, I know I can't spell, so does most the other people on this form.

[ToyCollector]

Trularin,

I am in the process of having a motor built, and the ARP stuff was supplied by a top engine builder here in the southeast that was the site of the dyno testing for Cracker's KC engine and builds everything from NASCAR to Powerboat race motors. I was amazed at the pains and special tools he uses when dealing with ARP grade studs and bolts to properly stretch them. It is by no means a process of taking a torque wrench to them and tightening them down. I found it most interesting and even my engine builder of many many years got an education. This is a science unto itself as evidenced by Carroll Smith's books.

[blykins]

Another good example of the above situation is Richmond's new race ring & pinion gears. They are tempered differently, so that when they reach their yield strength, they plastically deform instead of being brittle and breaking off.....

[rdorman]

I think it has already been said but the bolt should match the application. Blindly replacing fasterners will get you killed. Good luck with the nylock nuts on the brakes. I am going home now and tear down my motor and switch to all fine threads. Just want to be safe.
Rick

[Toby]

Several years ago while hauling intergalactic freight for the federation I learned that you should use whatever bolt you could possibly find in an emergency!!

You might be surprised to learn that grade 8 bolts are hard to come by on Rigel 4,s 5th moon, but they do have a Lowe,s.

Just a little humor there.

Toby

[ItBites]

Folks,

If you're not a trained and experienced Engineer familiar with bolted joint design, you should not be dictating which fastener to use for a given application - that includes you jerrywacker. You are a moron if you think that it is never OK to use grade5. I think what you are saying is that since you have no idea how to correctly characterize the loading in a bolted joint, you have to fall back on a philosophy to just overkill all joints. This is actually not a bad approach, given a lack of any better understanding. But you can't claim that a grade5 bolt in a given application is not satisfactory unless you've determined the loading in the joint, the resultant required preload to prevent gapping, the pullout capability of the mating threads, the spring constant of the grip length, the effective spring constant of the clamped members, etc... I have seen no evidence you have any capability to perform these calculations, so stop giving advice.

I do have to add that a statement that only fine threads are acceptable is ludicrous. I'm not sure where jerrywacker got his engineering degree and training, but any Engineer worth his salt will tell you this is not true and each application should be evaluated and then a proper fastener selection can be made.

Sounds to me like this jerrywacker is just a typical carbuilder hack that does no real Engineering and relies on some general rules of thumb that will (admittedly) generally result in a joint that does not fail. This approach is not Engineering or scientific.

Now, I must say that I do not know if the Kbros are any better, but they did not purport to be some bolt expert, giving stupid advice to folks here.

[CowtownCobra]

Anyone want to talk more about bolts, that was getting interesting...

[Brent Mills]

It would be interesting to see what grade/type fasteners should be used for the various components.

Anyone able to break that down?

[computerworks]

I would like to hear, also, what goes where...from the experts.

My 2 cents...

[mikiec]

Ron or Brent,

Can you move the relavent(SP) responses from the other thread here? Please leave out what is not on topic.

Thanks,
Mike

[Specialk]

I know that Shelby uses grade 8 fine thread fatseners for everything 1/4" and above. I always took it that Carroll Smith was god on this, but Jerry seemed to have a different opinion than Mr. Smith had.

[scootter]

If it is critical to the operation of the vehicle, use grade 8 ( steering, suspension, motor mounts, trans mounts, seats, seat belts, ect). Then if you put nylon lock nuts, those are not grade 8. You have to get crushed nuts to get grade 8. And they hurt. You will probably not get those at your local hardware store. Don't forget locktite