Grade 5 vs Grade 8 bolts

[Brian]

Dennis, the fasteners in cars like BMW's are often completely specific to the application, are sometimes unusual threads and sizes, and can have specific strength requirements. I hate to say it but taking a bolt that has already failed out of a front suspension upright, retapping to SAE and stuffing in a grade 8 SAE fastener is, to me, an unacceptable repair on a street car. Knowing if it's a adequate repair would mean figuring out what the loads are and why the original bolt failed. If you think about it, the bolt is going through a hole in a thing, and being threaded into another thing. What if the fit of the bolt shank in the thing is critical? If it's a steering arm, maybe it is...

Brian

[Jphoenix]

The FAA just put out a notice regarding hydrogen embrittlement crack failures in MS 21042 nuts. Batch sources vary significantly and the recommendation is to test 10% of your fasteners by torquing to the correct spec for the fastener diameter and leave it torqued for one week. If the test article fails, toss the batch. This failure characteristic is symptomatic of hydrogen embrittlement - that condition results in a very low cycle fatigue failure. This means that if your nuts are a season or two old, they're probably ok, if you are installing new ms21042 nuts, you should load them up for a week, they will fail if bad.

Hydrogen embrittlement is typically caused by failing to properly bake after cad plating.

If I can find my copy of the notice, I'll post it. I did see an internal FAA PowerPoint on some MS21042 nut failures discovered in helicopters and some large and small aircraft and the failures are visually impressive with a complete crack through the nut radially. Typical hydrogen embrittlement. The notice states that this problem has increased since the military is doing less testing of AN/MS and most of the commercially available hardware is reliant on industry specs and testing.

[Christopher Crowe]

Has anyone actually had a bolt failure? If so, what and where?

Thanks,

Chris

[Daryl DeArman]

Yes, grade 8 bolt on Crossle32 from chassis to motor mount.

[Modo]

WHoops ....... grade 5 ok according to C Smith, AN Best

[racerdad2]

Counterfeiting bolts is highly profitable and difficult for the consumer to detect. Is there a reliable & verifiable source for high grade hardware ? Personally, I don't trust anything from China. ... just sayin'

[lancer360]

I get my fasteners from my local Fastenal. Socket heads I get are made by Holochrome, a high quality US manufacturer. Critical bolts I use AN bolts. Fastenal can provide material test reports for most stuff they carry if you ask for it when ordering.

[BLS]

When I decided to replace most of the mismatched/wrong type/wrong grip length bolts on my car I found Aircraft Spruce to have the lowest prices on AN bolts/nuts.

[budawe]

I tend to overspec my fasteners and reduce the loads accordingly. Granted unsprung weight on a vee is over the top anyway. Only failures I have seen are from accidents.

[Brian]

I get my fasteners from my local Fastenal. Socket heads I get are made by Holochrome, a high quality US manufacturer. Critical bolts I use AN bolts. Fastenal can provide material test reports for most stuff they carry if you ask for it when ordering.

This is exactly my point. This is backwards. Use the AN bolts for the routine stuff and use the Holo-krome for the critical applications. H-K SHCS are significantly stronger in all ways than AN and with 180KPSI tensile strength are the equivalent of MS 21250 12-point head aerospace bolts, which are the highest strength that Coast Fab even carries in their catalog. H-K bolts are completely traceable. About the only problem with them is they usually aren't bright plated. But going to the lowest strength fastener available in the AN/MS/NAS catalog for your "critical" applications is just not right. It might not be wrong since that depends on luck and the application, but it's not the right attitude to have.

Brian

[Neil_Roberts]

It might not be wrong since that depends on luck and the application, but it's not the right attitude to have.

High strength fasteners are not always the best choice because they have less ductility and less impact toughness than AN fasteners.

[Brian]

I just don't believe that. I think it's a myth grown from poor quality fasteners used where they should not have been. I have bent quality grade 8 and allen type bolts in circles and I have never fractured one. At the end of the day, do what makes you happy. I have solid engineering behind my choices, and I understand the engineering.

Brian

[rfn026]

Here's an article I wrote about bolts for Vintage Motorsport magazine last year. It's in 2 parts.

Richard Newton

[Christopher Crowe]

Has anyone ever had a bolt break?

Ever?

Thanks,

Chris

[DanW]

Has anyone ever had a bolt break?

Ever?

Thanks,

Chris

Yes. Low stress/high cycle fatigue on halfshaft donut bolts, 7/16-20 SAE Grade 8. The bolts did not have enough clamping force/stretch greater than the cyclic loads.

All that whirling metal and rubber makes a big mess of at least one rear corner and the rear of the chassis.

The bolted joint is one of the most misunderstood mechanical devices.

[lancer360]

Yep. Had a 5/16" hex head bolt shear on the drive shaft adaptor on my F600. The head fell perfectly between the spinning adaptor and the block. Fractured the cast aluminum block taking out the oil pump and the water pump with it. Needless to say that was the end of that engine. Bolt was in a very difficult location to tighten properly. I switched all the bolts out for socket head cap screws as it would allow me to use a ball end Allen key socket on my torque wrench and tighten properly. Never had a problem after that.

[Neil_Roberts]

I just don't believe that. I think it's a myth ...

There is no reason to base an argument on personal beliefs when discussing topics for which empirical test data is abundant.

[Richard Learned]

The problem as I understand it is that there is no certification for what is exactly a GR5 or GR8. What a manufacturer states is grade whatever depends entirely on that particular manufacturer's definition. There is also a great deal of latitude on the frequency of testing end products. One company cycles a quality test every 100 products, another every 1000. I'd error on the side of requesting certs for the AN and MS route. IMHO, no certs...that company should not be selling to our group of mechanics and cannot sell to the aircraft industry under any circumstances. Yes, the certification paperwork will in some instances cost you money. I'm lucky, I live in Wichita where Beech, Boeing, Lear and Cessna build a whole lot of airplanes, gives me access to a ton of "surplus" sales when inventory levels get out of hand at any or all of those factories.

[lancer360]

Grade 2, 5, and 8 bolts do have a specification the is well defined. It is SAE J429. If you can get them with a material test report (MTR) you should be OK. They are some of the most widely counterfietted bolts so I wouldn't use them if you can't get MTR's on them. You do need to watch the shoulder length if you are putting them in shear and try to keep the threads out of the shear plane in highly loaded joints.

[Mike Scanlan]

This picture is of a couple of AN bolts that found concrete at Rd America a few years ago. They were the rear upper trunion bolts, they took a torque when we put the corners back together at the track and raced the next day. The car came back apart at the shop and now those bolts sit on my desk as a reminder to use AN.

[Brian]

I wanted to clarify my thinking on this whole fastener issue. I understand and completely agree that AN bolts are excellent quality safe choices for most applications on a race car. My point is that they are 125KPSI tension strength bolts and so are not all that strong - they are the aviation equivalent of SAE Grade 5. They are not miracle bolts. I have no issue at all with using them in almost all applications on a typical car that we would see. My issue is with the thought that they are in some way an ideal bolt for critical applications, as some have said. My point is that if you think you have a critical application, you have to then define it's criticality and fully explore the engineering requirements of the application to develop a definition of what the bolt needs to do. If it turns out that an AN bolt is the most applicable choice for that application then in my opinion it's by definition NOT a critical application. Extremely important, maybe, but virtually all fasteners on a race car are extremely important.

I thought for a while about what bolt applications are "critical" on a car, in the context of needing a special fastener. Rod bolts, head bolts, main cap bolts, flywheel bolts, and CV joint bolts came to mind. The engine bolts most everyone understands need special fasteners, but I've seen a lot of CV joints held on with AN fasteners or with AN washers under the bolt heads and people not understanding why they come loose every session. Volkswagen used a special 12 point head metric bolt that was the equivalent of MS20250 or a typical socket headed cap screw - the ubiquitous Allen bolt - and torqued it to around 35 ft lbs, used case-hardened load spreaders under the bolt head, and they did not come loose. I've also changed head gaskets at the track for people who used AN washers under head stud nuts.

My whole point is that people should not just assume that an AN bolt is the only choice or the best choice for any application. An AN bolt is a mundane choice for a mundane application, and the aviation industry likes it that way - foolproof in all intentions of that word. People should examine the requirement and make a decision based on mechanical requirements. The vast majority of requirements are great for AN bolts and equally suitable for a Gr 8 SAE bolt or for an ANSI socket head cap screw. The car I am currently working on has a large number of "Allen" bolts and you can't put anything else in - the head of the bolt is inside a recess that a hex head socket won't fit in. "Critical" locations like the single shear inner A-arm bolts are 3/8" or 7/16", and a nail would be strong enough (FWIW a 3/8" Gr 8 bolt has a shear strength, in single shear, of over 11,000 lbs). I don't think an AN bolt is going to add function or safety to a anti-roll bar link when the bar has about 75 lbs of spring rate and the bolt is 5/16" diameter. That's my point.

One of my vintage cars has the front brake discs held on with six 10/32 countersunk bolts on something like a 3.5" diameter. You can bet I did the math on that before I put it back together. The engineer who designed it knew his strength of materials... it was fine.

Brian

[mousecatcher]

Grade 2, 5, and 8 bolts do have a specification the is well defined. It is SAE J429. If you can get them with a material test report (MTR) you should be OK. They are some of the most widely counterfietted bolts so I wouldn't use them if you can't get MTR's on them.

Educate me as to why a MTR can't be counterfeited as well?

[Daryl DeArman]

They could be, but then so could any material certs...

I'm not a ME and figure that there is a lot of margin built in to many of the fasteners and rod-ends on older small bore formula cars, so if the old one didn't fail under normal use I figure it was up to the task. If I want to save weight by going with smaller fasteners, I'll consult the calculator and spec sheets.

I'd rather have something bend than break, at least that gives me a headsup that I need to fix it and I could likely finish without a DNF or other additional damage. I'd rather have a "weaker" bolt of the proper grip length than some stronger, less flexible bolt with threads loaded in shear. Especially on bolts loaded in single shear---which I always thought was taboo but see them all the time.

[lancer360]

Educate me as to why a MTR can't be counterfeited as well?

MTR reports can definitely be forged, but people are much less likely to forge a MTR document when they have to sign their name on it. A forged paper trail with your name on it is a real easy way to get sued.

[lancer360]

I wanted to clarify my thinking on this whole fastener issue. I understand and completely agree that AN bolts are excellent quality safe choices for most applications on a race car. My point is that they are 125KPSI tension strength bolts and so are not all that strong - they are the aviation equivalent of SAE Grade 5. They are not miracle bolts. I have no issue at all with using them in almost all applications on a typical car that we would see. My issue is with the thought that they are in some way an ideal bolt for critical applications, as some have said. My point is that if you think you have a critical application, you have to then define it's criticality and fully explore the engineering requirements of the application to develop a definition of what the bolt needs to do. If it turns out that an AN bolt is the most applicable choice for that application then in my opinion it's by definition NOT a critical application. Extremely important, maybe, but virtually all fasteners on a race car are extremely important.

I thought for a while about what bolt applications are "critical" on a car, in the context of needing a special fastener. Rod bolts, head bolts, main cap bolts, flywheel bolts, and CV joint bolts came to mind. The engine bolts most everyone understands need special fasteners, but I've seen a lot of CV joints held on with AN fasteners or with AN washers under the bolt heads and people not understanding why they come loose every session. Volkswagen used a special 12 point head metric bolt that was the equivalent of MS20250 or a typical socket headed cap screw - the ubiquitous Allen bolt - and torqued it to around 35 ft lbs, used case-hardened load spreaders under the bolt head, and they did not come loose. I've also changed head gaskets at the track for people who used AN washers under head stud nuts.

My whole point is that people should not just assume that an AN bolt is the only choice or the best choice for any application. An AN bolt is a mundane choice for a mundane application, and the aviation industry likes it that way - foolproof in all intentions of that word. People should examine the requirement and make a decision based on mechanical requirements. The vast majority of requirements are great for AN bolts and equally suitable for a Gr 8 SAE bolt or for an ANSI socket head cap screw. The car I am currently working on has a large number of "Allen" bolts and you can't put anything else in - the head of the bolt is inside a recess that a hex head socket won't fit in. "Critical" locations like the single shear inner A-arm bolts are 3/8" or 7/16", and a nail would be strong enough (FWIW a 3/8" Gr 8 bolt has a shear strength, in single shear, of over 11,000 lbs). I don't think an AN bolt is going to add function or safety to a anti-roll bar link when the bar has about 75 lbs of spring rate and the bolt is 5/16" diameter. That's my point.

One of my vintage cars has the front brake discs held on with six 10/32 countersunk bolts on something like a 3.5" diameter. You can bet I did the math on that before I put it back together. The engineer who designed it knew his strength of materials... it was fine.

Brian

You have some very good points. AN bolts are not some magic strength bolt that some people make them out to be. Lb for lb. they are no better than a matching SAE bolt. What they do provide over SAE bolts is a very large variety of shank lengths in small increments. This makes it very handy. AN bolts also always have the paperwork to back them up. You do pay for this convenience. In some applications you can find a SAE bolt with an unthreaded shank length that will work and you can just cut off the excess thread. This is what I do for the lower motor mount bolt on my F600. The bolt is metric and over 7" long. I don't even want to know what that might cost me for a AN fastener if they even made one. However I can get a socket head cap screw in the necessary unthreaded shank length to make sure the threads aren't in the shear plane and the MTR's are available.

[Steve Demeter]

Also, the shank diameters on AN bolts are held to much tighter tolerances than your garden variety SAE bolts. Hence, less slop in your bolted connections.