Bolts/Nuts-#8 or 5? NF or NC?

[dfndr]

I'm stocking up. Please educate me on when grade 8 should be used instead of grade 5. What about about NF or NC? Thanks

 

[t. jones]

Most important is know the manufacturer. There is still a lot of substandard counterfeit junk floating around. I use 3/8 coarse grade 8 more than anything else. ...Thanx Trevor

 

[Ign]

There's huge debates and many theories on Gr5 vs Gr8. A web search would turn up a lot.

The short answer is that Gr8 is stronger nearly everytime, in both shear and tensile strength. Generally it's not much more expensive, so for anything less than production cost is a small issue.

As for fine or coarse thread, in most applications coarse thread is great. It won't clog up as easily w dirt, sand, or rust, and works better even if you ding a couple threads by accident - I often have a problem w scraping automotive u-bolts as I pass them thru a spring plate.

Usually if I'm tapping thin plate I like fine thread - more threads per inch equals more strength and less possibility of stripping, but I'm usually tapping mild steel so grade is less relevant.

Here's a link w some info too:
http://www.accuratescrew.com/TechTips/?TipNO=6

If I were stocking up I'd get Gr8 coarse, and for the rare occasions when I want fine thread I'll go buy it specifically.

 

[1969]

Gr8 is much stronger and a much harder steel. Personally I use Gr5 for the everyday run of the mill stuff. If there is a question about safety, go with Gr8. In my neck of the woods, I have found Gr8 to be a bit more costly, thus my reasoning.

 

[Full Size 66]

The application is a big factor. Yes 8 is stronger and harder than 5 but sometimes you want a little stretch, rather than a hard bolt that would just snap off from shock load. This may sound convoluted but most engineer guys have tried to explain to me before. Some times factory car stuff is 5 by design and meant to be wearable. If you put in a hard 8 you might wear out the other way, or break off totaly.

 

[sberry]

Grade 8 for highly engineered connections where more clamping power is needed, Its a rare day when I need one, I usually up size if there is doubt about strength.

 

[kerryt1]

Either will provide the clamping strength you need, but you would need a larger diameter grade 5 fastener than a grade 8 to achieve the same strength. You might select a grade 8 over a grade 5 if you had geometric constraints when designing your bolted joint. It's not a bad idea to go grade 8 for extra insurance, I suppose.

As mentioned, the fasteners are only as good as your trust in the manufacturer because you obviously can't test the material strength of a fastener before using it, and there are crappy mislabeled fasteners available.

 

[38Chevy454]

Grade 8 are heat treated (through hardened), Grade 5 are not heat treated and are stronger due to cold forming of the bolt. Both typically have rolled threads which is better than cut threads.

Fine thread is better for self-locking and ultimate clamping force. Coarse thread is more popular.

For home use, if you can buy in bulk, get grade 8. Overstrength is not a problem. Understrength is a problem. Grade 8 has superior mechanical properties vs Grade 5, so use the stronger bolts and nuts given the choice.

 

[Falcon67]

Whatever fastener you pick, make sure you use the correct torque in a loaded joint or risk failure of the fastener.

http://www.portlandbolt.com/technicalinformation/bolt-torque-chart.html

 

[Eds_tls]

For the record, I've been a fastener design engineer for 12 years. Both designing the tools that make them and the fastneres themselves. I have never once in that 12 years seen proof positive of fradulent graded fasteners. Graded meaning with head markings.

There was a time about 15-20 years ago where there was a problem with that. It wasn't widespread but it happened enough that a federal law was passed. Its called the Fastener Quality Act. It is a federal crime to sell or distribute grade marked fasteners that do not meet specification. American companies take it very seriously now. I have not read or heard of one instance of it happening since the law has been passed.

As for the other questions, most of the above answers are right. Grade 8 is stronger than Grade 5. The only time I can think of when a Grade 5 fastener might be perferred is when using a short screw. Short fastners don't stretch as much as long fasteners, so in that case you may want a softer screw that will allow stretch at a lower clamp load. Bolt stretch is what keeps joints tight. Without it, you have fasteners comming loose

As for which to use on what applicaiton, that has too many variables. Grade 5 is very strong and is used in many, many applications. But when in doubt, you're usually better off erring on the side of overkill and going with a Grade 8

Fine thread has higher tensile and shear strength because of the greater cross sectional area. But the trade off is slower assembly time, (more rotations to seat the part). But that only really matters in a production setting.

 

[Eds_tls]

Grade 8 are heat treated (through hardened), Grade 5 are not heat treated and are stronger due to cold forming of the bolt. Both typically have rolled threads which is better than cut threads.

Grade 5 is absolutly heat treated. Grade 2 is not heat treated. The cheap-o pan head machine screws in the blister packs are Grade 2

 

[38Chevy454]

Grade 5 is absolutly heat treated. Grade 2 is not heat treated. The cheap-o pan head machine screws in the blister packs are Grade 2

I disagree with you and am a metallurgical engr that has dealt with many fasteners over many years. Grade 5 are NOT heat treated. Grade 2 are a lesser grade of steel and do not work harden to the same amount as grade 5. Grade 5 rely on the cold working to increase the strength and not on heat treating.

A grade 5 *could* be heat treated, but it is not economically smart. A proper cold heading die and subsequent cold working of the metal will produce a bolt meeting grade 5 properties without heat treating.

 

[Eds_tls]

Go look up SAE J429 tomorrow at work. Thats the spec for heat treat for Inch fastners. Its all listed in that spec how to process.

You can argue with me all you want. There is nothing to disagree with. You cannot have a Grade 5 fastener that is not heat treated. I literally work with fastener specification for 9 hours a day, every day for the last decade plus of my life

See page 2 under Mechanical Properties..

http://www.nucor-fastener.com/Files/PDFs/TechDataSheets/TDS_009_Hex_Cap_Screws.pdf


Mechanical Properties
The mechanical properties of finished Cap Screws are covered in the Society of Automotive Engineers
(SAE) J429, “Mechanical and Material Requirements for Externally Threaded Fasteners Specification”.
This specification covers the materials, manufacturing methods, mechanical properties, and testing
requirements for cap screws. Grade 2 (A307A) Cap Screws are low strength fasteners made from carbon
steel, achieving their strength primarily from cold forming. Grade 5 (A449) Cap Screws are medium
strength fasteners that achieve their strength from the medium carbon steel and a quench and temper heat
treatment. Grade 8 Cap Screws (A354 GR BD) are high strength fasteners made from alloy steels that
also achieve their strength from the alloy steel and a quench and temper heat treatment. Standard
mechanical tests are performed in our A2LA (American Association for Laboratory Accreditation)
Accredited Laboratory.

 

[38Chevy454]

Ed, we can respectfully disagree, not all grade 5 are heat treated. I am out of this discussion. Have good evening.

 

[dfndr]

Ok, looks like I'll get Grade 8, NF for most of my stock. Not that much more for amount I'm getting.

BUT, my local industial supply place told me their bolts/nuts came from China! Is that a problem? How do we know they are to spec? Who sells American made fasteners? How do we know if they are made in USA?

 

[DHCrocks]

Grade 8 might be stronger but that is not always better. For instance I've been told numerous times that for tow hooks/****** points on 4x4s you should only use grade 5 bolts and not grade 8. The problem is that when snatching a stuck truck out sudden high shear stresses are developed and it can snap grade 8 bolts and send the hook flying like a missle. Grade 5 bolts do not exhibit this property and has more give and will not snap, but deform.

 

[balddave]

Fine thread has higher tensile and shear strength because of the greater cross sectional area. But the trade off is slower assembly time, (more rotations to seat the part). But that only really matters in a production setting.

This is true, except that fine threaded bolts have lower thread shear areas, meaning they will pull out easier.

In general:

If you are loading a bolt in shear...fine will be better

If you are loading a bolt in tension...coarse may be better depending on length of engagement

 

[Graham08]

I do pretty much all race car stuff. For general use, I keep an assortment of Grade 8 NF, and when I need to restock, I order from McMaster-Carr. I have had really good luck getting Made in USA fasteners from them. Hex head bolts are normally Lake Erie brand, and have "LE" right on the head.

The sad thing is, Made in USA stock from McMaster is normally cheaper than walking into Fastenal and buying Taiwan bolts.

 

[Eds_tls]

Ok, looks like I'll get Grade 8, NF for most of my stock. Not that much more for amount I'm getting.

BUT, my local industial supply place told me their bolts/nuts came from China! Is that a problem? How do we know they are to spec? Who sells American made fasteners? How do we know if they are made in USA?

Fine thread isn't as common as course. If you're buying the mating fine thread nuts, then thats fine. But if you're buying the screws for general purpose for "just in case" you need one, then you're better off getting course thread because NC internal thread is more common.

You didn't say what your using the stock for, so its hard to say if you should go NF or NC.

You can ask your fastener distributor the COO. All the big places should have paperwork on file and should be able to supply you with material certificaitons.

Please do ask for domestic made fasteners....it helps keep me employed ha!

 

[Kevin54]

Ok, looks like I'll get Grade 8, NF for most of my stock. Not that much more for amount I'm getting.

BUT, my local industial supply place told me their bolts/nuts came from China! Is that a problem? How do we know they are to spec? Who sells American made fasteners? How do we know if they are made in USA?

For most applications you wilkl want a coarse thread over a fine thread. Most standard bolts in automotive applications if not metric are coarse threads. Notice that I say most but not all. 1/4-20, 5/16-18, 3/8-16. 1/2-13, are your most common. I have run into 5/16-24 and 3/8-24's on occasion.Then the most common of the number sizes are 4-40, 6-32, 8-32, 10-24, and 10-32.

As far as grades, I would get Grade 8's as most applications you want strength. The only thing that I have ran into that purposely uses a lower grade bolt or thread is the U-Bolts that hold a differential on. When torqued to the proper specs, it pulls the threads and locks the nut to the U-bolt so it cannot work loose. Once torqued, the nut does not come back off without stripping the threads off. That is why they just recommend cutting them off and replacing them with new.

 

[sberry]

Ed, I am guilty of not reading a whole lot on bolts but was under the impression that 5's were also low carbon low alloy but obviously not, they are med carbon then? They weld very well, no cracking sometimes found with mild electrodes on medium steels. I guess I would have just assumed 8 were med carbon alloy.
Now we know, thanks Ed

 

[Falcon67]

Also as I have mentioned elsewhere, you can get Grade 8 bolts in common sizes at Tractor Supply. They sell by the pound, which is a billion time cheaper than other places. Their Grade 5 stuff was $1.99 a lb the last time I was there.

 

[Graham08]

I would be leery of using Tractor Supply bolts in a critical application. They are definitely offshore in origin. They're OK for general use, but I want to know manufacturer and country of origin for anything someone's life may depend on.

 

[Eds_tls]

Ed, I am guilty of not reading a whole lot on bolts but was under the impression that 5's were also low carbon low alloy but obviously not, they are med carbon then? They weld very well, no cracking sometimes found with mild electrodes on medium steels. I guess I would have just assumed 8 were med carbon alloy.
Now we know, thanks Ed

Grade 2 = 1022 steel
Grade 5 = 1038 or 10B21/10B22
Grade 8 = 4037 or 4140

 

[zmotorsports]

As a general rule of thumb I use the premise of Grade 5 fasterners are rated at 120kpsi (120,000 pounds per square inch) and Grade 8 fasteners are rated at 150kpsi. These ratings are based on the fastener holding in tension. Shear ratings are generally 60% of tension. Given a certain diameter (cross-sectional area) and strength rating, someone could calculate how much load a particular fastener could carry in both tension and shear.

The formula for calculating is
A= Pi X r2
Where A= Area
Pi= 3.1416
r= radius (1/2 of diameter)

If using this formula we can calculate the load a .250" Grade 8 vs. a .250" Grade 5 bolts in shear can support.

Grade 8
A= PiX(.125)2 or A= 3.1416X.015625 = .0491
60% of 150k = 90,000
90,000 lbs./in.2 X .0491 lbs./in.2 = 4419 lbs. of force in shear

Grade 5
A= PiX(.125)2 or A= 3.1416 X .015625= .0491
60% of 120k = 72,000
72,000 lbs./in.2 X .0491 lbs./in.2 = 3535 lbs. of force in shear

You can see that the Grade 8 will support nearly 900 pounds more load than a Grade 5 of the same size. The question that you have to ask yourself is what IS the load and generally the Grade 5 is still more than adequate. Most products manufactured use a Grade 5 due to cost. The fastener is still more than likely overrated for the job even at a Grade 5 and there is a price difference between Grade 5 and Grade 8 fasteners as most of you know. It may not seem like a lot when purchasing a dozen of so fasteners but when products are mass produced it can be quite a substantial difference.


This will usually not be practical to calculate on every job to determine what fastener you should use, however, on those particular jobs that you DO want to know or have a specific requirement it is useful. Another practice when the load is approaching the fasteners capacity is to step the fastener up a size or two.

Another thing to remember is a bolt/fastener is nothing more than a spring and must stretch a certain amount in order to hold securely. When a bolt is stretched beyond its yeild strenght it should be replaced. The yield strength is the point at which a fastener is stretched and begins to take a set and not return.

It is never good practice to reuse fasteners that have been subjected to loading or the elements. Corrosion can cause a fastener to fail well below its initial strength. Mike.

 

[dfndr]

Thanks Guys. If I was not a member of this board, with all you who know a hell of a lot, I would have just walked into Lowe's or HF and bought $50 worth of bolts. Now, as usually happens, after getting the benefit of your wisdom, I'll be spending about five times that. Thanks. Rick