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Grade 8 or Grade 5 bolts

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Ok so I’m curious.  What bolts are everyone using on their car.  I have grade 8 for all important parts, suspension parts, a-arms, shocks, seat mounts, motor mounts, etc.  I always thought grade 8 was the best way to go.  But for some reason I started thinking about it the other day and was thinking grade 5 might be better since it’s my understanding it has more shear strength.  Shear strength  would seam to apply more to sand cars.  So what’s everyone using on the different parts?

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F911 on most suspension bolts and Grade 8 on everything else. Grade 8 is overkill for most the stuff but it easier to buy bulk hardware and use the same stuff everywhere. 

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Mark’s Standard Handbook for Mechanical Engineers lists Grade 5 fasteners as 120 ksi fasteners. This means the tensile strength is 120,000 lbs per square inch. It also lists Grade 8’s as 150 ksi fasteners meaning the tensile strength is 150,000 lbs per square inch. Also, the ultimate shear strength of a fastener is typically about 60% of its ultimate tension strength. So given a certain diameter (cross-sectional area) and strength rating, someone can figure out how much load that fastener can carry in both tension and shear.

boltex

Let’s look at an example of where grade 5 and grade 8 bolts are subjected to single shear loads (winch plate reference).

Using a .250-inch diameter grade 8 fastener gives you the following shear capability:

A = Cross-sectional area of the fastener size (since bolt bodies/shanks have circular cross-sections, use area of a circle) = Pi x r2 where R (radius) = .250/2 = .125, therefore A = Pi x (.125)2 = .0491 square inches (in2)
Capability in shear = 91,000 lbs / in2 x .0491 in2 = 4468 lbs

Using the same .250-inch diameter grade 5 fastener results in the following:
Capability in shear = 75,000 lbs / in2 x .0491 in2 = 3683 lbs

That’s a difference of over 750 lbs or over 1/3 ton. In this example you can clearly see that using a grade 8 fastener has a superior advantage over the grade 5. Therefore the result is if someone is using grade 5 bolts in a shear application like the winch plate example, they will fail almost 800 lbs earlier.

There’s an argument that grade 8’s are more brittle than grade 5’s and that’s why you shouldn’t use them. Well, first you need to understand what the term “brittle” really means. Brittleness in bolts is defined as failure at stresses apparently below the strength of the bolt material with little or no evidence of plastic deformation. Typically, fasteners are not brittle below 180 ksi ultimate tensile strength. Grade 5’s have an ultimate tensile strength of 120 ksi and a grade 8 fastener has an ultimate tensile strength of 150 ksi. This is why brittle is a relative term. Nearly all fasteners are considered ductile except some made from PH 15-6 Mo, 17-4 PH and 17-7 PH.

Going back to the D-ring on the face of the bumper example, you would want to know its tensile carrying capability. Calculating the tensile capability is not as easy as shear since the thinnest portion of the bolt is at the minor diameter of the threads (bottom of the thread “V”). So you need to know the nominal minor diameter of that particular fastener. That’s where military specification MIL-S-8879C comes in. It is titled “Screw threads, controlled radius root with increased minor diameter, general specification for”. It lists that and a lot more for almost all possible fasteners. MIL-S-8879C lists the nominal minor diameter of a .2500-28-UNF at .2065 inches. We can now calculate the A (area) of the cross-section:

A = Pi x r2 = Pi x (.2065/2)2 = .03349 in2
Grade 8 bolt capability in yield (stretch) = 130,000 lbs / in2 x .03349 in2 = 4354 lbs minimum
Grade 8 bolt capability in tension (failure) = 150,000 lbs / in2 x .03349 in2 = 5024 lbs minimum
Grade 5 bolt capability in yield (stretch) = 92,000 lbs / in2 x .03349 in2 = 3081 lbs minimum
Grade 5 bolt capability in tension (failure) = 120,000 lbs / in2 x .03349 in2 = 4019 lbs minimum

Again, you can see that the grade 8 will support over 1000 lbs more or a 1/2-ton more. But there’s something more important to note. The grade 5 fastener has already reached its ultimate load and FAILED BEFORE the grade 8 starts to yield or stretch. Therefore, the argument that you should not use grade 8’s because they are more brittle than grade 5’s is not a true statement in most applications.

Toughness is an important feature of a fastener. It is the opposite of brittleness and gives you an idea of how it will handle abuse without being damaged and eventually weakening the fastener or can cause fatigue to appear much earlier than normal. One way to “measure” toughness is by looking at the hardness rating of a fastener. The higher the number (Brinell, Rockwell …) the harder the material is and the tougher it is to damage. According to Marks’ Standard Handbook for Mechanical Engineers, Grade 5’s typically have a core Rockwell hardness of C25-C34 whereas a grade 8 typically has a core Rockwell hardness of C33-C39. Based on this, grade 8’s are tougher than grade 5’s.

Fatigue usually doesn’t play a big part in grade 8 or grade 5 fasteners since most steels are good for 2 million to 10 million cycles. Far more than you will ever winch or pull on. Here is a quick point about fastener fatigue. Almost all fastener fatigue failures are the result of improper (almost always too low) torque. Too low a torque will cause the fastener to pick up more load more often and eventually cycle it to failure. Therefore, you want to make sure you torque your fasteners to the appropriate level using a torque wrench and make sure to torque dry, clean threads.

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Not only does a grade 8 have higher shear strength, the higher tensile strength means you can torque the bolt more, resulting in greater clamping force, resulting in higher shear loading prior to overcoming the clamping force of the joint and actually testing the fastener’s shear strength. 

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One of the most overlooked advantages to grade 8 bolts is the resistance to galling during assembly or disassembly versus a lower grade bolt.

Also generally speaking  grade 8 bolts are made with a higher attention to quality over a grade 5 which results in a much easier task to drill one out that was sheared off or broken. Lower quality bolts are not as consistent throughout the bolt for hardness. Often times when drilling out a grade 5 bolt you will get into it a little and the hardness will change causing your drill bit to want to “grab” making a shock load that will either chip off the hardened tip of the bit sticking it in the hole which further complicates getting a hole through it or breaking the bit entirely. Consistentency throughout the bolt makes the procedure to drill it out much easier, hence you can use the same speed, pressure and lubrication to drill all the way through it.

Do not mix grade 8 bolts with grade 5 nuts or vice versa,  stainless steel with non  stainless steel, etc, etc.

Always lubricant bolts during assembly. I prefer an anti seize but common oil is better than nothing. Never use aluminum based anti seize (it’s silver) in aluminum. Copper based anti seize is the most universal (it’s gold).

Try to avoid Chinese made bolts altogether. They are the worst for consistentency. 

I prefer to make sure any bolt that is in a bulkhead is not placing the load on the threads. In other words the shank or shoulder of the bolt (the part without threads) is where the load is displaced rather than on the threads. This is a common reason a-arm and trailing arm bolt holes wear out prematurely. Properly built mounts should have a spacer that takes up the excess threads rather than the bolt hole which is supporting the load.

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F911 and grade 8.

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I think Rockwell and Romans and all sum it up very well, although I would add a couple points, Nylock nuts are your friend and still quite effective even if you use copper-based antiseize (preferred), but they should always be replaced after removed.In most cases, lock washers pound out in Off road use and fail compared to nylocks. Proper TQ and bolt stretch will keep everything tight, too much TQ will actually weaken bolts (I see that done a lot with 1/4 and 5/16 bolts).

I prefer using 12pt DB (US made Darling Bolt company) wherever possible, they have a higher tensile strength than grade 8, but not as brittle as 911, and Allens Fasteners pricing on those is cheaper than hardware store grade 8's.

When you cut bolts to ensure "shoulders" are in tabs rather than threads, try to use a Carborundum blade rather than a "saw" less heat and far cleaner IMO.

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53 minutes ago, fullthrottleguy said:

I think Rockwell and Romans and all sum it up very well, although I would add a couple points, Nylock nuts are your friend and still quite effective even if you use copper-based antiseize (preferred), but they should always be replaced after removed.In most cases, lock washers pound out in Off road use and fail compared to nylocks. Proper TQ and bolt stretch will keep everything tight, too much TQ will actually weaken bolts (I see that done a lot with 1/4 and 5/16 bolts).

I prefer using 12pt DB (US made Darling Bolt company) wherever possible, they have a higher tensile strength than grade 8, but not as brittle as 911, and Allens Fasteners pricing on those is cheaper than hardware store grade 8's.

When you cut bolts to ensure "shoulders" are in tabs rather than threads, try to use a Carborundum blade rather than a "saw" less heat and far cleaner IMO.

And don’t forget to put the nut on, then cut it so the threads are cleaned and trued when the nut is removed. If the cut is crooked, redo it so it’s straight. 

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5 hours ago, Rockwood said:

And don’t forget to put the nut on, then cut it so the threads are cleaned and trued when the nut is removed. If the cut is crooked, redo it so it’s straight. 

Thanks Rockwell  - I always forget that is not obvious. Keep a non-nylock grade 8 nut around to use for this. and if possible clean the cut edge up with a small file before removing the nut to keep the sharp edges down before putting the bolt into service 

I always paint the end of the bolt as well with rustoleum or POR15 to prevent corrosion when done

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Thanks guys.  Some interesting info.  I have used the silver anti seize on my stainless bolts, and nothing on my grade 8’s.  I’ll have to get some gold for my grade 8’s.  Maybe just use that for my stainless ones also.

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During my full build I’m using all f911 bolts for everything. All bolts are measured to the proper shoulder length so there’s no threads on anything but the nut. Its way easy to do if you have the entire car apart and are slowly putting it back together. Beyond that I’ve also thought about getting custom hardware made from RPI and cataloging everything in an excel spread sheet with every measurement, torque value and ordering information for the off season preps so there’s no reused hardware from season to season 

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49 minutes ago, bansheemantb said:

During my full build I’m using all f911 bolts for everything. All bolts are measured to the proper shoulder length so there’s no threads on anything but the nut. Its way easy to do if you have the entire car apart and are slowly putting it back together. Beyond that I’ve also thought about getting custom hardware made from RPI and cataloging everything in an excel spread sheet with every measurement, torque value and ordering information for the off season preps so there’s no reused hardware from season to season 

That sounds like a great idea.

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Where can I get some F911 bolts? I'll be tearing down my car for powder and new heims. 

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My opinion here...a correctly installed and tightened grade 8 is completely adequate. I believe F911 is more bling than anything. I try not to delve too deeply into the engineering aspect of something because 1) I'm not an engineer and 2) most of the technical information I can get on the internet is just opinion from armchair experts. 

All I can say is my personal sand car is assembled with grade 8 bolts of unknown origin, the 1/2" suspension pivot bolt shanks go completely through the tabs and I use nylocks. I have never broken a bolt.

Read at your own risk. :smilie_flagge13:

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