Hey there everyone, step right up to the never ending bin of automotive fasteners! This magical bin is filled with every nut, bolt, washer, or screw you could ever possibly need. Don’t worry about grades or threads, any nut will do…or so you might think. In reality, the circus side show of fasteners is fraught with peril if you choose the wrong one. A minor mistake such as thread pitch will send you back to the store, but a major one like using a Grade 2 zinc bolt where a Grade 8 is needed can lead to a dangerous situation. Make sure you have the right fastener for the job using this guide.
Making The Grade
There are three common grades for SAE (fractional) and four common grades for metric fasteners. These are 2, 5, and 8 for SAE, and 5.8, 8.8, 10.9, and 12.9 for metric. Each has specific strengths.
SAE Grade 2, Metric 5.8
These bolts are the common shiny silver zinc bolts that are used for general purposes. They are made of low to medium grade carbon steel, the head has no markings. Sizing ranges from ¼” up 1.5”. Strength ratings are as follows-
Proof load (max non-deformation): 55,000 psi (33,000 for ¾-1.5”)
Yield strength (min deformation load): 57,000 psi (36,000 for ¾-1.5”)
Tensile strength (shear load): 74,000 psi (60,000 for for ¾-1.5”)
Common uses for Grade 2 bolts include general low stress items like door handles, interior trim, low-stress brackets, etc. Never use a Grade 2 bolt in place of a higher grade fastener.
SAE Grade 5, Metric 8.8
These bolts are medium carbon steel, that has been quenched and tempered for strength. They feature 3 radial lines on the head. Metric bolts have the grade number formed on the head face. Strength ratings are as follows-
Proof load (max non-deformation): 85,000 psi (74,000 for ¾-1.5”)
Yield strength (min deformation load): 92,000 psi (81,000 for ¾-1.5”)
Tensile strength (shear load): 120,000 psi (74,000 for for ¾-1.5”)
Common uses for grade 5 bolts include bumpers, engine brackets, brake booster/master cylinders, seats (not seat belts!)anything that has moderate stress.
SAE Grade 8, Metric 10.9
These bolts are the highest yield fasteners for carbon steel. They feature 6 radial lines on the head and are made from medium carbon alloy steel which has been quenched and tempered. Metric bolts have the grade number formed on the head face.
Proof load (max non-deformation): 120,000 psi
Yield strength (min deformation load): 130,000 psi
Tensile strength (shear load): 150,000 psi
Common uses for Grade 8 bolts- suspension, steering, brakes, motor/transmission mounts, high-stress items. The rule of thumb here is this- if it breaks, can you die? If the answer is yes, then use a Grade 8 fastener, period.
These fasteners are tempered and quenched alloy steel and quite strong. They are stronger than Grade 8 bolts, and used in the same instances. Metric bolt heads include the number grade.
Proof load (max non-deformation): 140,000 psi
Yield strength (min deformation load): 160,000 psi
Tensile strength (shear load): 177,000 psi
It is important to note that fasteners come in sets; that is the bolt grade must match the nut grade. In cases where the bolt is threading into a part, such as a cylinder head or engine block, this does not apply. Additionally, the washers should match the grading as well. Grade 2 washers are non-tempered, whereas Grade 5 and 8 are tempered, so they do not deform like Grade 2 washers will. Over time, a Grade 2 washer under a 5 or 8 bolt will deform and lead to a loose fastener, which is not a good thing.
Combining washers and lock washers is not advisable in most situations. If the operate calls for a lock washer, just use a lock washer, a second flat washer can cause some clamping issues.
Then there is the issue of thread countpitch. Most fasteners are sized using two sets of number, the diameter and the thread count or pitch. For example, SAE bolts come in coarse and fine thread, which is referred to as thread count, such as ¼-20 and ¼-28, 28 being the fine-thread variant. Fine-thread bolts are typically a little stronger and yield a higher clamping force than their coarse thread brothers, but this comes at a cost, namely in galling and fouling of the threads. Because the threads are closer together, any debris or rust will cause more problems with removing or tightening a fine-thread bolt. Additionally, fine thread bolts need more thread engagement to provide the same clamping force, you need a at least full width worth of thread engagement for a fine thread bolt to yield full force, where you can get away with less on a coarse thread bolt. Whenever possible, the goal should be a full width worth of thread engagement to yield full clamping strength. Unless specifically called for, the standard thread is coarse.
SAE Thread Chart
|Diameter (Inch)||Course Pitch||Fine Pitch|
Metric Thread Chart
|Diameter (Millimeter)||Pitch (Course)||Pitch (Fine)|
Fine-thread metric fasteners are more difficult to locate, and there are typically three thread versions. Metric fasteners use pitch instead of counts, there is standard, fine and extra or super fine. This is expressed in a metric number like 1.25 or .8. This references 1.25 threads per 1 mm of length. This makes determining the pitch more difficult, as an 8mm bolt can be 1.25 or 1, and 10mm can be 1.5, 1.25 or 1, each is a different count. Metric bolts are typically labeled as standard or fine, but not always.
These basics should help you next time you have to replace nuts and bolts on your project. When in doubt, use the next higher strength bolt, just to be on the safe side.
To learn more about NAPA AutoCare, visit www.NAPAAutoCare.com.
Source: NAPA Know How