Most mechanics pull apart a component to repair or replace it and they just reuse the bolts. So, most of the time, thinking about bolt grades isn’t even a thought. However, when you have to replace a bolt or doing something custom. It is highly important to know what bolt grades to use.
I have seen DIY and professional mechanics, use a low-grade bolt for head bolts, and yes, disaster happened!
It is even more disastrous when using a low-grade bolt for pulling an engine with a hoist. How many are guilty of using a low bolt grade on an engine stand mounting a big block?
If you are wanting to know more about Bolts by Grade and Torque, my informational page can supply you help guide you.
Determining Bolt Grades by Head Markings
Bolts Grades on Top of Bolts-1.jpg
Many think that a bolt is a bolt, but this is so far from the truth. This can also be a dangerous mistake if an inferior bolt is used in the wrong place.
On the head of every bolt, there are markings that are telling you what strength the bolt is. This applies to nuts as well. If using a nut and bolt combination and the nut isn’t the same strength as the bolt. You are just asking for an issue.
Determining bolt grades by head markings involves identifying the symbols and numbers stamped on the bolt head.
These markings indicate the bolt’s material composition, strength, and manufacturing standards. By understanding these markings, you can select the appropriate bolt for specific applications.
This ensures safety, performance, and compatibility in assemblies and mechanical projects.
Bolt Grades by Markings Chart
You will find the strength of your bolt by the marking on the head of it. This chart is for the most common bolts used when working on cars and trucks. However, there are some stronger bolts than the one listed below.
The stronger bolts are going to be used in heavy commercial use like for cranes and boilers and such. Let’s just focus on the automotive application to keep it simple.
SAE Bolts
Metric Bolts
Determining Nut Grades by Markings
There are many that haven’t noticed that nuts also have markings on them to identify the grade. Sadly, the markings are being changed on nuts, just to complicate things more.
Originally, for SAE nuts, they just put dashes on the nuts. Depending on how many dashes, would indicate the grade. Now, we have dashes and dots and you have to figure the degree angle the markings are to identify the grade.
This is about as stupid as going to the Torque Angle Guage, instead of just using a Torque Wrench.
Identifying Nut Markings
SAE Nuts
If a nut has no markings on it, then it is a grade-1, could be a grade-2. They do this nonsense with the bolts as well. I guess they figure that grades 1 and 2 are not worth marking since they are low strength.
Starting with a grade-5, this nut could have the following. The nut could have 3 dashes evenly spaced. It could have a dash and a dot, put at a 120° angle from each other. Or it could have two dashes at a 120° angle from each other.
A grade-8 nut, originally had 6 dashes evenly spaced apart. Then here is where it gets really stupid. Now, the nut can have a dash and dot, or 2 dashes like the grade-5. However, these are at a 60° angle from each other, instead of 120°. I mean why?
NOTE: If it is easier, you can view these marks as hands on a clock…
Metric Nuts
Metric nuts aren’t any better, they too have dashes and dots. All metric nuts have dashes and dots, unless it is a grade-12, then it is 2 dots side-by-side.
A grade-5 is 135° apart. Grade-6 is 180°; grade-8 is 120°; a 9 is 45° and a grade 10 is 60° apart. A grade-14 is like a 12, however it is 2 dots together and a dash at a 60° angle.
Grades in Washers
At times, washers had markings on them. But in reality, I wouldn’t count on identifying a washer by marks. You rarely see markings, and even if you do, the markings aren’t consistent with one another.
There are however, different grades, and there is a difference between a SAE washer and metric. So, if you are thinking a washer is a washer, you are mistaken.
It is very important to use the correct grade washer to match the grade nuts and bolts. Washers have grades just like bolt grades.
By using too low of a grade washer, or lock washer, on a higher-grade nut or bolt. The washer could crush and make the fit become loose.
However, if you are using lower bolt grades with a higher-grade washer, that is perfectly fine.
If you are unclear what the grade means, let me clarify this. The grade is the type of metal the nut, bolt and washers are made of. Such as soft steel, all the way up to high tensile strength steel.
The lower the number, the softer the steel. Higher in the number means it is a more harden steel.
So, if your project or part requires an excessive high torque, ensure that your washers are strong enough. They may initially torque down correctly. But being too soft of steel, in time will crush, causing your torque setting to change.
I would strongly suggest, always go buy new washers at the grade you need. This will eliminate any possibility of getting the wrong washers under your nuts and bolts.
Bolt Grades Determine How Much Torque
The reason a bolt isn’t just a bolt is, strength and how much torque you can apply with it. Granted, there are assemblies are not so much torque, as the assembly may have movement.
You don’t want a soft steel bolt on a joint in a moving part. However, parts on cars like brake caliper brackets, motor mounts or control arms. The bolts have to be high tensile strength or they will shear or break.
There will be an occasion in every mechanics life, that torque specs won’t be available. You will be putting something together and have no idea just how strong the bolt needs to be. This is where you go for Grade-8 or higher, just to be sure.
Now, the industry has interjected this completely STUPID method of tightening bolts with a torque angle gauge. It just makes things far more difficult and less accurate. Although they claim it is far more accurate due to thread stretch.
Any long-time expert at building engines will tell you. They like to torque a cylinder head down in many steps of torque. If a head bolt torqued at 80ft-lbs, you get the best results by doing 20ft-lbs increments until you reach the spec.
Not anymore, now you tighten it to a Foot Pound setting, then turn it some additional degrees.
This is where knowing Bolts by Grade and Torque come in handy. If you know your bolt grades and the size of your bolt, you can guestimate the torque spec.
This doesn’t mean if you have a bolt that can take 100ft-lbs, you should torque it to that limit. You should always research what you are tightening down. But bolt grades can give you a close ballpark.
Bolt Grades Conclusion
Anytime you are dealing with a car or truck, always use grade-8 bolts. For heat shields, or shields period, you can go with grade-5 or even less. However, for everything else, I would stick to higher bolt grades.
If you run across something you can’t find a torque spec for. Or you don’t want to rely on the torque angle gauge spec. Knowing you bolt grades and the size, can give you a really close idea.
For engine torque specs, when rebuilding an engine. Look up the specs for an older engine before they started using the torque angle method and guestimate.
Just remember, if you are doing something like a cylinder head. Just because your bolt can torque 120ft-lbs, doesn’t mean that is where you would torque the head bolts too.
Remember, using the correct grade washers are also as important as using the correct bolt grades. With that, using the matching grade on nuts is also extremely important.
Never use a low to no grade bolt when pulling an engine out. Or using any type of overhead crane with inferior bolts. Keep yourself safe by spending an extra dollar or two to buy good bolts.
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