You can usually figure out the grade, or strength, of a bolt by looking at the markings on its head. English bolts (U.S.S. or S.A.E) are categorized by grade, from grade 2 to grade 8, where the grade represents inch strength. The higher the grade, the stronger the bolt. English bolts have radial lines on their head to identify the grade, or strength class. The number of lines on the bolt head is 2 less than the actual grade. So, for example, a grade 5 bolt will only have 3 radial lines on its head. A grade 8 bolt will have 6 radial lines on its head. Metric bolts are a bit more straight forward, usually having their strength class number stamped right on the bolt head. Some metric nuts also have the strength class number stamped on their face. The below diagram shows examples of English and Metric bolt and nut markings.
1. Grade 2 English Bolt
2. Grade 5 English Bolt
3. Grade 7 English Bolt
4. Grade 8 English Bolt
5. Class 9 Metric Nut
6. Class 4.6 Metric Bolt
7. Class 4.8 Metric Bolt
8. Class 5.8 Metric Bolt
9. Class 8.8 Metric Bolt
10. Class 9.8 Metric Bolt
11. Class 10.8 Metric Bolt
| US (English) Bolts | ||||
| Grade and Material | Nominal Size Range (inches) | Proof Load (psi) | Min. Yield Strength (psi) | Min. Tensile Strength (psi) |
| Grade 2 - Low or medium carbon steel | 1/4" - 3/4" | 55,000 | 57,000 | 74,000 |
| 3-4" - 1-1/2" | 33,000 | 36,000 | 60,000 | |
| Grade 5 - Medium carbon steel, quenched and tempered | 1/4" - 1" | 85,000 | 92,000 | 120,000 |
| 1" - 1-1/2" | 74,000 | 81,000 | 105,000 | |
| Grade 8 - Medium carbon alloy steel, quenched and tempered | 1/4" - 1-1/2" | 120,000 | 130,000 | 150,000 |
| Grade A325 - Carbon or alloy steel with or without Boron | 1/2" - 1-1/2" | 85,000 | 92,000 | 120,000 |
| 18-8 Stainless - Steel alloy with 17-19% Chromium and 8-13% Nickel | Up to 1" | 20,000 Minimum 65,000 Typical | 65,000 Minimum 100,000-150,000 Typical | |
| Metric Bolts | ||||
| Class and Material | Nominal Size Range (mm) | Proof Load (MPa) | Min. Yield Strength (MPa) | Min. Tensile Strength (MPa) |
| Class 8.8 - Medium carbon steel, quenched and tempered | Up to 16mm | 580 | 640 | 800 |
| 16-72mm | 600 | 660 | 830 | |
| Class 10.9 - Alloy steel, quenched and tempered | 5-100mm | 830 | 940 | 1040 |
| Class 12.9 - Alloy steel, quenched and tempered | 16-100mm | 970 | 1100 | 1220 |
| A-2 Stainless - Steel alloy with 17-19% Chromium and 8-13% Nickel | Up to 20mm | 210 Minimum 450 Typical | 500 Minimum 700 Typical | |
Proof Load is the axial tensile load that the product must withstand without any evidence of permanent set.
Yield Strength is the maximum load at which a material exhibits a specific permanent deformation.
Tensile Strength is the maximum load in tension (pulling apart) that a material can withstand before breaking or fracturing.
1MPa = 145 pounds/inch²
Approximate Equivalent Chart for Metric and English (USS/SAE) Bolts
| Metric Class | SAE J429 Normally Used | Grades | ASTM Grades |
| 4.6 | 4 or 5 | 1 | A307, Grade A |
| 4.8 | 4 or 5 | 2 | |
| 5.8 | 5 | 2 | |
| 8.8 | 8 | 5 | A325, A449 |
| 9.8 | 9 | 5+ | A193, B7, B16 |
| 10.9 | 10 or 12 | 8 | A490, A354, Grade 8D |
| 12.9 | 10 or 12 | A540, B21 – B24 |
I have a Toyota bolt securing a bul bar with “11” stamped o . it. What would this mean?
Can you email us a picture of it? You can send it to [email protected].
I have the same thing would love a explanation
I’ve got 11 stamped on chrysler head bolts as well. I’ve been sent 10.8 bolts as replacements from online, they have a cheap finish on them.
I wanted know if they are going to be ok for replacement of 11 head bolts.
Thanks
The “11” is a manufacturer mark. You would want a Grade 8 bolt to replace it.
if this is for a Chrysler 2.2 or 2.5 used from early 80’s – mid 90’s I believe the 11 represents 11mm for the thread.
The blocks went from 10mm thread to 11mm thread for the head bolts in 1986 IIRC. Therefore they marked the bolts on the head so the techniacian would not use the wrong size as they were replaced frequently.
I have a number of bolts of various sizes on a 1960 Lancia, an Italian car. They are marked OAV 100. I understand the manufacturer is Agrati, but cannot find information about the “100” marking. I assume it has to do with grade or class. Have you any information on this? Thanks.
Hi John. I would agree with you that it probably has to do with class, but unfortunately we do not have any reference info on the Agrati bolts.
I rebuilt a Lancia Fulvia Sport and you need to note that the car was over-engineered; Many bolts are metric fine thread which are not easy to find (more threads per length of bolt) and secondly many of the bolts are specific to one application on the vehicle, so don’t throw them away or cut them off because you may find replacements hard to come by. Also they use bolts (only partially threaded) rather than cheaper set screws (fully threaded).As an example, the steering box has three bolts attaching it to the sub frame and are all different lengths. There are also some bolts where the smooth ‘body’ of the bolt (the non-threaded part) is a larger diameter (shank) than the diameter of the thread. This costly over-engineered approach was a casualty of the FIAT takeover in the 1970’s when FIAT’s own stamped bolts started to replace the Lancia ones.
I think the clue is in your comments …. they were over engineered and it sent them broke. Fiat used more standardised product and were still going. Maybe there is a lesson in that!
I found a one inch bolt on the garage floor and am concerned it came off of my sons 2015 Buick Verano. Through my research I have found out the manufacturer of the bolt does indeed supply Buick. The markings are SPF 8.8. Do you have any clue as to what purpose this might serve on a car. I am concerned it either comes from the master cylinder or drivetrain.
Hi Adam. It’s hard to say as it could be off of a number of things. I would start looking around the area of the car where you found the bolt.
Main bearing cap bolt for 235 chevy 1936 truck
The bolt denotes a (Y) on head of bolt what
grade is it.
The “Y” is most likely a manufacturer mark. It could possibly a Grade 5 notation if it’s three lines and not connected in the center, but I don’t think so due to its age. Feel free to post an image of it and someone may be able to help, though.
I have backup plates to spindle mounting bolts. One half inch fine thread by 3/4 of an inch long. They are marked 30 – 37 on the head. Is that a strength grade? Some of these bolts were handmade. I am not knowledgeable of alloys or heat treating processes in 1931. I’m guessing at best this half inch bolt is SAE5 to SAE6, 60 to 85′ pounds torque. Do you have any information?
I have a carriage bolt with 3 L D marked on the head of it, curious to what that stands for?
I have a 2012 Toyota Sienna Front axle control arm bolt with the marking ” II”. I dont know if its 11 or the alphabet “ll” or capital “ii”. I am using a 10.9 W.T bolt for replacement because it shredder threads on the original bolt with impact gun :((((( I wonder if 10.9 bolt is as strong as this “II” or “11”. Any info is greatly appreciated. Thank you!
I have a 1/2″ carriage bolt with B U B stamped on it. Anyone know what that means? Thanks
This was a helpful article to see where my 8.8 bolts fit in. Does anyone know if they can be torqued to 40nm?
What grade of bolt is used to secure a cylinder head to the engine block of a 4.2 L (258 cu.in.) Jeep engine?
I have two different length 7/16×14 bolts taken off that engine and cannot find like sized 7/16×14 bolts anywhere.
Grade 8 or higher as it’s a head bolt.
The black oxide finish or the yellow or silver color finishes don’t matter as long as the threads are correct. (Personal experience talking to Tech’s over the years.)
I have a some 5/16 unc bolts 1/1/4 long it has f in the centre and 7 lines around the head.. is this stronger than a grade 8 bolt? I was thinking of using them for clutch bolts
That sounds like a manufacturer’s mark, so I can’t say if they are as strong as, or stronger than, a grade 8. I would probably just get some new grade 8 bolts and not chance it.
I have an issue the original bolts are stamped with Q/9.8 the replacement bolts that were used are stamped with BB/10.9 would that cause an issue?
Hi Aaron. The “Q” and “BB” markings are most likely a manufacturer’s mark. Based on the numbers, it looks like these are metric bolts. If that is the case, then the 10.9 would be a stronger bolt than the 9.8. 10.9 is similar to a Grade 8.
Hi the head bolt tool o am looking for is suppose to be an M9 head bolt extractor but i am not sure it has multiple angled flanges…it is not hexagon as identified as being an M9 extractor…could you please identify it for me? I do have a picture of it but couldnt attach here.
Feel free to send us an email, with pic, at [email protected]. thanks
I have a couple of bolts that I can’t identify the hardness on.
They are metric, 8mmx1.25 threads and have a 7 on top of the head, with a tiny triangle or delta above it.
They also have attached lock and flat washers and are yellow cad plated.
Any clue to their hardness and torque values would be greatly appreciated!
Thanks,
don.
That sounds like a manufacturer’s mark, but we can’t say for sure. Unfortunately, we don’t have a reference for this particular marking.
I have some metric bolts with no numbers on the head just some dots some have 2 others have 4. 3 together & 1 opposite. What does this mean?
It seems like those would be some sort of manufacturer marks.
Hi there. I am trying to find a grading off a bolt. Its of a Nissan patrol Y62. these 4 bolts hold in the rear subframe K cross member on the chassis. I have some pics of the bolt head but the stamping marks mean nothing from what I can find. We have managed to find that the torque Nissan says is 240 NM.
The bolt size is 14mmx 1.5
Can you help.
Im happy to send some pics of the bolt
Hey Aaron, please send pics to [email protected] and we’ll take a look at them. Off the top of my head, I’m guessing these would be Class 10 due to their location/use, though.
what is the grade equivalent to a 12.9
Class 10 or 12 typically equate to Grade 8.
I have a bolt that has 6 lines from the 10 o clock to the 2 o clock position with a capital A towards the bottom. What is hardness and is it metric or standard
John,
Typically 6 Radial lines would be a Grade 8 Bolt. The A on the Bolt is most likely a makers mark they can also be used for things like type of metal as well ( I can’t determine the maker by the mark unfortunately ). Generally when it comes to identifying if they are metric or standard, you can determine that by the markings for the grade. Standard will most likely have lines, like the 6 radial lines yours has to determine grade. Metric generally only uses numbers to determine grade like 8.8 or 10.8 etc.