Bolt Head Markings: What do they mean? | Fasteners 101

Fasteners bear all forms of markings on their heads, from number and letters, to shapes and hash marks. But what do they mean? In this video our fastener specialist covers several more common head markings on bolts and what they can tell you about the fastener.

[anthonytuck]

Great vid. One comment. The stainless bolts are not delineated by higher or lower grades. Each stainless alloy is typically designed to do a specific job in preventing corrosion. 304 is a basic 18/8 alloy that has a broad range of uses and is reasonably priced. 304 is also typically used in you house, kitchen sink, knives, forks etc. 316 has a small amount of molybdenum in it. Typically used in the food industry as it has excellent resistance to the acids found there. When using stainless bolts it is wise to understand strength of the specific alloys as their properties differ vastly from carbon steel bolts. The L appended to the end of the grade (IE: 304L) means that the alloy is low carbon and meets that specification. Happy days

[holliwood]

IFI = Industrial Fasteners Institute. Just like any industry association their purpose is not to make it easy to decipher their markings. Hence "no marking = grade 2, 3 lines = grade 5, 6 lines = grade 8. Common sense does not apply when dealing with standards organizations.

[drgolfjim]

Thank you for getting right to the point.

[bubblywaters]

I just learned that you add 2 to the number of slashes on a bolt head to get the grade. 3 slashes add 2 and you have a 5 grade. Pretty cool. Always add 2 to the number of slashes for actual grade.

[solobassoon]

I have a specification on bolts saying: 'STUD BOLTS C/W 2 A4-70, ISO 3506-2, SF, HH...etc...'. May I ask what does 'SF' and 'HH' stand for?

[hoey]

Could some explain what f and 7 lines on a 5/16 unc bolt mean? Does it mean is stronger than a 6 line grade 8 bolt?

[macbethdemilo]

My stainless steel hex bolt has no dashes. Its has F593C JBK THE on the head. What grade is this bolt? Grade 2 since it has no dashes or something else? Thanks.

[MoeLykit]

The Letter Codes on Bolts at Big Box stores from Bulk displays are Speed Codes for the Cashier to be able to identify them at the register

[danielnegrao]

Is cool, how about metric bolts? And Astm?

[francoistombe]

I have a bolt with 12 radial marks and an F in the centre. Where does this go on the ranking?

[doncallangher]

Thanks for clearing this up for me, its something I've wanted to know since adolescence but too lazy too look up.
So I guess, when in doubt, get a bolt with as many slashes as possible!

[rickconstant]

You don't mention metric bolts at all, which have a numerical system for identifying grades. Even in the US, most cars now are built using metric fasteners.

[CaniLupine]

I have metric bolts with 4 lines that secure my bottom motor mount, and I can't find any information on them.

[meassassin]

How about "7T DS" where the 7T is abve the DS. Closest I coe is JIS, but appears non-metric 5/8-18? Very Confusig (on a tractor wheel stud)

[kiengmeng]

Could you help me explain about symbol M, 8.8, and MICon cap bolt of china product?

[Finlandiabeach]

I have purchased Bolts marked "Fed-Alloy" With 12 markings. They were sold to me by Acklands-Grainger Industrial supply on sale. Are they the strongest bolts available?

[dnolder]

How would you know what hardness a stainless steel bolt is?

[phiethoangthe]

I have a bolt with big letter H on Head, what does it mean? Thanks

[lofivibes-techanimestudios]

Hi, I am new in the fastener sales journey and I'd politely ask for advice to where start Selling Stainless Steel Fasteners :)

[crabjoe]

What is the difference between an A2 and A4 bolt?

I replaced a bolt marked SRS 8.8 with one marked XXYX A2 S304. I was told to find one marked A4, but A2 was all I could fine. This is going to see a lot of road salt in the winters. The bolt is used to keep a DEF injector mounted to a diesel exhaust system. I figured any stainless bolt would be better than a carbide steel bolt, which I think was originally in there.