Threaded fastener grades (Part 2)

Order Reprints


The grade system for nuts is a number that represents 1/10 of the specified proof load stress (in kgf/mm2). The proof load stress corresponds to the minimum tensile strength of the highest grade bolt that can be used with that nut.


Although it is common for Grade 5 bolts to feature three slash marks and for Grade 8 bolts to feature six slash marks for identification purposes, it’s important to note that not all Grade 5 or Grade 8 bolts will feature these marks! Quite often, specialty bolts (made for an automaker’s specific requirements) may not feature any marks at all, or may feature a unique symbol. If you’re performing a restoration, it’s best to stick with factory-original bolts where grade may be in question. Otherwise, if the bolt head isn’t marked (blank head), and if you don’t know where the bolt came from, you should assume that it features a hardness grade of less than 5 and shouldn’t be used. If the bolt was produced by a known quality maker, such as ARP, you can be sure that the bolt is correct for the application recommended by the maker. I’m simply saying that if you don’t know where a bolt came from, and it’s not marked, throw it away and buy known-quality bolts instead.[PAGEBREAK]


Threaded fasteners are graded or classed to identify their hardness and tensile strength. When choosing fasteners for a performance car or race car build, never blindly assume that harder is always better. Bolts, screws, studs and nuts should be chosen for the application at hand. Lower grade or lower class fasteners will tend to stretch more easily than higher grade or class fasteners, and higher grades/classes will be more resistant to stretching but may provide either overkill or damage to components that need to move more easily under thermal conditions. By the same token, don’t always assume that you can use chromed or stainless fasteners anywhere you like, since they may not be up to the task. When bolts are chrome plated, they may become more brittle and less elastic as a result of hydrogen embrittlement during the electroplating process.

Stainless fasteners, depending on grade, may not be able to withstand forces encountered in some engine and chassis applications. Stainless steel is an alloy of low carbon steel and chromium, offering enhanced corrosion characteristics.
A common misconception among hot rodders is that stainless steel is stronger than regular steel, which isn’t the case. Because of the low carbon content in stainless steel, stainless steel cannot be hardened.

When compared to steel, stainless steel is slightly stronger than an un-hardened Grade 2 steel fastener, but is significantly weaker than hardened Grade 5 or Grade 8 fasteners.

Grade 2 is a standard hardware grade steel, generally unacceptable for any automotive applications.

Grade 5 bolts are hardened for increased strength and are the most common bolts found in automotive applications. Grade 5 bolt heads feature three radial lines as a marking.

Grade 8 bolts have been hardened more than Grade 5 bolts, and feature six evenly spaced radial lines on the head as a marking. Grade 8 bolts are stronger than Grade 5 bolts, and are typically used for demanding applications such as suspension components.

Alloy steel bolts are made from a high-strength steel alloy (not to be confused with stainless steel), and are further heat treated for added strength. Alloy steel bolts are typically not plated, resulting in a dull black finish.

In other words, don’t pick fasteners based only on appearance, especially for applications such as engine assembly, suspension, steering and brake systems. Do your homework and use the specific metals and grades recommended for specific applications.

(Note: Grades are used to identify fractional fasteners; while class is used for metric fasteners.)

GRADE 2…….Made from C1006-C1022, C1215, 12L13 or 12L14 steel. For use in only low-strength applications. Minimum Rockwell hardness B49. Minimum tensile strength is only 44,225 psi.
GRADE 5…….Made from steels such as C1008-C1026. For use in medium-strength applications, with a Rockwell hardness of C25. Minimum tensile strength is 120,000 psi.
GRADE 8……Made from stronger alloy steel such as C1021-C1045, for high-strength applications. Minimum Rockwell hardness C24. Tensile strength is minimum of 150,000 psi.
GRADE 9……Commonly called L9. Rockwell hardness C38-42. Minimum tensile strength 180,000 psi.
CLASS 4/4.8/5.8…..Comparable to Grade 2. Minimum Rockwell hardness is B71. Tensile strength is 60,900 psi minimum.
CLASS 6………….Comparable to Grade 2. Minimum Rockwell hardness is B89, and minimum tensile strength is 87,000 psi.
CLASS 8………….Comparable to Grade 5. Minimum Rockwell hardness is C20. Minimum tensile strength is 116,000 psi.
CLASS 10…………Comparable to Grade 8. Minimum Rockwell hardness is C32. Minimum tensile strength is 150,800 psi.
CLASS 12…………Exceeds Grade 8. Made from alloy steel C1035 and C1045. Minimum Rockwell hardness is C39. Minimum tensile strength is 176,900 psi.

Related Articles

Threaded fastener grades (Part 3

Threaded fastener grades (Part 1)

Threaded fastener grades (Part 4)

You must login or register in order to post a comment.