As someone who has seen more than a few “that’ll do” repairs, I think spending a bit of time understanding the nuts and bolts of nuts and bolts is a good idea.
With standard or SAE (Society of Automotive Engineers) American bolts, we find those fasteners using standard tools to tighten or loosen them. Sizes of these bolts might include 1/4 (which generally needs a 7/16 wrench), 5/16 (1/2 wrench), 3/8 (9/16 wrench) and so on. Standard bolts come in two thread types: coarse and fine-thread. Fine-thread bolts have more threads per inch and are generally found in settings where greater holding power is needed. I generally ran into fine threads when splitting tractors (separating the engine from the transmission at the clutch housing or the speed transmission for the differential).
Metric bolts of course require metric tools, although sometimes you can cheat a bit and use a six-point standard socket to remove a metric in a pinch. I do not recommend this, however, as it is too easy to round a bolt head off by using a “will fit” tool (see “that’ll do” repairs). Metrics also come in different thread pitches. Truthfully, I find metric pitches much more difficult to differentiate from just looking at them, but that is most likely because of less personal experience on my part.
Bolts are made of different grades of steel. The greater the steel’s tensile strength, the more torque the bolt can take and the tighter the bolted joint will be. Standards for bolt strength grades are set according to a system devised by the SAE; this marking system uses raised dashes on the bolt head to indicate strength. Other systems use raised numerals on the bolt head to indicate bolt grade.
According to the SAE, the SAE marking standard starts with grade 2, indicated by a bolt head with no markings whatsoever. A grade-2 bolt made of low-carbon steel has a tensile strength of 64,000 pounds per square inch or less. Tensile strength is the amount of pull the bolt can withstand before breaking. A bolt head with three raised dashes in a radial pattern marks an SAE grade-5 bolt of tempered medium carbon steel with a tensile strength of at least 105,000 pounds per square inch.
The strongest commercial-quality bolt is grade 8, marked by six raised dashes; its medium-carbon-alloy steel has been quenched and tempered to achieve a tensile strength of 150,000 pounds per square inch.
Before we move into metric bolts, let’s stop just a second and visit one point that might stop some real heartburn. Several implements around the farm (especially those that are PTO-driven) will use shear bolts to help protect the drivelines and other moving parts of the machine. The theory is: If the machine (for example, a silage blower, baler or mower conditioner) experiences a sudden load that stresses the machine, the shear bolt will fail, saving the machine or tractor from much more costly damage.
Since the bolt must shear, you might be led to believe a softer, lower-grade bolt would fit the bill. This is not the case. A softer bolt will be more likely to bend instead of shearing, allowing the machine to continue to be stressed and eventually suffering damage. Therefore, the highest-tensile-strength bolts are needed as shear bolts.
Metric bolt strengths are set by the International Standards Organization (ISO). The strength grade of a metric bolt, known as its property class, consists of two numbers separated by a dot. The property class is expressed in raised or depressed numerals on top or on the side of the bolt head. The first number represents the load in megapascals (a pascal unit of measurement for internal stress … and yes, I did have to look that one up) required to break the bolt. The second number represents a ratio between breaking load and bending load.
Since we have already discussed that higher-number (strength) bolts may be needed, how do metric numbers and SAE grades compare? A metric bolt designated as ISO class 6.8 roughly corresponds in strength to an SAE grade-2 bolt. Bolts of ISO class 8.8 and the slightly stronger class 9.8 roughly correspond to an SAE grade-5 bolt. An ISO Class 10.9 bolt roughly corresponds to an SAE grade-8 bolt.
Another widely used system of bolt strength grades comes from ASTM International, formerly the American Society for Testing and Materials. Its strength grades are indicated by the letter A plus three numerals stamped on the bolt head. Common ASTM grades include A307, which roughly corresponds to SAE grade 2. An ASTM A325 bolt is roughly equivalent in strength to SAE grade 5 and an A490 bolt is about equivalent in strength to SAE grade 8.
As you can see, selecting the correct size and strength bolt to do the job is an important step to differentiating between jobs “being done” or “being done right.” Even a bolt that seems to thread up and tighten nicely could be setting you up for replacing a gearbox or repairing a tractor with a snapped PTO shaft. 
Andy Overbay holds a Ph.D. in ag education and has more than 40 years of hands-on dairy and farming experience.
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Andy Overbay
- Extension Agent
- Virginia Cooperative Extension
- Email Andy Overbay
