If you’ve spent much time looking at hardware you’ll note that you can find ungraded hardware, grade 5 hardware and grade 8 hardware. Each of these is denoted on the top of the bolt, nut, or fastener. The higher the number, the harder the fastener and the more torque and load it can withstand without breaking. The commonly referred to Grade 5 and Grade 8 specifications are for UNF based fasteners only (fractional sizes).
To determine the tap drill size for metric fasteners, simply subtract thread pitch from the fastener diameter and drop all but the first decimal place.
Example: M12 - 1.75 = 10.25 is a 10.2 tap drill size.
Fastener Strengths/Grade Markings
Fastener markings are designed to provide the user with two important pieces of information:
1. The specification to which the fastener was manufactured
2. The manufacturer's identification mark.
Camcar's Identification marks for Camcar fasteners may appear on inch-sized fasteners as "...." (4 dots); "Camcar"; or as three radial lines under the head. On metric-sized fasteners. Camcar's Identification marks may appear as "....12.9"; "Camcar 12.9"; or as three radial lines under the head.
WARNING: Inch standard and metric property class 12.9 socket head cap screws are considered comparable. However, in countries that use the metric system, the manufacture of socket head cap screws to property classes other than 12.9 is permitted. If your application requires socket head cap screws to property class 12.9, it is very important you verify with your supplier that those socket head cap screws you are buying meet those requirements.
To determine the tap drill size for metric fasteners, simply subtract thread pitch from the fastener diameter and drop all but the first decimal place.
Example: M12 - 1.75 = 10.25 is a 10.2 tap drill size.
There are metric equivalents for these and the Grade 5 equivalent is metric grade 8.8 and grade 8 translates to Grade 10.9. There are harder bolts than grade 8 or 10.9 but these are very expensive and are usually reserved for aeronautics and other demanding applicatoins. It is highly unlikely you will ever break a grade 8 or 10.9 bolt unless it is grossly undersized for your applicatoin.
The other matter here is threading. You’ll note that in almost all applications on your truck, you have coarse threads. This is typically fine for all but the most demanding applicatoins. In some cases, fine threading is called for. Fine threads allow for more torque to be applied to the bolt to hold a joint together.
Pitch/diameter charts:
In some cases, fine threading is called for. Fine threads allow for more torque to be applied to the bolt to hold a joint together. You might want to use this in high stress applications such as the drive shaft bolts or when mounting a winch or winch bumper. You may wish to refer to the following reference to get an idea of what kind of loads fine vs coarse threading can offer.
This PDF document from Umbrako, a leading manufacturer of socket head cap screws, is basically a treatise on threaded fasteners. It offers insight into strength, failures, thread classes, grading, and has conversion and sizing tables, etc. A very worthwhile read for those interested in this subject.