Grades and rust resistance
Nuts and bolts should have information on them to explain their grading. A metric bolt, for example, will have a number displayed on the top of the head which indicates its tensile strength. The higher the number, the greater the bolt can be tightened, making it more useful for applications where a high torque setting is required such as caliper mounting bolts and suspension mounts. Typically you will find 4.6, 8.8, 10.9 and 12.9 grades, though they may only be marked up with the 4, 8, 10 or 12 on nuts. The 4.6-grade was generally used for attaching light objects such as trim, but these have been deleted from most manufacturers' catalogues these days and 8.8 is now the standard choice for most applications up to and including structural and safety- critical components. You will find 10.9 in some especially critical areas, but mainly on more modern cars, while 12.9 is the grade for socket cap screws used primarily on racing and extremely highperformance motors.
Unified fastenings use SAE grades 2, 5 and 8, which are the equivalent respectively of the metric 4.6, 8.8 and 10.9. However, rather than having the numbers cast on the bolt heads, these use a system of radial lines – none for SAE 2, three for SAE 5 and six for SAE 8. Confusingly, the equivalent nuts to go with these bolts use a lettering system – 'A' grade nuts go with SAE 2 bolts, 'B' grade with SAE 5 and 'C' grade with SAE 8.
As for imperial, Whitworth and BSF also use a lettering system, but theirs is totally different and the grade boundaries do not match those used for metric. Starting at P, this is the equivalent of metric 5.6/5.8, while S covers the lower end of 8.8 and T the upper end. V is a match for 10.9, and X (75- 85 ton/ in2) and Y (80-90 ton/ in2) between them cover the entire 77- 88 ton/ in2 range of metric 12.9 fastenings. Simples!
Stainless steel fasteners will generally be either A2 which has around 18% chromium and 8% nickel to offer a high level of corrosion resistance, or A4 which is considered a better standard than A2 – it contains the same chromium and nickel, but also 3% molybdenum and offers even better corrosion resistance but generally at a higher price. However, the basic advice with stainless is not to use it on safety- critical fastenings, but only where looks are more important than performance.