Metal finishes

Whilst nuts and bolts can be made from a variety of materials, the most popular on classic cars is steel. However, there are a number of different finishes, some of which last longer than others. Zinc plating and passivated coatings are the most common and among the cheapest to apply. Nickel has a harder, more resistant finish, but it is more expensive. You used to be able to get chromium and chromate coatings that offered a brighter finish with even better resistance to corrosion, but those processes have since been banned.

Stainless steel is popular because it offers good corrosion resistance. However, it is not as strong as steel, so it is generally not recommended for fitting to safety- critical components such as mounting points for the brakes, suspension, drivetrain or subframes. You can get stronger grades of stainless, but they are not common. Brass nuts are often used on fittings such as exhaust manifolds. This is a softer material than steel, so the nut has to be deeper to compensate. However, it is used because brass doesn't rust and so these will not bind to the stud despite the harsh operating environment. Zinc plated nuts and bolts are resistant to corrosion, but be aware that the even more durable zinc coated fastenings exist for industrial use; these have a thicker coating than plated fixings, so the nuts are 15-thou oversize and will flop about if used on a regular plated bolt with the thinner layer of zinc.

Different finishes give nuts and bolts different properties. So does the material it is made from – note the brass manifold nut.

Zinc plated nut (on right) is corrosion resistant, but zinc coated nuts (on left) have a thicker coating and are for industrial use.