Threads and heads
The age and make of your classic car determines what sort of spanners, sockets, nuts and bolts you need for it. A Britishmanufactured classic from the 1960s up to the 1980s will probably use unified fastenings, with the most popular spanner/ socket sizes being 1/ 2in, along with 9/16in and 7/16in. By the 1990s, many of these cars were using metric fastenings (measured in millimetres, or mm for short), which the continental manufacturers had already been using for a few decades. Nowadays the Americans have largely retained unified sizes, but most other manufacturers have adopted metric.
Before unified was introduced, there were the likes of Whitworth and BA (British Association), which you'll find on pre-war British classics. Moving to a newer standard of thread was made easier in some cases by using nuts and bolts that had the new thread but retained the older standard for the head – a metric or unified thread on a nut or bolt with an imperial head was not uncommon in the 1930s, 1940s and 1950s as it enabled factories to keep using their existing tooling. This was the case with the Morris Minor which had the then-modern UNF/ UNC threaded nuts and bolts with older Whitworth-sized heads, and with the likes of MG with their early T-Type engines which had metric fine threads with Whitworth heads.
Recognising what type of thread you have and its size isn’t always easy, especially if a vehicle has been updated with later nuts and bolts. This is where the head markings can come to the rescue, while a thread gauge can provide additional information. On imperial and unified fastenings the gauge will give you the number of threads per inch ( TPI), but on metric fastenings it will only give you the pitch (angle of the teeth) – crucial info as there are three different metric pitches to choose from.
However, if you are ordering new nuts and bolts, then forget the spanner size because you still have to measure the diameter of the threaded portion
– the supplier will need to know both this diameter and the TPI or pitch. We must emphasise that to a specialist or an engineer, it is the diameter of this threaded portion that describes the size of a bolt, not the size of spanner you use on it. For example, if you ask for a 1/ 2in UNF bolt because that is the size of spanner that fits your old one, then you will get one that has a thread diameter of 1/ 2in and takes a 3/4in spanner. Similarly with metric fasteners, a 10mm spanner typically goes on an M6 bolt, the thread diameter being 6mm.