Workshop Manuel explains: sorting out why your bolts won't thread, and your nuts are stuffed

Have you ever been working on your car and gone to fit a part, only to discover you can't do up a bolt or a nut won't thread onto your car? You're probably trying to put the wrong format fastener into play, which is like trying to speak a foreign language to a deaf person at the Drive-Thru.

Car-based fasteners are generally sized referring to the diameter of the thread, and the thread count (also referred to as “pitch”). The pitch is – roughly – the shape and spacing of the threads, which affects how they tighten down. But there are several different styles of fastener, as they have been developed in different parts of the world.

SAE format is also known as “imperial” is an American thread pitch format, but ISO or “metric” is by far the most popular fastener format used in Australia, Japan and most other places who like numerical systems that range from 0-100 instead of -67.4 to +38.9 bald eagles. Unless you’re working on very old English cars you shouldn’t come across BSW (British Standard Whitworth) fasteners, but these are a different format again and can provide bulk headaches as they can visually appear like an SAE fastener but won’t always seat properly in an SAE thread.

Metric fasteners are offered in “coarse” and “fine” thread formats, sized by their diameter (in millimetres), with the pitch displayed as a decimal number. SAE bolts also come in coarse (UNC) and fine (UNF) options, with the diameter measured in fractions of an inch, or ¼-20 and ¼-28, with 28 representing the fine-thread version of a quarter-inch bolt.

So, how do you know what type of bolt you have? The easiest way is to pick up a bolt and thread-checker, as these will tell you what size and format fastener you’re holding. If that isn’t an option you need a set of vernier calipers so you can measure the diameter of the threads to see if it is a parallel (straight) or tapered (narrowing) fitting. Next, grab a ruler and measure the threads-per-inch (or threads-per-millimetre for a metric fastener) and compare the numbers to a fastener pitch chart which are easy to find on Google.

What is the difference between coarse and fine pitches? Generally speaking, fine-thread bolts are stronger and offer a higher clamping force than a coarse format thread. However, because the threads sit closer to each other the risk of galling or fouling (leading to cross-threading) is increased. Any dirt, swarf or rust will make it harder to tighten or remove a fine-thread bolt, too.

Liquid fittings, like those found in fuel systems, are measured in a few different ways. NPT is a contraction of the term National Pipe Thread, the American format for pipe thread pitch-measurement, while the British Standard Pipe (BSP) format comes in BSPP (parallel) and BSPT (tapered).

You’ll also have seen “AN” fittings while watching Mighty Car Mods, and that refers to when the American Army and Navy standardised hydraulic and fluid fitting sizes in World War 2. Sized by AN numbers (also referred to as “dash” numbers, from -2 up to -32) the sizing refers to the outside diameter of the fitting, which uses a 37⁰ taper to seat (unlike SAE fittings which use a 45⁰ taper). It is also important to note there are AN-format bolts but these are sized differently to AN fluid fittings.

Next time I'll run you through the process of making a fuel or breather line using AN fittings.