Screw Threads Used in the Z Magnette


During restoration as well as routine maintenance and repair operations it is often necessary to replace fixings like nuts & bolts and even to retap holes. In order to do so one needs to be able to define the screw thread, unless a specific replacement part is listed by the parts supplier.

The majority of the threads used on the Z Magnette are from the Unified range. This standard was developed after WW2 so that manufacturers on both sides of the Atlantic were producing interchangeable parts. Unified threads come in three types: fine "UNF", coarse "UNC" and extra-fine "UNEF". It is the first two that most concern the Magnette owner and, of those, the fine UNF is the most often used. Coarse is generally confined to situations where a screw, bolt or stud made of steel is threaded into a softer material like aluminium, where a finer thread might strip too easily.

One also needs to distinguish between 1) a screw, where the thread goes all the way up the shaft of the fixing, and 2) a bolt, on which there is an unthreaded section below the head, generally because it must pass through other components before the thread is needed.

Unified fixings are normally identifiable by a circular depression machined on the head or the face of the nut and nuts sometimes also have two interlinked circles stamped onto one of the flats.

Other threads are used on the Magnette. Small screws less than ¼ inch diameter are normally threaded to a  BA (British Association) standard. There are also examples of Whitworth and BSP (British Standard Pipe) used in relation to hoses and hydraulic systems.

Threads are defined the diameter of the rod from which they are machined (major diameter) and by the “pitch”, which defines the number of “threads per inch”. So, faced with an unidentified fixing, one can normally define it by checking for the depression and then measuring the diameter and the pitch. Armed with this information, mail-order fixings suppliers can usually supply a replacement. This information is also needed if you have to buy taps and dies.

Some fixings used in safety-critical applications are “high-tensile”, which means they have more strength. It is important that such items are always replaced  like-for-like and for this reason, spares suppliers will often list them as specific parts for that application.

In a few locations, where the rotation of a component might tend to unscrew a threaded fixing a left-handed thread is used. That is to say, a thread that tightens with rotation to the left. The workshop manual will normally flag up these situations.

Many screw-threaded components need to be tightened so that a specific force is applied. This can be so that enough downward pressure is applied where a seal exists (e.g. cylinder head nuts) or to ensure that excessive force is not applied where it might be counter-productive (e.g. when screwing a steel fixing into an alloy component). Rotational force is known a torque and a torque wrench is used to apply a measured force to a nut or bolt. Torque is measured by a combination of force and distance; in the imperial system the measure is "lbs/ft" (pound/feet). 1 lb/ft is the force equivalent to putting a pound of force on the end of a lever 1ft long. The metric equivalent is Nm or Newton/Metre. If you intend carrying out a good deal of mechanical work, it is worth investing in a good quality torque wrench. An engineer's screw-thread gauge can also be helpful in measuring thread pitch.

When using a torque wrench, be sure that the threads are clean because rust or paint in the threads can increase the amount of torque needed to tighten the fixing and a false reading is obtained at the wrench.

Fixings suppliers often offer a "lucky bag" containing a selection of nuts bolts and screws of a particular thread type. Whilst these are superficially attractive, I have often found that they contain a disproportionate quantity of "make-weight" items that are of limited usefulness. It is better to analyse which sizes are most commonly used and buy a quantity of each to keep in the garage. 

For a very detailed article on screw threads, click here

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