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	Threading on bolts, nuts etc.

According to a circular letter (a "note technique") from Citroën to the dealerships from the 11th of May 1970, they previously used an (essentially) French standard for threading, called SIM (Système International Modifié). This was replaced by the now used (essentially German) ISO (International Standardization Organization) standard.

The two standards are listed in the table below.

Diameter SIM ISO Pitch Width across flats Pitch Width across flats Screws Nuts Screws Nuts 3mm 0,60mm 5mm 0,50mm 5,5mm 4mm 0,75mm 7mm 7mm 0,70mm 7mm 7mm 5mm 0,75mm 8mm 8mm 0,80mm 8mm 8mm 6mm 10mm 10mm 10mm 10mm 7mm 12mm 12mm 11mm 11mm 8mm 14mm 14mm 13mm 13mm 9mm 14mm 14mm 14mm 14mm 10mm 17mm 17mm 17mm 17mm 12mm 19mm 21mm 19mm 19mm 14mm 21mm 23mm 22mm 22mm 16mm 26mm 24mm 18mm 29mm 27mm 20mm 32mm 30mm

Here, the ISO standard is taken from the circular letter mentioned above, but in practice, there may be deviations, such as 10mm screws and nuts having 16mm width across flats instead of 17mm as they should have had according to the standard.

These changes are most commonly experienced when you are working with 7mm screws and nuts on a Citroën from the 60's, where the original parts have 12mm width across flats, whereas all those that have been replaced over time, usually have 11m width across flats. Generally, this has no consequence, except that you need to have both 11mm and 12mm spanners handy at all times.

More dangerous, however, is the change in pitch of the 5mm threading. If you are not paying attention and using new 5mm screws when mounting for instance the triangular valence panels on a 2CV or the front lining down of a D-model you damage the threading in the nuts that are welded on the car. The screws are strangely hard to twist and once you realize your error, it is hard to do anything about it.