Sourcing M5 x 0.75 Pitch Nuts

NUTS IN MAY! 

I've not posted anything for a while. The winter months were certainly quieter, but since then I've been pushing ahead with a number of things that have now been completed. In fact April and may have been very, very productive months. For a start, I've had some nuts made for use on my DS!

New 5mm x 0.75 pitch nuts

But 'why?' you might ask......Well you probably appreciate that, being Citroen and so French, the DS is assembled with metric sized fittings. These days, the standards that governs the size of metric fittings are 'ISO' ('International Standards Organisation') standards. ISO 261 governs the outer diameter and 'pitch' for bolts/ set screws. A metric bolt that is 5mm 'wide' (has a 5mm outer diameter) and is designated as an 'M5'. ISO 68-1 defines the shape/ angle of the cut threads and 'pitch' refers to the width of each thread. You can think of that as the distance between 'peak' of the thread. 

Metric fittings can have 'fine' or 'coarse' threads - and the finer a thread is, the smaller it's 'pitch' value is  - as there are more turns of thread per centimetre. For example:

• Coarse thread M5 fittings have a pitch of 0.8mm
• Fine thread M5 fittings have a pitch of 0.5mm

Many metric fittings are widely available in DIY shops or on line and it's not at all hard to find M4, M5, M6, M8 and M10 fittings. And with a little more effort you can find these sizes in either coarse of fine thread. You'll see bolts for sale advertised as 'M5 x 0.8' for example - that's an 5mm wide bolt with a coarse thread. 'M5 x 0.5' would be a 5mm wide bolt with a fine thread.

The Citroen DS is typically assembled with a large number M5 but also M7 and M9 sized fittings.....For late Ds, you can easily buy M5 hardware, but M7 and M9 are harder to find. The DS parts suppliers have these but they are very expensive per item, compared to the more common and widely available sizes.

You can even buy new M5 and M7 bolts with the little symbol on top - just like the original ones used by Citroen.......

Replica Citroen set screws in M5 and M7

If you have a late DS you are probably blissfully unaware, however if you have a pre-April 1970 DS you have an added complication: Citroen only adopted the 'ISO' standard for thread sizes in April 1970! Prior to that date, Citroen used fittings made to the 'S.I.M' standard. (I haven't even been able to find out what 'S.I.M' stood for....).

Here are examples of the Technical Note' sent out to dealers/ repair agents announcing the change of standards they would adhere to and how they would phase out old stock.....:

Technical Note to German dealerships

American version of Technical Note

If you look carefully, you will see that the note says that for fixtures sizes above 5mm, the threads are about the same and so compatible. But, for 3mm, 4mm and 5mm they are not - the threads don't work together.

Why is this? well taking 5mm fixtures as an example, under the new ISO standard, the coarse pitch of an 5mm 'M5' bolt became 0.8mm. Whereas under the previous S.I.M standard, the coarse pitch had been 0.75mm.

What does this mean?

• If the pitch is 0.75mm, then there will be 13.3 turns of thread per centimetre
• If the pitch is 0.8mm, then there will only be 12.5 turns of thread per centimetre

So that's a difference of about one rotation of the fixture per centimetre. That doesn't sound a lot but, this is what that kind of looks like. Kind of.....

Illustrative purposes only: differences in thread pitch

In practice, if you try to fit a 0.75 pitch 5mm nut onto a 0.8mm pitch M5 bolt, then after just a couple of turns, it will start to get very tight.....And the more you tighten it, the harder it will become. You will only be able to tighten it further by damaging threads - either on the nut, or bolt, or both. 

On a DS - and in terms of 5mm/ M5 fixings - you will have cause to use more bolts/ set screws than you will nuts. And if your car is pre April 1970, 5mm x 0.75mm bolts are available. But like the M7s and M9s, they're not cheap......

M5 (5mm) x 0.75 pitch bolt

Some folk just use a tap to re-cut any welded or captive 0.75mm pitch nuts to 0.8mm to take a modern M5 bolt.

Re-tapping a welded nut

If you do this, you cut off metal and weaken the thread. Take a look at the 'wave' diagram above. It takes about 10cm for the peaks of 0.75 pitch and 0.8 pitch to 're-align'. That means that after about 5cm they are completely out of alignment - the peak of one thread is aligned to the trough of the other thread. If you were re-tapping, then at that point all the metal forming the 'pea'k of the tapped thread would be removed to turn it into a 'trough'.

Of course nuts are only 3mm or 4mm deep - so the threads don't go too far out of alignment, however even over such short distances, re-tapping leaves the fittings wobbly and with less 'bite'. Now 5mm sized fixings aren't really used in many mission-critical areas (other than hydraulic plate joints of course).....

M5s securing hydraulic pipes

......but it does mean that if you are a bit forceful and over-tighten your fitting, the remaining thread in the welded/ captive fitting might strip - leaving you with a bigger problem.....That's not guaranteed - just a 'maybe'.

On older Ds, there are also few 5mm bolts or studs that need/ expect 0.75mm pitch nuts. Here are some examples on my 1968 car......

In theory you could use a die to re-cut the studs to 0.8mm pitch....

Die for cutting male threads

...but in practice you tend not to have the room to use a die. Options then are to:

1. build up a store of 0.75mm pitch nuts from old cars and use those
2. re-tap 0.8mm pitch nuts to make them 0.75mm
3. use brute force or ignorance and make a 0.8mm pitch nut fit
4. seek out any old or new 0.75mm pitch nuts you can find.

Having exhausted option 1, and tried option 2, I became an 'option 4' guy. I searched high and low for a supplier. On that point, I got lucky and managed to buy a load of original NOS Citroen 0.75mm bolts. But no nuts...

You'd have thought that - since new 0.75mm pitch bolts can be bought, that the same suppliers would have nuts. But it seems not!! The closet I came was find very thin nuts used to fulfil a specialist role on a Dellorto carburettors. They were costly and at 3mm, not really deep enough to replace the 5mm nuts used on a DS.

Closest I found - and still expensive

And THAT'S why I ended up having some nuts made!

I was encouraged in this endeavour by fellow DS owner Robert Harris. He approached a few engineering firms here in the UK to see if they would make some nuts up and gave me a few leads to follow up too. These tended to draw a blank simply because of the numbers of parts i was likely to want made. I didn't want to have to have many thousands made and then seek to recoup the expenditure by trying to sell them on. there is some demand - but its very 'niche'. 

In the end I put out a plea on a UK engineering forum and someone hesitantly said they'd be willing to make up a batch. He had a computer controlled CNC machine that could be programmed to make a number of nuts to the same standard and dimensions automatically. We settled on a minimum quantity of 350 nuts......

All of the 5mm x 0.75mm pitch nuts I've seen on my DS are 5mm deep - that's a noticeable difference when compared to ISO M5 nuts - and that's the size I went for. I hadn't found anything other than 5mm thick nuts on my DS, and my catalogue only listed one 0.75 pitch nut.....

.....but I have been told by fellow DS enthusiast Gert Larsen that Citroen also made them in other thicknesses:

• ZD9231900U is height 5 mm
• ZD9232300U is height 4 mm
• ZD9232000W is height 3.5 mm

In all cases, these were just plain steel finish - not yellow cadmium plated. At least that meant that I could keep the cost of mine down by not having them yellow zinc passivated. 

I sent my engineer  my last few 'perfect' 5mm deep nut.......

 ........and set up his CNC machine and ran a couple of new nuts off for my approval. 

They looked great, so I gave him the go-ahead and off he went.

They are here now. The results weren't cheap but - based on the price of the 0.75 pitch bolts that are sold - I felt the 'per unit' cost was acceptable.

A lovely bag of 5mm x 0.75 pitch nuts.....

I only really need to keep no more than 100. I don't have the time or inclination to be selling them at 10 a time, so at the D rally in June I will see if one or more of the DS repairers is interested in taking the remainder off my hands at cost price. That then leaves them room for a bit of mark-up if they offer them for sale..