Restoration of a 1968 Citroen DS21bvh Pallas: A Quick Word About………Hydraulic Fixings and Seals

I won’t pretend for a moment that this is the definitive article on DS pipe seals, but if you are a DS owner – and so potentially a home mechanic - it’s useful to have some level of understanding of the kinds of hydraulic fixings and seals you will find on your DS.

Pipe seals........

Download manual 843 or 844 “Hydraulics – Course Notes” if you’re interested in finding out more about the theory and princples of Citroen hydraulics. You can find a link to it at the bottom of the list here:

Manual 844 - Hydraulic Course Notes

There’ll be a test on Monday.

Flare Nuts

These are probably the most common and recognisable pipe fixing you will spot on your DS. These threaded ‘male’ ends are found on the ends of individual lengths of hydraulic pipe. They are used to fit pipes to hydraulic components, to junctions, or simply to other lengths of hydraulic pipe.

A pipe with flare nut in situ

The ‘nut’ is the hex nut bit (obviously) and the ‘flare’ is the bulge on the pipe near the pipe end. because of the flare, the nut cannot be slid off the end of the pipe. This can be a problem if the nut is chewed up and you want to replace it.

Nut and flare

Citroen flare nuts work by compressing a small, tubular, rubber (more about that in a minute) seal in the recess between the flare nut and the item the pipe is joining to. The flare on the pipe end provides a lip for the hex nut to be screwed against so that the pipe is pushed into the receiving piece – with the compression provided by the nut then also compressing the seal around the pipe and into the thread grooves.

Two sizes of nut are usually found on a DS – defined by the gauge of pipe they are used with:

Flare nuts for 4.5mm pipe – widely used on the DS. These have a 9mm hex end.

Flare nuts for 6.35mm pipe – only used in a couple of specific situations/ locations. These have a 12mm hex end.

Citroen type flare nuts

When the 9mm ends rust, the hex can become rounded (especially if you try to undo it with a normal spanner) - making the nut very difficult to loosen. Some parts sellers have replacement nuts for 4.5mm pipes but with a bigger, chunkier 10mm hex ends.

Flare nuts with 9mm and 10mm hex ends - both do the same job

Pipe seals.

The compound used to make the seals for use with LHS fluid cars reacts to LHM. Similarly the compound used for LHM seals is not compatible with LHS. In short use the right seals with the right hydraulic system: seals suitable for use with LHS are colour coded red. Seals suitable for use with LHM are colour-coded green. Just to confuse things, Citroen also made some seals (usually 'O' rings) that are colour coded white. These are suitable for use in both LHS and LHM system!

Red, green and white-marked seals

What I don’t get is that, if they could make seals that can be used in both systems (and they certainly appear as part numbers as far back as 1966), then why did they bother to introduce LHM-only seals at all? Why not all white?

From Parts Manual 562 - 1966

Tools

The tool of choice for fitting and removing flare nuts is - a flare nut spanner! It’s more than an open-ended spanner but less than a ring spanner – but only a bit less.

Flare nut spanners

They have a narrow gap which allows them to be slipped over a pipe, but then have sufficient inner surface area to grip the hex end more than an open-ended spanner would. Less spanner slipping means less rounding……

The spanner slips over the pipe being fitted

Flare Nut Removal

If you plan to completely undo a flare nut, DEPRESSURISE YOUR HYDRAULIC SYSTEM. If you don’t know how to do that then you probably shouldn’t be messing with your pipes. Wipe the area around the nut first to remove any grit and road dirt. The aim is to stop debris falling in the hole once the nut’s removed - or when the pipe is refitted.

Clean away dirt before removing

When in use, it’s all too easy for the area of a pipe immediately above the hex end, to get knocked and become bent. It’s important to avoid this. Firstly because it weakens the pipe, and secondly because it makes removal more difficult: it prevents the hex nut sliding up the pipe - with the risk of thread stripping or pipe bursting.

Bent pipe ends make flare nut removal and fitting more difficult

Flare Nut Fitting

Whenever you undo a flare nut, it’s good practice to replace the rubber seal before doing it back up again as the seals get chewed up and might not provide a good seal second time around.

Chewed up seals after removal - note loose slivers

Before fitting a new seal, it’s also good practice to check the female receiving thread. This is because thin slivers of the old seal tend to be sliced off when the nut is fitted and/ or removed and these remain trapped in the threads. These need to be removed otherwise they can prevent any new seal fitting properly and/ or may get into the hydraulic circuitry and cause malfunctions and blockages.

The correct way to fit a flare nut is to push the pipe seal on the pipe end first, leaving about 2mm of pipe end exposed. This exposed pipe end is to help ensure that the pipe engages correctly in the receiving aperture.

Place the seal on the end of the pipe

I dab the appropriate hydraulic fluid (LHM in my case) on the end of the pipe and the seal before fitting. I also dab a little fluid in the receiving hole.

Fitting a pipe with seal

When the pipe is pushed into the receiving aperture, the exposed pipe end locates and the seal rides up and partly over the flare.

The pipe is located straight in the fitting

Once located, you should be able to tighten the nut by hand by at least a couple of turns and engage it. If you cannot, it either means that the pipe (and so the nut) is not in straight, or that the pipe and seal have not been pushed far enough into the receiving hole.

The flare nut should be able to be easily turned by hand

Once the nut is engaged, use a flare nut spanner to tighten the nut down.Tightening the flare nut pushes the pipe further into the aperture and compresses the seal in the threads below the nut. It does not have to be too tight. Citroen say "moderately tight" - 1m.kg (15 ft.lbs). Don’t worry if all the thread of the male part is not taken up by the female part.

Flanges and Sealing Plates

Where more than one pipe need to meet in a small area, there is probably insufficient room to allow for tappings for receiving flare nuts and then insufficient room to get a flare nut spanner on them to tighten them anyway. The solution? Terminate the pipes in a flange. The pipe ends are brazed onto/ into receiving holes in a flat plate (the flange).

Gear selector pipes terminating on a flange plate

These flanged pipes are often found to have a green plastic tag (or red for pipes for LHS systems) attached to one of the pipes. This indicates that the pipework joins at the flange have been pressure tested.

"Pressure Tested OK" tag on a flanged pipe

The receiving holes continue through the flange.

Holes continue through the flange - 3 bolt holes and 5 pipe holes in this case

These pipe holes will marry to orifices on the receiving part or block. But how to provide a hydraulic seal? Solution: a ‘sealing plate’ is used. These plates have holes that align to the fluid passageways - but which have a bigger diameter. Into these, sealing ‘O’ rings are placed.

Sealing plate with 'O' rings. Note the white markings

Sealing plate on the CRC

Original seals are marked in white on one side and are meant to be fitted so that the white side faces towards the source of pressure. I'm note sure why. perhaps the profile is not circular and they have a subtle shape that resists leakage? Anyway, modern replacement seals don't carry a white mark.

Original ring seals orientated according to hydraulic flow

The plate then goes between the flange and the receiving part. The thickness of the sealing plate is less than the thickness of the ‘O’ rings – meaning the 'O' rings are compressed when the flange is bolted down – so creating that all important hydraulic seal around each hole.

Flange and sealing plates on gearbox pipe

Other Types of Seal

The other common type of seal you are likely to find is the kind used to seal a hydraulic sphere. These are a form of 'ring seal'. In this application they are fitting in the bottom and inner circumference of the female threaded/ receiving end, but are compressed down and out by screwing the sphere on to form a seal.

As well as for sphere sealing, theses 'crush' seals are also found inside various hydraulic components - such as the centrifugal regulator.......