Daf front strut rebuild

Servicing of the front suspension strut on the daf range is covered in the Autodata manuals, based on replacement of the strut internals using "service kits".  That was all well and good when they were written but service kits are a little hard to come by now so there has to be another way!

Isabel the red 33 had trouble with both front struts seizing before we got her and, as a temporary "fix" I's slackened off the top gland nuts slightly.  This freed them up initially but the n/s one went very tight again quite quickly and also started to leak:

Front strut

 

So, it was time to investigate the insides.

 

This can be done on these without actually removing the strut from the car by undoing the top mount:

Remove split pin

 

Undo top strut nut

 

Undo mount securing bolts

 

Disconnecting the track rod (this isn't essential but makes access a little easier):

Disconnect outer balljoint

 

And pressing the piston rod down through the hole in the wing:

Push piston rod down

 

This allows the top of the strut to swing clear of the wheelarch.  At this point the leak is a little more obvious than it was:

Leak at the top of the strut

 

The top gland nut can be undone with either a suitable (and very strong!) C spanner or a hammer and punch.  If you use a punch, expect to cause damage to the slots but within reason this shouldn't matter as it's above the top of the strut tube on refitting.

According to Autodata, under this nut there's a complicated seal system involving about 12 assorted washers and fibre seals but, in this unit, there was just a rubber lip seal in the nut itself and an O ring to seal the threads.  This needs to be lifted out to withdraw the piston rod:

Carefully lift out O ring

 

The rod will now withdraw along with the inner cylinder, which can simply be pulled off.  At this point, empty all the old oil and clean the main strut bore and the inner cylinder.  There's very little to go wrong with either of these so, unless there's obvious accidental damage, drop the inner cylinder back into the strut ready for reassembly.

One part that Autodata doesn't give instructions for is cleaning the piston or valves.  These are part of the legendary service kit but are very unlikely to have suffered any damage.  they will, however, almost certainly be blocked.

To remove the piston, hold the flat on the top end of the rod in a vice and undo the nut at the bottom end.  Turn the rod over in the vice and support the pison on the vice jaws, without clamping the rod.  A few gentle taps with a soft mallet will bring the pison off the rod.  The assembly consists of:

  • the nut
  • a washer
  • a thin, flexible valve disk
  • the piston
  • another, larger, valve disk
  • a spring

Parts of the piston assembly

 

The piston itself has a ring of holes drilled in it and two more holes inside these.  These holes are part of the valve system and likely to be clogged with dried up oil and general crud:

Clogged holes in the piston

 

As the strut is compressed, the oil is forced through the ring of holes and lifts the larger valve disk against the spring.  This lets the strut compress with relatively little damping.  As it extends again the large disk seals the ring of holes and all the oil has to force its way through the two holes and the un-sprun (but flexible) disk.  This creates a lot of resistance to rebound.

If these holes are blocked then the damping rates will be wrong.  In the extreme, completely blocked holes will make effectively seize the shock by preventing fluid from flowing fast enough.  They can be cleaned by carefully probing with Magic-Mendy Wire (green plasic coated garden wire - every toolbox should have a roll!):

Cleaning the valve with Magic Mendy Wire

 

 

On this strut, the sticking was caused by a fairly seriously bent piston rod.  The specifications give no more than 0.1mm (4 thou) runout on the rod but this one had a fairly clear bend.  By mounting it in a lathe and using the tool tip as a datum the extent of the bend can be seen.

Adjust the tool so it just touches the rod when turned to the "closest" side, moving the tool longitudinally to find the point of maximum bend:

Find maximum bend point

 

Then turn the chuck 180 degrees to see how much bend there is:

Turn to see bend

 

You can then wind the tool in till it just touches at this point and use the lathe's scale to measure the runout.  In this case it was a fairly alarming 3.8mm - that's about 38 times the maximum allowed!

At this point you're supposed to throw the rod away and replace with one from a "major service kit".  That really isn't much use nowadays but it's a strong bit of steel working with mainly compressive loads in normal use so straightening it is an option.  In this case the bend was so severe that it could hardly be made worse by trying!

With it still mounted in the lathe, turn the rod slowly by hand and wind the tool in until it just leaves a "shine" mark on the rod.  You're not looking to remove any metal - just to pinpoint where the high-spot of the bend is.

Having found this spot, turn so that the shine is on top and apply a correction using a soft-faced (nylon is good) mallet.  Apply it hard!!!

Applying a correction

 

Find the high-point again and measure the total bend to see if it's reduced.  Repeat as required.  Initially it will take a lot of very hard hitting to reduce the bend but, as you get to below a millimetre or so, start to take it easier to avoid going too far.  If you do slightly overdo it, you'll see the high-spot suddenly switch to the opposite side of the rod.  That's a good sign that you're virtually there so go easy from then on.

Finally, having got it as close as you like, spin the lathe up at a low / medium speed to check.  The rod should run visibly true with virtually no "blurring" of it's edges:

Check final runout by spinning the rod in the lathe

 

Once the rod is straight, and the piston / valve assembly clean, the whole thing goes back together.  Before inserting the piston rod, pour 250ml of standard shock absorber oil (available from some motorbike shops) into the strut.  Automatic transmission oil makes a reasonable substitute but will give firmer damping than original - it's equivalent to a "heavy duty" (ie: sports type) shock oil.

 

Screw it all back together and enjoy a vastly improved ride!

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