written by Gavin Ligget
# 28 Mar 2003 few minor layout modifications by Sean750
# 25 Mar 2003 initial version
Having anticipated the dreaded leaky rear strut problem for some time, I aquired an old set of struts that had come off of an E32 of unknown model and vintage.
When my struts started leaving
puddles in my garage I started looking at rebuilding the struts
mainly because of the exhorbitant price the dealer wanted for new
ones and the fact that I had driven an E32 converted to
conventional suspension and was not at all impressed with the
resultant handling. I originally assumed the struts were welded
together. I mistook the pressed marks on the lower spring
retainers for spot welds and originally ground them off only to
find that the spring retainers can be tapped off with a nylon
Remark from Sean750: the arrow of the picture above shows the pressed marks. But I didn't understand how Gavin separated both parts of the strut. So I asked Gavin more info about separating those:
The three marks that look like
welds are in fact just crimps to stop the retainer from rattling
on the cylinder. Hold the cylinder in one hand by the aluminium
part and hit the spring retainer with a nylon mallett and it will
slide off in the direction of the top of the strut. In other
words you hit on the bit sticking out below the nylon ring in
your pic and the portion that the nylon ring is on will come off
nylon ring and all. See pic below and hit in direction of the
Once tapped off the seal block
will be revealed which is retained by a spring clip. You will
need to use a suitably sized socket to tap the seal block down
into the strut cylinder:
so that you can remove the
spring clip using a small flat screwdriver:
With the clip removed pull
smartly up on the strut shaft which will eject the seal block.
Be carefull to keep the shaft aligned with the cylinder so as not to damage the piston seal on the bottom of the shaft.
There are two seals contained in the seal block. The upper one is simply a wiper seal which serves to keep crud out of the cylinder. It is retained by a taper. This seal does not prevent the egression of Pentosin. Should you want to replace it you will need to machine a tapered tool with which to reverse the taper and another with which to retaper the lip once you have replaced the seal. Frankly given the hassle involved in removing these scraper seals and the difficulty involved in finding replacements I have not renewed these as long as they still display a wiping action on the shaft.
A few observations before continuing with the rebuild:
My car is a 1991 E32 730i (remark by Sean750: in some countries LAD was an expensive factory option and could be ordered on a standard i model. Not in the US, where only iL models are equipped with LAD). I firstly rebuilt the spare struts I had found and once I had exchanged struts I rebuilt the original units. I found a number of differences, some incidental and some very pertinent the rebuilding process.
I do not know exactly what model my spare set of struts came from. The spare strut cylinders were longer as were the lower spring retainers ( by about 10mm). There were significant differences internally as well. The spare struts were equipped with rebound springs which my originals didnt have and the wiper seal dimensions were completely different making them harder to source and in part prompting me to not worry about replacing them. The shock absorbing mechanism ( arrowed in the first image) also had internal differences relating to the method of sealing. The spare set utilised conventional (nylon) hydraulic seals and the originals an o-ring type design. I couldnt find anyone who could remanufacture the nylon seals and I didnt bother trying to dismantle the o-ring type shocks. I found through trial and error that replacing the o-ring component of the nylon hydraulic seals was sufficient to prevent further leaking. (This worked on the top seals as well)
Turning the seal block over will
reveal the nylon hydraulic seal that does all the work:
The u shaped cutouts in the
retaining washers allow Pentosin to exert pressure on the nylon
seal in proportion to the internal hydraulic pressure, thus
improving the sealing action as pressure increases. You will need
to remove the three burred edges that retain the washers to
reveal the seal and o-ring. On my original struts the washer was
cupped and the burred edges were thus not evident.
The nylon seal I had machined by a specialist seal supplier who also supplied the o-ring. The o-ring is pressed over the nylon ring and both are then pressed simultaneously into the recess in the seal block. The overall diameter of the two components when thus joined is the critical measurement. I found that when the nylon ring was still a tight fit on the shaft, replacing just the oring solved the leaking problem. However these seals are of a dimension well within the capabilties of specialist seal manufacturers so rather replace them along with the o-rings.
Moving to the bottom of the
strut shaft you will find the piston ring:
The spare struts had a dual ring design, the original units the single as pictured. I have been told by those knowledgable in hydraulics that these rings are not seals but guides for the piston. Either way I did not replace these rings at all. Note that the rings have a groove on their inner face which corresponds to a groove in the piston. Take care to orientate correctly when assembling.
Assembly is a reverse procedure. Take care to keep the shaft aligned when inserting into the cylinder to avoid damage to the piston seal/guide. Lubricate sparingly with Pentosin when assembling.
Should you remove the shock
units from the cylinders note that they screw off conventionally
(right hand thread). Be carefull what you use to turn the
cylinders. The walls are relatively thin and dent easily. I found
a pipe wrench damaged the cylinder and used a strap spanner
second time round applying the tool at the extreme top of the
cylinder where the piton doesnt travel. There is an o-ring down
there that provides the sealing, so disassemble only if you have
The struts may leak from a
number of places:
1. The top nylon seal in the seal block.
2. The outer seal block o-ring.
3. The lower o-ring where the shock assembly attaches.
all solved by the above rebuild or
4. At the bottom shock assembly cap.(see following pic:)
As noted previously I noted two sealing designs in the damping unit. The one pictured above utilises the same design as the top seal block ie nylon/ o-ring seal combo. The other design was difficult to investigate given the fact that the assembly was not designed to be taken apart. Again given a tight fit on the shaft you will get away with just replacing o-rings. I could not find anyone to make up these nylon seals because they are quite small. I'm sure someone will do it it will just take some time to find them. In the absence of a leak at the botttom of the strut I would leave well enough alone. These seals dont work as hard as the top ones.
Its important to note the orientation of the nylon seals when you remove them. Pay attention to which way the inner lip faces as this provides the seal. This goes for the top seals as well.
My first rebuilds are doing well and the total cost was under US$20.00 for seals and o-rings.
Story by Gavin.
Email Gavin with any questions.
If you would like to add any comments, remarks and/or corrections to this procedure, feel free to email Mike Oswald and we'll put it on our site. Share your experiences with us so others may benefit from it.