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  • Bilstein into Macpherson strut install

    Title edited, thank you mods/admins.
    Purpose of this thread is to provide a single place where you can list tips for installing yellow B6 & B8 type mono-tube Bilsteins into Audi B3 & B4 chassis Macpherson struts because although very worthwhile, a successful install is a little different from how the original factory & other OEM (twin-tube) style dampers (including Bilstein B4) are assembled into the original struts.

    1.) Tools and Equipment Required:
    - Confirm your Bilstein cap-nut (aka: gland-nut) wrench is of the appropriate size for your application; and is in perfect condition. If not contact Bilstein;
    - Leather work gloves;
    - Proper penetrating oil; hack saw; bench-mounted vise; wooden blocks to contact and brace the strut with;
    - Stillsons, or a small chain or pipe wrench. Caution, for use only gently around the old cut cap nut; and only - after - it is loose;
    - Hand-held wire brush; navel jelly; lapping compound; varsol; lacquer thinner (or electrical contact cleaner); spray paint (primer and 2 colours);
    - Loctite & Torque wrench; &
    - Nail-polish (in a contrasting colour to your strut paint) & wax-oil rust-proofing compound.

    2.) Removal of Old Stock Damper from Strut (springs, etc. are already off the strut at this point):

    No Torches, Etc. Method:
    To get the likely seized old stock original style damper-retaining cap-nut off the strut, yet prevent any possible crush damage to the strut-tube itself from the vice or pipe-wrenching etc.!

    Method Outline:
    In preparation for cutting the cap-nut by hand, brace the strut, cradling it in the vice using the wooden blocks such that it is held steady -without- putting any pressure on the tube. Next remove the OEM cap-nut as follows: Hacksaw as detailed below, then soak with oil; and only then finally with just your gloved hands, twist it off.

    a.) How your nuts get stuck and why use this method:
    Your strut cap-nuts may be fused onto the strut-tubes by years of corrosion activity. The originals were installed without anti-seize compound or Loctite. This rusting often seals the lower edge of the cap-nut on the strut; closed, preventing any penetrating oil from entering. Because of the possibly very large total threaded surface area which may be involved (perhaps all ten internal threads of the cap-nut, times the diameter of the strut tube, equals a potentially huge rust grip area that) you will not easily break the cap loose from, or successfully remove it with just the vice, pipe-wrenches, and torches; without also damaging the thin-walled strut-tube. With this "no torches, etc." method, any strut damage is easily avoided.

    This method involves using a plain ordinary hack-saw to cut diagonally across and around the O.D. of the cap-nut as required to next get penetrating oil on -all- ten of those hidden engaged internal cap-nut threads . It's fast, simple and easy.

    b.) How to turn your OEM style nuts off with just your hand:
    First cut, on a diagonal (relative to the strut axis), from the top to the bottom, around the O.D. of the cap-nut, -just- partially through to the roots of the cap-nut's internal threads. Depth-wise, stop when you have made a series of holes appear in the bottom of the slot you have cut, through which you then apply penetrating oil. Continue to cut the diagonal slot right to the lower edge of the cap-nut and it will actually spring open slightly. If not loose at this point, do this again in 2 or 3 spots spread around the O.D. and the nut will turn off by hand (Caution: Cap OD now sharp!). By careful use of just a hand-held hack-saw, any damage to the strut tube's threads can be avoided altogether.

    Below is a post link which further describes the task list above & repeats how to cut into the thread interface between the cap-nut and strut tube.

    3.) Mac Strut Body Prep:
    a.) Thread re-condition as follows:
    Considering what nut removal may have involved, you may want to take steps to ensure this does not happen again. This will also aid in the correct cap-nut torquing later on.
    - Wire brush the threads;
    - Likewise Naval jelly as per its' instructions;
    - Paint (other than, but up to threaded area)
    - Further strut tube thread cleaning. I used the just removed old & now cut (allowing clearance adjustment by squeezing the cut sides) OEM cap-nut as an effective thread-chasing die with some lapping compound;
    Importance of thread-cleaning: The Bilstein cap-nut recess, which secures & centres the top of the yellow damper tube, appears to have a slight taper, so any slack in its final positioning will allow the damper to wobble up top; and move out of position at the bottom! Re-condition the threads until they operate "as new", this will allow both proper capture of the damper top and torquing of the cap-nut.

    b.) Drain Holes:
    I put my drain holes on center thinking that would be the low spot and least likely to be blocked by the damper's presence, I also enlarged them to an 1/8" after finding one plugged with debris.

    c.) Clean Strut Body:
    Ensure no debris inside where damper fits into base; and clean threads to receive Loctite.

    4.) Dry Run: Empty Assembly of Strut & Bilstein Cap Nut:
    You want to be able to fully and easily seat the cap nut on the empty strut tube with just your bare hands. This will ensure a couple of things:
    i.) That you have sufficient free cap-nut travel to properly capture and hold the damper when installed later; &
    ii.) That your applied torque will later all be employed to pressing down on the top of the yellow tube; and not partially lost fighting thread friction.
    Paint any exposed threads on the strut tube below this point. This will give you an initial reference point that you should almost but not quite reach later with the damper base correctly placed inside and the yellow tube-top properly captured in the nut.

    5.) Understanding Fixturing of the Mono-tube type Bilstein within the Mac. strut:
    Unlike the original Boge or other OEM (twin-tube) style damper, the yellow mono-tube Bilstein is held centered in place -only- at the very top and bottom ends of the yellow tube, with an air space possible all around it's O.D. The possibility for errors here increases with the difference between your particular strut-tube's largest effective I.D.; and your damper's O.D., so less of a possible concern if these two diameters nearly match. The base has an abbreviated (less than 5mm) and shouldered slightly spherical protruding shape to fit the mating depression in the strut base, and its correct function is dependent on three conditions:
    a.) Damper base sphere centered correctly in the strut base recess;
    b.) Top of yellow tube correctly captured within the Bilstein cap nut recess (less than 5mm deep); &
    c.) Cap-nut correctly fastened with Loctite & reco'd torque (see torque# on Bilstein wrench).
    Examples of what can go wrong:
    i.) Damper-base not centered in strut-base but off-set to one side and sitting up on shoulder. This will allow damper-base to rock laterally during driving;
    ii.) Yellow tube-top not correctly captured in nut recess. Nut torque may relax during driving allowing damper to both rock laterally and hammer vertically within strut. Nut torque may deform yellow tube-top edge which may allow water entry.

    6.) Successful Placement, Damper into Strut:
    a.) Damper must be installed by lowering into strut whilst both are held vertical.
    b.) Do a dry full vertical assembly to ensure a couple of things:
    i.) You are going to get a feel for how everything fits together; note the new final installed height of the cap nut, now nearly but not quite touching the strut-tube top ref. mark.
    ii.) You are going to mark the strut O.D right below the installed cap-nut w/the 2nd paint colour to give you a final ref. point to confirm correct final assembly below.
    iii.) Prior to final assembly is where I coated the strut tube interior with the rust-proofing. With a drain-hole of sufficient diameter, any excess will drain out the bottom without plugging the hole.

    7.) Successful Final Assembly:
    Involves just a few simple but carefully executed steps, including:
    a.) Loctite & full vertical assembly including cap-nut position at the above ref. mark;
    b.) Torque application & see also "Bilstein wrench mod";

    Bilstein Wrench Mod makes torque application a one-handed job:

    There are a couple of photos in my album there, one of the wrench & tube; and one of the mod'ed tool in use. I can't re-activate them in the post at the moment. Although Bilstein (North America) has always been very helpful in sending me wrenches (quickly and at no cost!) the wrench is a delicate stamping and it is not robust enough to remove the mono-tube's cap-nut if it is rusted, over-torqued or already damaged. By using the modified wrench, successful removal is more likely.

    c.) Match-mark & rust proofing (nail-polish & wax-oil);
    By marking the cap-nut and strut tube with a single (nail-polish) mark in a spot you can most easily view when installed on the car, you can confirm your assembly in future. The wax-oil can be used to fill the remaining exposed threads below the cap-nut.

    Concluding Remarks:
    I'd like to think that the above task list would allow one to not have to re-visit the damper install, however you may want to understand hydro-locking, this particular damper's susceptibility to it; and know the symptoms which may indicate it's onset, I've certainly been wrong about this issue before, please also see my response to GT500's post titled "Bilstein Killed" on this forum and work back to the link:

    Bilstein Killed

    Photos: a) & b) Example of Bilstein gland nut & wrench. Both the nut and seal are available separately. One can see that the wrench is only a stamped unhardened item & therefore fragile. As received , the two wrench teeth may be a little to long &/or sharp to easily/safely fit over & clear the chrome shaft's OD. The shiny wrench finish will help to highlight the deformation of the wrench when you are over-doing it. The corners where your square-drive fits & where the teeth emerge from the ring will deform first.

    I think that's most of it, hope this helps. And certainly I may have missed something, please chime in.
    Attached Files
    Last edited by Lago Blue; 30th January 2019, 01:00.

  • #2

    Bilstein wrench mod - inserted tube shadow.png Bilstein wrench mod in use.png

    a) Bilstein Wrench mod (note how inserted tube can push directly against the back-sides of the wrench's bicuspids);

    b) Sorry, but this is perhaps just about all your current (shameless plug for "Strut-Stops"!) un-aided OEM top-mounts are good for;

    c ) An atypical case of Bilstein frozen strut syndrome?

    Last edited by Lago Blue; 11th January 2019, 18:41.


    • #3
      Bilstein yellow mono-tube with a from the factory, 10mm strut drain hole mod label.png

      A Brief Look Back into History at the Origin of a (Not so Litttle!) Black Hole.

      (From the Bilstein Factory no less, the Official Mac. Strut Drain-Hole Mod Sanction.)

      If you've ever wondered if a Bilstein mono-tube can truly exist for any length of time (beyond the next event horizon, i.e.: the far edge of the next pot-hole), and yet remain self-contained in an un-vented strut body universe, this mod's for you. Almost lost to the vagaries of beyond life-time warranty to the original purchaser, not quite obscure string theory, but in plain German, French & English. It must have been deemed important to warrant its' own illustrated warning label, this was the august August BiIstein GmbH's response to Audi owners who'd clearly experienced "big-bang theory" issues; in practice.

      If you haven't already, perhaps you'll now with this permission, "Drain the Swamp!" Or better yet...prevent one. Stumbled onto this photo recently (while looking for something else), it's from another forum (AW) in a post by audipete with the tag-line:

      "Bilstein used to put a label on the struts stating that drilling the hole is a must"

      Original thread is here, which BTW, I think proves Professor Hawking's multiverse theory, our doppel-gangers are out there, ...just on a different forum. Its funny how much good 'ol stuff just gets lost to the circularity of time, comes around again; and sometimes is or has to be, re-discovered.

      The other thing is you have to be really specific when speaking about some of these items, for instance the near identical B6s and 8s aren't ever too harsh for your daily commute (you can easily compress them between your bare hands!) unless you've got/had water inside already; and just don't know/didn't realize it...yet. Lots of myths out there. Although quite clearly published here, the label was unfortunately a limited edition not repeated on all subsequent & related production.

      The mod itself was never really forgotten, on the contrary and for good reason, but it should be more widely known; and never got the respect it is due either, was that due to a lack of a known official sanction? It was the mod's origin that was forgotten. I never knew it had such all along. Anyhow, personally I've seen enough to now know a strut drain-hole is -always- a good idea in our Mac. struts, regardless of -whichever- damper brand or type you may run.

      Not sure of what car the above P/N refers to (is it simply an older superseded numbering scheme?), but clearly its a mono-tube (Bilstein's classic up-side-down wasserpumpen große mutter wunder-damper). What I think is important here is to recognize that this is a damper type vulnerability, as opposed to any specific chassis application. That the factory recognized this as an issue is telling, as is the size of reco'd hole's diameter. When I first learned of this mod years ago, I only drilled a 1/16" wide one; and it self-plugged later on, till I did it over larger.

      Yes, some of these (B6 & 8 Bilsteins) are vented on the bottom (UrS-sized ones) and others not (North American B3), no matter. Water -can- get sucked into the interior of all of them, despite that exactly how is debatable, variable and perhaps multi-faceted. It can occur at the shaft's worn gland seal if its a mono-tube; and it gets in the bottom vent too if its an UrS-sized type. Here's the thing, it can get into -every- strut, and even if its just in there, with just an original Boge twin-tube type inside which actually resists water ingress quite well, it will still have its bad effects. All preventable. The important point is you may want to try to eliminate the known and suspected vulnerabilities.

      Orange label above even appears to have a drawing of said mod, which can't quite be made out, too bad. Site the hole at the bottom center of the strut-tube base. French text confirms you don't have to take it apart to drill!, I didn't do it that way; I'm not that good or brave!

      So, consider yourself duly warned. Some might say beyond this the possible damage is self-inflicted.
      Remember. Only -you- can prevent "Frozen Strut Syndrome"!
      Hope this helps.
      Last edited by Lago Blue; 11th January 2019, 18:47.


      • #4
        A timely reminder of what time nearly forgot - my struts have just gone off to be dipped and stripped prior to painting and reassembly with B6 dampers.

        I read the link for the thread on quattroworld and eventually understood how water was getting into the bottom of the damper, it was nearer the end of the thread that someone mentioned vent in the body but prior to that i could only reference against the strut insert i took out being a completely sealed unit.

        I opened up the B6 box and had a look - yep, lo and behold the bottom of the tube has holes in it.

        I'm not entirely sure though how so much water was getting into the tube in the first place. The gland nut has a seal and i have the bellows style boots over to go over the top of that and was intending on the advice read somewhere to tie wrap / clip / secure them in place someway.

        How does water get in? is it condensation?

        The old inserts came out o.k and there was no water in my struts and they didn't have the benefit of a top cap with seal holding it all together.

        If i do end up drilling the drain hole, i will me making sure the inside of the tube is as protected as the outside - the thought of modifying in a way to let water in just sounds like an invite to corrosion to cause trouble from the inside out


        • #5
          "How...?" I'm not entirely sure either, but you may be on the right track with condensation as a part of it.

          Please also see post #2 in this thread which links to another AW thread where I list half a dozen or so preconditions in the 2nd last post (response to rmccomiskie) which I think applied in my case back then.

          Mance (VAP) also commented there on the odd variability of the issue's occurrence within his own household fleet, which begs the question: "When?", as in under what -outside- conditions does this occur? Which leads me to think I was lucky in the sense that both times it happened to me (looking back. unknowingly I was really asking for it to happen), I went from no detectable issue to total solidity in a very short while; making it easy to detect the performance change, and at least the 2nd time; act before harm was done.

          More ordinarily I think it happens much more slowly, such that it more likely goes undetected for longer, and therefore puts the strut at greater risk of an impact rupture, for a longer period also, making permanent damage all the more likely. Furthermore, in that way (i.e.: slowly, like say your condensation would if at work here) permanent damage is again more likely because even without fully filling the cavity, without impact damage and without freezing, even a little water in there for a long time can't be good.

          With the large exterior surface of the exposed chrome tube inside the boot, significant condensation could gather on the gland nut seal.

          Similar to the above, I think the other thing that happens is whenever the roads are wet, the tire is throwing off spray at a great rate, and that as the boots must be vented (just like Mance predicted above), that turbulent cloud in the fender permeates the boot interior and rains down on the slightly recessed gland nut seal; and sits.

          From there, between gravity, inter-tube (chrome to yellow) clearance movement and cavity suction (i.e.: with no drilled strut drain hole!) on rebound (cavity air may have been burped out on compression previously), any water present on the seal lip gets pulled down into the damper cavity (slurp!); and can't leave.

          One can imagine how the above effects may be coincident.

          Something like that would do it, I think.

          Now what remains is how did my struts, with non-vented Bilsteins, and no strut drain holes, also fill with water the 2nd time?

          Bilstein yellow B6-B8 cut-away with water ingress detail rmccomiskie.png

          Photo: Yellow Bilstein inverted mono-tube type B6/B8 damper cut-away with water ingress notes (this was the most difficult reference to find, I think originally by rmccomiskie;
          Last edited by Lago Blue; 11th January 2019, 18:49.


          • #6
            Comprehensive reply as ever! ​​​​​​

            The issue of suction sounds like a good candidate to me - the gland nut/top cap does or should effectively seal the strut tube as that is its job but there is a vented piston sliding up and down in that sealed space. Makes sense that a tiny amount of vacuum being generated at the shock vents on full rebound could be enough to suck water past the gland seal.

            I think I will make a small hole and make sure there is sufficient paint/wax or whatever inside the strut to protect. I'm certainly not going to make a 10mm hole in the strut!


            • #7
              With apologies to Galileo: "...and yet it leaks" in, ...sometimes.

              "Should" may be the operative word here, and perhaps this delicate looking cap-nut (seal available separately) seal's lip itself is the item which needs to be confirmed to be in perfect shape periodically, particularly by those who have yet to take the maker's advice and drill to hand. If installed dry, or with a deleterious lubricant, or worn or torn, or an edge lifted by a particle of dirt, or a chip in the chrome, or perhaps when cold it's too stiff and can't seal as it ought as it just doesn't adapt fast enough to the two tubes side-ways rocking, being none too generously sized, or a combo of a couple of these, could be the opening's cause?

              On the other hand, it may be that a good gland can be effectively sealing the cap to chrome tube gap, and yet at the same time not sealing the yellow to chrome tube gap, or the threaded strut-tube to yellow tube gap either; as follows:

              Assume for a moment the above cap to chrome-tube gap seal function is perfect. Without the reco'd Mac. strut body vacuum relief / water drain hole, and a Bilstein cap-nut otherwise torqued (even to the upwardly revised target number as per the tech bulletin), if the nut is installed -without- Loctite, on strut-tube threads taken back to fine deutsche stahl by by a well-meaning owner; the brushing, etching, blasting, lapping, and/or filing after a quarter century of rust gives up a re-newed but slightly more clearanced thread that, especially when pulled down hard against the yellow tube-top (there's got to be clearance above the strut-tube's threaded top at this point, correct?), must now also allow an enlarged double-helix shaped air passage at the engaged thread's root to crest clearances; all the way through that joint.

              This may provide both a perfect gently sloping condensation trap space and detour passage-way (erstwhile vacuum equalization port) back into the strut body's interior, and either via an imperfectly clamped shut yellow tube-top, or even an un-vented North-American yellow-tube screw in base, or both; a route back connecting the strut interior to the damper's lower chamber; under the nut and by-passing the cap seal.

              Even while you're walking away from the car after a good run, the damper's hot gas and air chambers are cooling and beginning to pull at that helical passage and that next single drop of water hanging on cap-nuts lower edge.

              I think this was in part my issue the second time around.

              It's all just theory mind you.
              Last edited by Lago Blue; 28th March 2018, 03:33.


              • #8
                Bilstein yellow mono-tube NOS with a from the factory, 6 point pink -Attention- label.png
                Another NLA factory label from a NOS yellow mono-tube. Nothing earth-shattering, just good to know.


                • #9

                  Strut Drain-hole Mod photo (not mine, but thanks to Chris Brydon):

                  This photo is lovely because the white paint on the strut-bases highlights both the drain hole size (be at least this generous) and location (to get it centered rather than thereabouts, scribe it, center-punch it, then drill). Separately, you can also see that the welded on strut-bases are just slightly domed, hinting at their interior shape which will with care, center and keep captured; the damper bottom. Captive in that fashion, the damper base won't be ringing the strut tube like a clapper does a bell!. Sometimes less is more, this tiny hole, part air pressure equalization port / part water drain hole is going to save you much time & trouble, this no cost near invisible mod is not going to improve the un-sprung side of the weight equation much, but in hindsight it will prevent a major lightening of your wallet. I'll re-attach a larger version of the photo when able.
                  Last edited by Lago Blue; 15th July 2018, 07:43.


                  • #10
                    Exactly where I drilled my holes on Sunday. ​​​​​​


                    • #11
                      Won't the drain hole be blocked by the base of the shock body?


                      • #12
                        No, because it's an upside down chock with airvents in the bottom. If there is no drain hole in the bottom of the chocktower then if water comes thru the top seal it can't get out and it will destroy the chock.

                        Audi UrS4 Avant 2.5 20vt - twincharged - Project
                        Audi 80q B4 Sedan 2.2 20vt - Daily user
                        BMW 320d Touring e91 M-Sport - Daily user
                        BMW 740iA e38 - Project


                        • #13
                          Thanks for clearing that up.


                          • #14
                            Lago Blue - I was the original poster of the vintage Bilstein shock which was a UrQ heavy duty (HD) insert.
                            Peter S

                            1990 ErsatzS2 - Pearl/Black track toy
                            1991 Coupe Quattro Silver/Black(forever awaiting a 3B swap)
                            1991 Coupe Quattro Silver/Platinum
                            1991 Coupe Quattro Red/Black
                            1995 S6 Sedan Cashmere
                            1995 S6 Wagon Silver


                            • #15

                              Very interesting bit of history sir (AudiPete?), thank you, I'm grateful that you did. I'd heard the UrQ HD was NLA, that would explain the currently un-listed P/N. This moves the issue start date back a generation. Do you recall whether the UrQ HD had a vented base like the UrS (shown on left, in Mance's photo below) or non-vented like the other, a B6/B8 type for a North American swaged-down strut? Thanks for taking the time.

                              Also of note in this photo WRT the RH smaller OD (for swaged-down to 50mm up top struts) unit, Mance said: "Note lightly scored wear ring on bottom of (the) Bilstein. That scored ring is the only place making contact with the bottom indentation (cup) in the strut tube (base) and ONLY when the Bilstein cartridge is PERFECLY aligned! Getting that alignment is NOT easy! If the strut tubes are turned horizontal to tighten the caps, it (the damper base) will fall out of that (strut) base cup before (the) threaded cap tightens against it to hold it in. Then (the) cap is tightened down on the cartridge while its canted at an angle and out of its base-retaining cup. A recipe for clunking and cap loosening."
                              Last edited by Lago Blue; 11th January 2019, 18:56.