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Bilstein into Macpherson strut install

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  • Bilstein into Macpherson strut install

    Title edited, thank you mods/admins.
    Purpose of this thread is primarily to provide a single place where you can list tips for installing yellow B6 and B8 type mono-tube Bilsteins into Audi B3 and B4 chassis MacPherson struts. This may be of use as although similar (and considered a worthwhile mod by many), an install that can be looked back on as successful longer-term, is a little different from how the original factory Boge's and other OEM (twin-tube) style dampers (including Bilstein B4's) were assembled into these Audi struts. Please note also that there are a few points herein which (all noted as such I believe), apply -just- to the Boge's, Bilstein B4's and similar conventional OEM (twin-tube) style dampers.

    Notes on parts and special tool, compatibility and condition:
    Along with dampers that fit, you will need the specific Bilstein cap-nuts and cap-nut tool (a.k.a. the Bilstein wrench) also correctly sized both for your dampers and strut-tubes. Important because:
    - Front struts of the North-American cars all had necked-down strut tube-tops requiring small O.D. dampers;
    - Some of the destined for Europe cars also came with front strut tubes of less than the more usual for Europe 55mm OD; and
    - Early on, you may want to measure both your struts to be sure; in case they are not both the same or the expected size.
    If re-using Bilstein cap-nuts, ensure the rubber gland it carries is in very good condition, as this seal not only protects the damper's greased interior sliding interface below (chrome shaft against interior of yellow tube) from water and grit, but also keeps closed an otherwise known water entry point which can lead to hydro-locking of the strut assembly (more on this below). If the Bilstein wrench is a used one, ensure it has not been previously damaged, fits easily over the damper shaft, and engages fully onto the nuts correctly, read more below. Although new Bilsteins should come with new gland nuts (with glands preinstalled), no wrench will be included. The nuts, either with or without the required glands, the glands themselves, and the tool are all available separately from Bilstein.

    Bilstein B6 : B8 M50 ring-nut , for 50mm front struts.png Bilstein B6 : B8 30mm seals, for 50mm front struts.png Bilstein B6 : B8 gland-nut complete with seal for 55mm front & rear struts?.png

    Jordan's photos (from M/G) of a 50mm gland-nut w/o gland, the glands for those North-American front struts & 52mm? nuts w/glands for rears.

    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, a hack saw, and proper penetrating oil (not WD-40);
    - Bench-mounted vise; wooden blocks to contact, and brace the strut with. Use vise judiciously with the wood blocks to secure the strut in without collapsing the strut tube, for drilling the drain hole; and to cradle the assembled strut in, both for cutting the original cap-nut; and torquing the Bilstein cap-nut.
    - Note: Stillsons, or even a small chain or pipe wrench should not be necessary and may result in strut-tube damage. If employed, use caution, and use only gently around the old cut cap nut; and only - after - it is already loose;
    - Hand-held wire brush; Navel Jelly (a rust remover gel); lapping compound; varsol; lacquer thinner (or electrical contact cleaner); spray paint (primer and 2 colours which contrast with strut-tube colour; and each other, a small paint-brush);
    - Loctite (blue), a 1/2" drive torque wrench;
    - Nail-polish (in a contrasting colour to your strut paint) & a wax-oil type rust-proofing compound.
    - Scriber, center punch and a ball-peen hammer; &
    - Electric hand-drill, a small diameter pilot-hole drill bit, a 1/8th" to 10mm bit.

    2.) Removal of Old Stock Damper from Strut (springs, etc. are assumed already off the strut at this point).
    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 the penetrant; and only then finally with just your gloved hands, twist it off.

    Mac strut damper retaining cap-nut cut-lines 2019-05-02 at 10.06.28.png

    Drawn lines in photo above are to indicate where to cut diagonally in 2 or 3 places around the outside of a seized cap-nut's circumference; to loosen the rusted threads. Once cut, penetrating oil can be applied directly into the affected threads and the nut will actually spring slightly open. Using a hand-saw only, cutting is sufficient when a series of small holes appear as you break through the roots of the cap-nut's interior threads. Done carefully, no damage to the struts threads (which are fully engaged and very close while you perform this task) will occur.

    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.

    Noye; 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. Don't use a power saw or wheel of any sort as you will mess up your struts if you do.

    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 small 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-nut 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.

    Here's a photo from a post in S2Central's Strut Cap-Nut Tools thread (link to complete thread near bottom of this post) where he made use of the above method:

    Diagonally cut strut cap-nut.png

    Here is an old post link (from 2010 on which also described the task list above, and 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. However more immediately, this will also aid in the correct cap-nut torquing later on as torque consumed in turning the nut onto a rusty dirty strut-tube is automatically subtracted from the torque applied to the damper body (where you want the torque to be felt). Note also that there was a factory bulletin (see link below) which upped the torque because of issues observed due to cap-nut loosening.
    - Wire brush the threads;
    - Likewise Naval Jelly as per its' instructions;
    - Paint (other than, but up to threaded area)
    - Further strut tube thread cleaning. You may now use the just removed old and usefully cut OEM cap-nut (allowing adjustment by slightly squeezing the cut sides in as required) as an effective thread-chasing die with some lapping compound.
    Note re importance of thread-cleaning: The Bilstein cap-nut recess, which secures and centers 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", as this will best allow both proper capture of the damper top and torquing of the cap-nut, helping to ensure the damper will remain correctly centered and fixed inside the strut-tube. Again, when applying torque to the nut, consider that any force consumed in turning the cap due to poor thread condition; is torque that is -not- applied to properly clamp the damper in place. This is not fixable later until the main springs are again removed.

    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 tube-top external threads to best receive Loctite.

    4.) Understanding Fixturing of the Mono-tube type Bilstein within the Mac-P. strut:
    Unlike the original Boge or other OEM (twin-tube) style dampers (such as Koni), 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 perhaps less of a concern if these two diameters nearly match, but any misalignment (between strut-tube and damper center-lines) will result in movement while underway and at least likely damage to the cap-nut's gland seal. The damper base has an abbreviated (less than 5mm) and shouldered slightly spherical protruding shape to fit the mating depression in the strut-tube base, and its correct function is dependent on three preconditions:
    a.) Damper base sphere correctly centered in the strut-tube base recess;
    b.) Top outer edge of the yellow tube correctly captured and centered within the Bilstein cap-nut recess (less than 5mm deep); &
    c.) Cap-nut correctly fastened with Loctite; and done up to the recommended 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 interior base shoulder. This will allow damper-base to rock laterally during driving which will at least make noise, very annoying (and possibly begin cap-nut loosening and vertical damper hammering); and everything must come apart to fix it;
    ii.) Yellow tube-top not correctly captured in the cap-nut recess. Nut torque may relax during driving also allowing damper to both rock laterally and hammer vertically within strut. This will quickly kill the gland-nut seal; and the nut torque may deform yellow tube-top edge which may allow water entry into the strut.

    5.) Dry Run #1: Empty Assembly of Strut-tube & Bilstein Cap-Nut:
    You want to be able to screw on and fully seat the cap-nut on the still empty strut-tube easily with just your bare hands. This will ensure a couple of things:
    i.) That you have sufficient free cap-nut travel to properly capture, compress and hold the damper when installed later; &
    ii.) That your final applied torque will all be employed to pressing down on the top of the yellow tube, securing the damper; and not partially lost fighting thread friction (Note: Be aware there was a factory TSB which upped the torque on the original cap-nut due to observed loosening issues which occurred here, see more on this at bottom of post).

    Paint-mark this fully down point on the exposed threads of the strut-tube just below the nut. This will give you an initial reference point (Ref. A in the strut tube-top photo below (photo by audi*line, thanks) that you should almost but not quite reach later with the slightly taller damper correctly in place with both its' base properly centered inside; and the yellow tube-top properly captured and centered in the cap-nut.

    Strut-top cap-nut fit reference points.png

    6.) Dry Run #2: Ensuring Full Correct Damper Placement, and Fixturing into Strut-tube:
    a.) Unscrew the cap-nut and place the damper inside the strut-tube carefully centering its' base in the bottom of the strut-tube; (Note: Damper -must- be installed by lowering into strut whilst both are held vertical). Note the damper will sit lowest in the strut-tube when it's base is properly centered, and -not- up on the circumferential shoulder of the strut-tube's internal base.
    b.) Next screw the cap-nut back on and hand-tighten fully onto the strut-tube again ensuring the yellow tube-top is properly centered and captive inside of, and -not- up on edge of, the cap's recess or catching the edge of the yellow tube.
    c.) Paint a 2nd broad mark (well apart from the 1st) w/the 2nd paint colour, that can be easily seen while your working, horizontally just below the bottom edge of the nut at its' now final install position (Ref. B in above photo). The difference between these two paint marks should mirror the effective and observed height difference between the top threaded edge of the strut-tube; and the installed damper's yellow tube-top's (slightly taller) height (Ref.C).
    d.) Now paint the 3rd and 4th marks vertically w/the 2nd paint colour again, one on the Bilstein cap-nut and one on the strut-tube to give you a good vernier indication of the nut being exactly fully home correctly (see Ref.D). If the top or bottom of the damper are not correctly positioned inside the strut, you will not be able to easily by hand turn the cap-nut down to this point during final assembly below (Hint: Paint this one such that you can see it in future by lifting the damper boot when the car is all back together).

    Doing both the above empty assembly and the practice full vertical fixturing is going to help ensure a couple of things:
    i.) With damper now in place you are here going to be able to confirm that because the cap-nut bottoms out -before- reaching Ref. A, that the damper will be effectively clamped in place (this is particularly important strut-tube effective internal length check for those who have done Mance's BBSE mod, and it confirms you dampers are the correct length if your struts are unmodified also) before the cap-nut bottoms on the strut-tube at Ref. A.
    ii.) You are going to get a feel for how everything fits together properly; and by noting the new final hand installed height of the new cap-nut , which will now approach Ref.B and be complete at Ref. D, you are able to more easily confirm success as you go during final assembly below, ensuring that the damper is correctly installed into the strut, rather than discovering it's not, after it's all back together; and you try dry driving on it. You now need to be thinking about how you are going to brace the assembled strut for the torquing of the cap-nut without damaging the strut-tube, the cap-nut, the Bilstein wrench or (see post #2 below); yourself!
    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.) Apply just a little Loctite in one spot & complete full vertical assembly again including bottoming cap-nut by hand into position at the above final ref. mark (see Ref.D);
    b.) Brace strut for torquing and zero damage to strut and you!
    c.) Torque application & see also "Bilstein Wrench Mod, from 2007" (photos in following post). You may need to re-do your assembly match-mark paint slightly after torquing.

    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, already damaged or if one uses the incorrect Loctite or even too much blue Loctite and/or doesn't warm-up the Loctite prior to the attempted loosening. By using the modified wrench, successful removal is more likely, but not guaranteed.

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

    Example of both the Bilstein cap-nut with gland & the wrench (photos below). 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 damper's chrome shaft 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.

    image_71419.png image_71420.png

    Torquing the original style strut cap-nuts (with the 6-sided internal opening. seen in para. 2.) above, and the sockets for them:
    Here is a link to an old write-up (from 2008 on regarding the above mentioned factory Technical Service Bulletin re the cap-nut torque increase. This may be of particular interest to those who may have been reading here about strut dis-assembly; and are going to be re-installing the original style cap-nut on their struts, used with conventional OEM (twin-tube) style original Boge's, and replacement Sachs, Koni's and Bilstein B4's, for instance:

    Recapping cap-nut torque with the factory TSB...

    Further (and a little farther off-topic), here is a photo (courtesy of Orsum) of the factory tool (both the VAG 2069 and VAG 2069A appear similar) necessary for torquing strictly the above original style cap-nuts onto the struts (Orsum rightly notes this socket's annular recess which has been cut away at the end shown to avoid any contact with the installed damper's main seal, which may be of particular interest to those who might be fashioning their own tool):

    OEM style strut cap-nut tool VAG 2069 .png

    For more on these two different socket sizes, P/N's and various suppliers and makers of this tool, please also see S2Central's thread here:

    Tools for the Front & Rear Strut's Cap-Nuts...

    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, these particular dampers' 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..." (from 2011) on this forum and work back to the links.

    I think that's most of it, hope this helps. And certainly I may have missed something, please chime in. Questions?

    Lago Blue
    Last edited by Lago Blue; 25th April 2020, 15:52.

  • #2
    The Illustrated Bilstein Wrench Mod and intro to Frozen Strut Syndrome...

    1.) The Bilstein Wrench Mod allows you to then attach the wrench to the cap-nut, so it can't slip off. Makes the required torque application an easier, safer, and even a one-handed job:

    a) Bilstein Wrench Mod - see link in previous post (note how inserted tube can push directly against the back-sides of the wrench's bicuspids);

    Bilstein wrench mod - inserted tube shadow.png

    b) Bilstein Wrench mod in use. Sorry, but this is perhaps just about all your current (shameless plug for "Strut-Stops"!) un-aided OEM top-mounts are good for, with the wrench held fully engaged and secured to the cap-nut, perfect single-handed torque application;

    Bilstein wrench mod in use.png

    2.) An atypical case of B6 / B8 Bilstein frozen strut syndrome?

    Note: They don't have to freeze...
    Last edited by Lago Blue; 7th April 2020, 03:29.


    • #3

      A Brief History of What Time Nearly Forgot: The Little Black Hole That Is Not Just a Theory!

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

      Bilstein yellow mono-tube with a from the factory, 10mm strut drain hole mod label.png (May 2018 Edit: PeterS's photo of an NLA UrQ HD damper, which used the old Bilstein P/N'ing scheme, but most importantly shows it's tri-lingual factory warning stickers regarding the required strut-bottom drain / vent hole drilling.)

      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 galaxy ( far far away, 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.

      Notes on other than B6 / B8 Bilstein installs:
      Previously, I'd said a strut drain-hole was -always- a good idea in our Mac. struts, regardless of -whichever- damper brand or type you may run, but there is an alternative strictly for conventional (twin-tube) style dampers which may suit.

      a) Having found what could only have been freeze-crush damage to a conventional Boge, (the original factory fitment), this drove the above reco. Because the lower damper-body had been oddly crushed in some areas, that resulted in it being enlarged in others, to the extent that is was quite difficult to get the damper out of the strut-tube. There was a quantity of water still in the strut-tube on damper extraction. I suspect the water came in the 6-sided opening in the OEM cap-nut. The water wouldn't have been a problem except it eventually froze. This requirement for low temperature here differentiates this version of "frozen strut syndrome" from the B6 / B8 Bilstein type detailed above which need not involve any freezing to still take place.

      b) I read in Tractor Dave's UrQ thread, the Koni (a conventional OEM (twin-tube) style damper) install instructions say to put 50mm of anti-freeze into the strut-tube prior to installing the damper, some of which will overflow in that process. That this might aid in wicking away heat from the damper through the strut tube, may be useful and is a well-recognized method (save that motor oil or ATF is used) on older Porsche's and by other marques specifically for this reason. Being seamlessly encased and lacking a necessarily empty lower chamber, these conventional dampers, unlike the B6 / B8's, aren't vulnerable to failure by ingesting liquid.

      c) Like the drain hole, certainly use of a liquid unlikely to freeze will also prevent the above water ingress / freeze / crush event sequence, but I don't know if that consideration was either even a part of or the real point of Koni's thinking in writing their instructions, perhaps not. You will have to decide which method you'd prefer.

      Back to B6 / B8 Bilsteins:
      March 2018: Not sure of what car the above photo's P/N refers to (is it simply an older superseded numbering scheme?), but clearly its a mono-tube (Bilstein's classic up-side-down wasser-pumpen 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 (for North American B3 & B4 chassis), 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; 7th April 2020, 12:18.


      • #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 inside the fender-well 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; 9th January 2020, 13:44.


          • #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; 9th January 2020, 13:47.


              • #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 / Vent-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; 28th February 2020, 03:39.


                  • #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

                              A very interesting bit of history indeed sir (AudiPete?), thank you, I'm very grateful that you did! I'd heard the UrQ HD was NLA, that would explain the currently un-listed P/N. This also moves the issue start-date back a whole generation. Do you recall whether the UrQ HD had a vented base like the UrS unit (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.

                              I was here, for clarity, also going to borrow Skull's photo (see 2nd photo in next post) of the smaller O.D. (50mm) Audi 80 B4 strut tube-top being measured (if I can't figure out why it won't up-load here!):

                              Also of note in the photo below WRT the RH smaller OD unit (again, for strut tube-tops swaged down to 50mm), Mance said: "Note (the) lightly scored wear ring on (the) 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; 19th February 2020, 23:10.