Bilstein into Macpherson strut install

[Lago Blue]

Bilsteins both going into and coming out of MacPherson struts...

a) 1st Screen-shot details some notes and explanations from the Bilstein catalog (I've high-lighted the requirement for no tension on the top-mount prior to removal). We are here (below) going to expand our understanding of that warning.

b) A cautionary tale about a UrS-sized yellow Bilstein damper half as projectile we should all be aware of...

https://www.s2forum.com/forum/techni...em#post2111150

c) A listed caution from Bilstein.com (although the subject damper construction details, configuration and specific eject-able part name may differ from the example shown in Bilstein's photo (1st screen-shot below), this factory warning is still applicable here), see note regarding risk of piston-rod ejection (clearly a factory known possible hazard, we simply have failed to pick up on their long pre-existing instruction), and Igor's experience (albeit an UrS one) above is a very lucky example. What comes to mind is that if you don't know whether your dampers were always installed (or particularly removed I would think!) using the approved two-tools by hand only factory method, you may want to proceed as if someone did use an impact-gun in the past.

While still on the car, B-series car's struts would (fortunately) only be able to blow-out downward, as they are constrained by the small hole diameter of the fender-turret receiver for the top-mount et al, while C-series cars strut's top-mounts appear to be remove-able from the top of the still installed strut, posing the possible hazard outlined above. In either case we now see that it may be wise to always remove all weight from the wheels, allowing the suspension to fully extend, so that tension against the top-mount can be checked as absent; prior to strut and top-mount removal.

Understanding that both ends of the the skinny and hidden internal piston rod are threaded, one into the base of the yellow tube and then staked in place to help prevent exactly this issue (see photo in post 15 above), the other is fastened to the piston with a nut (3rd, 4th and 5th screen-shots). and that the possible undoing of which (and the factory's unhappy experience) is likely the cause for the warning.

We must therefore always key the internal hex of the damper shaft to prevent it turning during top nut install and removal. Although somewhat counter-intuitive, it may also be that even during improper nut installation (with the use of an impact-gun for instance) that the factory thread-locking (at the piston to piston rod connection) is weakened or undone to the extent that, the damper becomes 'armed' for a possible future explosion at some point; if we are not watchful for something amiss during dis-assembly.

d) 6th Screen-shot below (from the excellent https://s2-audi.co.uk/tech_articles/front_shocks.htm) shows the factory method re the top nut.

e) 7th Screen-shot details (from) Koni top-nut torque targets and approved method, but also echoes an anti-impact gun warning / prohibition for this location.

Bilstein notes and explanations Screen Shot 2022-01-05 at 08.44.01.png Blstein install : removal tip Screen Shot 2021-11-13 at 12.50.50.png Bilstein piston attchmnt 1a Screen Shot 2022-01-05 at 12.45.45.png Bilstein piston attchmnt 2 Screen Shot 2022-01-05 at 11.56.43.png

Bilstein Mono tube piston to rod attachment cut away Screen Shot 2022-01-05 at 12.19.15.png https:::s2-audi.co.uk:tech_articles:front_shocks.htm Screen Shot 2021-11-13 at 13.47.58.png Koni top nut torques, method and warning Screen Shot 2021-11-19 at 11.08.46.png

[Lago Blue]

Disposal and Possible Dis-arming of Yellow-bodied Bilsteins posing a blow-out hazard on removal...

Not to forget that Bilsteins are eminently rebuild-able at authorized service centres world-wide, below are instructions and drawings from Bilstein WRT end of life treatment for several Bilstein types including the subject mono-tubes.

Like many other makes of high-performance dampers, Bilsteins are pressurized to 25 bar (I've seen 30 listed on the MSDS sheets for these), so the explosive potential for injury if allowed to blow out on removal is to be kept in mind when doing a removal.

The methods outlined here may be also be of interest to someone who fortunately is vigilant enough to recognize the possiblity of a blow-out on removal as above and stops (as per the discussion in the previous post) before such occurs, but is unfortunately then stuck as how to proceed. Although I have not done this, I simply suggest this as a possible exit strategy if one does encounter a damper that will not allow the top-mount to be without tension prior to removal, and which may want to blow-out if allowed to further extend. Even though encircled by the main spring, the procedures outlined below may still allow one to drill in between the coils and more safely de-fuse said failed damper inside; in a more controlled manner than otherwise.

Disposal of Bilsteins text Screen Shot 2022-01-05 at 08.20.27.png Disposal of Bilsteins drwg Screen Shot 2022-01-05 at 08.17.36.png

[igor136]

I thought a little. If UrS6 B6 shock absorbers have two ventilation holes at the bottom of the pipe, how can they breathe if the large mounting bolt has a rubber seal.
It is a closed system. It is logical to drill a hole in the housing through which the shock absorber will breathe. But then water enters through that hole. I don't think these shock absorbers are the best for our car. OEM Boge maybe?

[Lago Blue]

Hi Igor,

When contemplating the vent / drain-hole mod, it may be helpful to keep some history and physics in mind as follows.

- When Audi was first developing the original S1 quattro for competition (1980 or thereabouts), they hired Boge to develop the suspension, and chose Bilstein for the dampers to fill the struts which were made by Boge.
- This issue (and this fix!) were both clearly known by Bilstein way back (1980 - 1991) when they where putting those orange paper warning stickers (seen in post 3 above) on their B6s and B8s (called HD and Sport back then) for the B2 series UrQ cars (see text in posts 14 and 15 above).
- Years later (1991 - 1997), this issue was re-introduced more widely, brought to the attention of the Audi forums community in the U.S. by UrS owners who experienced the same damper failures until incorporating this fix, with no reported down-sides attributable to the mod.

Although your B6 or B8 dampers for the 55mm ID UrS struts do have factory drain / vent holes in them, consider also that my 50mm B6 and B8s do not (although the two off-centre holes in the base they do have (ostensibly for wrenching) look like vent / drain holes, they definitely are not (see photo in post 15 above). Because my struts (pre-vent / drain-hole mod) and dampers, could not collect water through the bottom, that means the water found inside, must have come in from the top of the struts somehow (or possibly via the threads of the damper base in the case of my dampers, I just don't know); somewhere (beyond this, it may not matter specifically how or exactly where). Whether one has a B2, 3, or 4 series car or a C4 series type 4A (1991 - 1994), such as the UrS4, 6 or 6 plus, irrespective of the damper inside, all these struts are known (for perhaps 40 years now!) to collect water. Still after all this time, unfortunately there are those who even with this knowledge, believe their install is somehow exceptional. However in view of the above history, I would strongly suggest your struts (just like other UrS and S2 owners have discovered) can collect water similarly; also from above. The Bilstein instructions to drill, detailed on that orange label are only the logical extension of those factory vent / drain holes; that your UrS Bilsteins already have.

Precisely because the strut's interior is as you say "a closed system", even with the drilled hole in the strut-base, that does not alter its performance WRT water possibly entering from below, but it does mean that without the drilled hole in the strut base, any water that does gather; cannot leave. Like a diving bell lowered into the water (or after one has driven into the lake till the water level is over ones axles and pouring inside over the door sills!), since the air is (almost perfectly) trapped in the strut chamber, the water cannot rise further and enter. More ordinarily, anything that does manage to splash in from below, (or more importantly - leak in from above, because although it is in fact closed, it is not a perfectly sealed system!); immediately falls out. That last bit is perhaps the most important consideration to keep in mind, that the water (post-mod) cannot remain. Only if the water could remain will damage occur.

So we see that this issue has a very long history of both what the too frequent result of the problem is, and what is the successful and so far un-problematic fix.

Q: "...how can they breathe if the large mounting bolt has a rubber seal?"

A: If you mean by "large mounting bolt" to be the damper's central piston rod, and by "rubber seal" to mean the internal bump-stop, then know that the parts are sized to allow a sufficient clear passage-way to the exterior. If you mean the gland nut which holds the damper in the strut, it may perfectly seal the strut closed when it's brand new but as it wears and ages, perhaps not so perfectly.

Attached is a close-up of a cross-sectional drwg. of two Bilstein 50mm motorsport struts (see other details in post 27 above). These are externally threaded all-in-one strut/dampers for adjustable spring perches and welded-on tabs to attach hub-carriers, but otherwise internally very similar to our familiar yellow B6/8 dampers. The top one is fully extended, the bottom one is fully compressed and details the from the factory (and quite large!) drain / vent hole feature (it is actually a separate part with a screen in it!), and one can clearly see the route from that lower air chamber through it to the exterior (blue arrow). The specific vent / drain path in our case (the damper and strut body are separate items) is discussed in posts 11 and 12 above.

Gravity is our friend here, if the avoidance of this classic hydro-locking issue (whose insidious cause accumulates undetected until spectacular damper failure is the result) down the road is sought, all we need do is follow Bilstein's tried and true preventative fix. The vent /drain-hole mod future-proofs your dampers from this possible fate.

To your last point, while only you can decide what the best dampers are for you and your car, know that even with the OEM BOGEs (or similar), they can also suffer (although differently, as actual freezing is required!) due to the absence of a strut vent / drain hole, see text of "Notes on other than B6 / B8 Bilstein installs:" in post 3 above.

Bilstein Mtrsprt strut partial x-sectional drwg Screen Shot 2022-01-07 at 08.20.26 copy.png

[Davi123]

Just taken apart my struts to install B6’s and note they are slightly shorter than the originals. I’ve installed one tonight and there isn’t much thread coming through to the top nut (slotted bush correctly torqued). If I remove the rubber on the top domed washer (811412319) it seems to fit better. Has someone solved this problem before please?

[Lago Blue]

Davi123
Hi Sir,

This is occurring with your rear struts, correct?

Re "problem". Good question, but you may not have an issue here. IIRC, there was not a lot of damper-shaft top threads protruding on mine when about to start the top nyloc nut, however the shaft was drawn up into the top nut upon tightening; without issue. There shouldn't be an issue here even if the incorrect (i.e. front style) spring cap was in use here, except your rear ride height would be elevated.

As you may have guessed, the function of the "retainers" (your domed washers), is to simply retain the strut tops from falling out of the wheel-wells. So if they are quite snug, as the rear struts don't really rotate, un-like the fronts do when steering, that the rubber is compressed on the install is actually desired; and works out well. Although there must be some clearance between the underside of the front retainers and the painted wheel-wells' top surface under the retainers, there need not be on the rears.

I would guess that one of the reasons for the factory material difference between the front retainer's plastic rings and these rear "rubbers" on the otherwise identical retainer washers was for noise prevention due to the rear mount's location being almost inside the cabin. The issue would be due to the eventual (but preventable - see Strut-Stops!) top-mount collapse, creating unwanted vertical slack / clearance; a noise maker. The front style hard plastic ones (designed to be low-friction, facilitating a drag-free rotation during steering changes if and when in contact with the top painted surface of the front wheel-well during slow-speed manoeuvres over uneven ground; precisely when just one front wheel drops), they are often found missing, as they fall off after cracking. I say 'noise-maker', as you also may find the little factory-fitted OEM glued-together hat-box shaped open-cell gray foam caps over the rear top-mounts (see top-most item (the foam cap) in drwg. below).

Audi 90 q Rr susp. drwg of LH side Screen Shot 2026-01-14 at 8.07.22 PM.pngAudi 90 q 20v sedan LH Rr top mount in trunk photo Screen Shot 2026-01-03 at 9.05.54 PM.png

[Davi123]

Thanks Lago Blue, I will just tighten them up then with rubbers on the rear and no rubbers on the front which is how they came off the car. Looking forward to trying them out now

[Davi123]

Okay so refurb all completed (many thanks to forum members for their threads and help) and MOT passed. But I'm not convinced about the front mounts. There is a gap between the domed cap and the painted top of the arch (obviously with wheels on the floor) of 9mm, that seems excessive to me. Ive checked all the torque settings. The new top mounts i used were SKF 731 657 233 1309 and according to auto doc we're compatible. Any one had this problem before?? Thanks Paul

[Davi123]

Here is a photo

[Nuvo]

Something looks off there, are you sure you have the spring ends properly seated in the strut and the top "spring disc"?

[Davi123]

Thanks Nuvo, yes top and bottom of spring alignment to the stops.

[Tractor Dave]

I don't think its too bad. Problem is that the original Boge top mounts are nla and I think the Sachs mounts ( who took over Boge) are the closest you can get, but are still slightly shorter than the original. If you fit the Lago Blue bush savers with the TT top washers, the gap is even bigger.

[Davi123]

Thanks Dave. It might be because I've not driven it for 2 years during the refub but it felt a bit spongy (side to side type motion). Might be the 4 wheel alignment will help.

[Nuvo]

Hi Davi,
How big is the gap measured between the tower and the underside of strut stop? Is that a white ring one of the Mount Saver components that is unseated from the interior of the top mount?

[Davi123]

Hi Nuvo,

It's about 9mm. The white plastic is something I just made as I understood the originals had a plastic washer. Just seen this product, anyone used it?