Alfa Romeo 164 User Manual

Rebuild Manual for the
Boge Electronic CDS
Strut (controlled damping
suspension) used on the
Alfa Romeo 164S, 164Q,
164QV

Authors: Steven Immel and Ben Bishop

Special thanks to Soren Snitker for donating
a pair of old CCD struts for the benefit of
this project.

first published: Nov. 3, 2011
last revision: Apr. 6, 2012

1

n.b. Manual now addresses the complete
breakdown of the strut including the removal
of the inner tubes, disassembly of the piston
and solenoid assembly, and salvaging of CDS
struts with non-CDS parts

The Boge CDS struts used on the Alfa Romeo 164 are of the highest German
manufacturing quality for their time and purpose. According to the manufacturer they
feature:

induction hardened, chrome-plated piston rods, machined and super polished to exact

•
roundness.

superior double lip, low-friction seals for long life and consistent performance

•
calibrated piston and foot valves

•
valves flow-rated, calibrated and tested to be within 1% of engineering tolerance

•
seamless cylinder tube according to the highest quality Swiss seamless steel tubing

•
technology

These positive features along with the horrendous replacement price for electronic-type
struts (ca. $1000/unit) are just some of the reasons that justify rebuilding these unique
components when they are leaking or are no longer performing well. Poor performance
is usually first noticed on the “auto/soft” setting with too much sway or “wiggle”, or when
the unique floating quality of the suspension becomes exaggerated; very seldom will the
“sport/rigid” setting (= default) lose its special stiffness unless the struts have lost too
much oil. There is one caveat to the rebuild: the piston rods must be in unblemished

condition without any significant pitting, scoring or loss of chrome and naturally the
solenoid must be functional. Regarding possibilities for salvaging a CDS strut with non­CDS parts, including rod and inner tube, see the Appendix 1 of this document.

The rebuilding process is particularly simple thanks in part to the fact that the internal
replaceable parts for front and rear struts are few and identical, not the case with most
other cars with CDS struts. A rebuild will cost as little as $50-$100 a unit. It will involve
dismantling, cleaning, replacing 3 essential seals—improving on the design of the top
mechanical seal—installing new synthetic oil, adding a Schrader valve and regassing
with nitrogen*. The addition of the Schrader valve in itself represents a significant
improvement, allowing monitoring of the pressurized system and periodic topping up if
necessary. For a final touch it is recommended that the strut be sent to a facility that has
a strut dyno (ideally the Roehrig dyno) so that the rebuild can be verified with a printout
graph showing its damping curve. You will then be able to compare the results with our
rebuild (see pp.17-18) and with others who will have posted (we hope) their graphs on
the Alfa Bulletin Board. It must be underlined that the aim of the present rebuild is not to
alter rebound or compression damping characteristics, only to “restore” the strutʼs
original performance specs as designed by Alfa and Boge engineers. While no single
graph can represent a “base-line” of performance, several graphs will get us very close.

2

*note on nitrogen: Based on an undocumented report from an individual involved in the development
of the BOGE CDS system, dry compressed air, was used to pressurize the original unit. However,
nitrogen is superior to compressed air because it expands and contracts very little with temperature
changes, It keeps the shock cooler, and its molecules, being larger, are less prone to leaking at the
seals. It also prevents the oil from cavitating (forming air bubbles) as itʼs forced through the valves.
The original charge of compressed air was reported to be 4-5 bar (58-72 psi). It is recommended that
the charge of nitrogen be kept the same.

Before commencing with the rebuild procedure it is best to summarize how the Boge
CDS struts work and to address what parts wear out. The Boge CDS strut is a twin-tube
design.* The inner, or working cylinder, is where the piston and shaft move up and
down. The outer cylinder serves as a reservoir for the hydraulic fluid. There is one
clever addition in the design of the CDS strut: an additional chamber for oil, made
available by a solenoid operated by-pass valve, that gives the strut two different
damping settings, a “soft” or comfort setting (affecting compression and rebound), and a
“rigid” or sport one. Also called “adaptive damping”, it was the Lancia Thema 8.32 that
first used this feature in 1987, followed by BMW in 1988 and Alfa shortly thereafter. Like
all twin tube designs there are fluid valves in the piston and in the stationary base valve.

The base valve controls fluid flow between both cylinders and provides some of the
damping force, especially on the compression stroke. The valve in the piston controls
most of the damping, most of it on the rebound stroke. The term “gas shock” usually
refers to the same twin-tube design, but with one improvement: low pressure nitrogen
gas is added to replace any oxygen air. The effect lessens aeration and performance
fade.

*see this useful website for explaining pros and cons of twin-tube design
http://www.ek9.org/forum/suspension/4741-mono-tube-twin-tube-suspension-info-tein.html

3

Figure 1: The Boge CDC Strut as depicted
in the Alfa Workshop Manual
(n.b. drawings are not to scale)

Valves seldom wear out, however, there are three important seals at the top of the Boge
strut that do, and the fluid itself, over time, breaks down. Age and contamination
particles in the hydraulic system are the most common reasons for the breakdown and
short service life of seals. Most of the particles reach the system via the rod. Thatʼs

why it is so important that you maintain the dust boots on your struts—itʼs by far
the cheapest way to prolong their life.

In the original Boge design the top wiper seal is combined in a single molding with a u­cup seal. This “double” seal rather than being housed in a seal holder is bonded to a
thick metal washer (4mm thick x 4.65 mm OD) that provides the hold down thrust of the
gland nut.

4

Itʼs best to see this as an assembly: nut, washer (it sort of acts as a bearing) with
integrated double seal, followed by a large o-ring (41.5 x 3mm) that is squished
between the washer and the top of the rod guide/“centering bung” (the bung locks the
top of the inner tube in a concentric position). While the double seal seals the rod from
leakage from the inner tube, the o-ring seals the outer tube, maintaing a closed system.

(nut side)

Since Boge does not provide any service whatsoever for these parts, and since no
company makes a clone of this unique bonded double seal, our remedy—thank you
Bilstein!—is to have a new washer/seal carrier CNCʼd to “hold” two separate high
performance 22 mm Bilstein seals from their successful 9100 offroad racing shock. Our
new seal carrier will have exactly the same function: there will be a “wiper” on top (to
keep dirt off the rod), and an oil seal on the bottom (keeping the strut from leaking oil
and maintaining its charge of nitrogen). Everything else remains the same. These
elastomeric seals are described by Bilstein as “new dual-stage” and “self lubricating”;
their polyurethane construction delivers up to ten times the service life of neoprene
rubber not to mention much improved sealing at low temperatures. Our carrier,
machined out of 6000 series aluminum, also offers a major improvement over the
“clunky” (stamped iron) washer used by Boge in that it forms a more positive seal with
the top of the large o-ring (a point of leakage on an aging strut), and will not rust.

wiper seal

(top position, lips

faces up)

5

oil seal

(bottom position,
lip faces down)

As you can see from this photo, two
stacked Bilsteins are the same height as
the original Boge double seal:

new seal carrier &
washer

drawing)

(cutaway

Rebuild procedure

STEP 1 - get all your parts lined up

n.b. before you start buying parts be sure your strut rods are in good condition; any damage to
the rod to the uppermost 5 cm will probably not be a problem as the this area is still above the
seals. However if the top of the shaft is chewed up due to vise-grips being used to remove the
top nut, be sure to sand down any roughness that could damage the seals during reassembly.

The list is quite simple, the quantities below will rebuild fours struts:

4 - Bilstein 9100 racing strut 22mm wiper seal (polyurethane 93)
part.no. 194044 $4.25 each (source: polyperformance.com)

SKF no. MPS-022-030-04,5-DT embossed on seal:

“SFK 1J72-1 22mm” (also available from Martin Fluid Power as
MPSU-022.0X030.0X07.0B)

4 - Bilstein 9100 racing strut 22mm u-cup seal (polyurethane 93)
part.no. 194432 $4.15 each (source: polyperformance.com)

SKF no. UBR-125-00.875-250 embossed on seal:

“SFK 1C25-1 .875” (also available from Martin Fluid Power as
UBR-125-00.875-250)

6

4 - o-rings M41.5 x 3mm Buna-N 70
$.76 each (source: theoringstore.com)

4 - Schrader valves
part.no. 91660192 $3.14 each
(source: polyperformance.com)
thread size: 1/8 - 27 npt

4 - droop stop (may not need replacing!).
use standard 7/8” ID pvc vinyl tubing, cut a
piece exactly 8 mm long, grind or file 6 radii
on one side about 2 mm deep to match
original

(original: 22 mm ID, 29.3 OD, 8 mm high)

2 liters of Pentosin CHF 11S
($53)

(Pentosin is one of the recommended oils for
BMW ECD struts; for 10% higher viscosity you
can use LiquiMoly 1127)

Special hardware (optional, for dyno testing only)

“Stirrup” for lower strut mount so strut can be mounted in dyno “in-line” (some speedshops may
charge extra for running a dyno test if they have to make this hardware). We designed this
bracket to be universal for either front or rear strut; you will also need one “rod end” (14mm x

1.5mm) to screw onto the end of the rod.

7

Welding by Chris Bernard

female rod end 14 x 1.5mm ($14.00)
(source: midwestcontrol.com)

Optional service tools:

gas filling tool
Bilstein part no. 193000 ($120)
(source: teopro.com)

(far right: another fill tool, $65)
(source: trailduty.com)

beaker - highly recommended!
(for precisely measuring
old oil and for refilling with new)
Bilstein part no. 193020 ($15)
(source: teopro.com)

Special Machine Shop work

1) Have machinist CNC a seal carrier to replace the original Boge washer-seal. The material

should be 6000 series aluminum and follow these specifications:

8

drawing and carriers by Ben Bishop

Finished carriers with the Bilstein seals installed (gland nut will slide over “top hat” of carrier)

N.b. As of 4/6/2012 several sets of carriers are still available for purchase. Contact

[e-mail]

for information

2) Have the machinist drill and tap a hole for the Schrader valve. Best position is just below

upper flange (see arrow below), on side that is opposite black cap of electronic solenoid),
this way the Schrader valve will be orientated on rear side of strut tube. First have machinist
drill a small pilot hole, then silver solder, braze or TIG weld a tab to the strut casing, then drill
larger hole, followed by tap (1/8” - 27 NPT). Ideally do this work with strut casing upside down

so metal shavings fall downward. Whoever does this work should pressure test the strut
to be sure there are no pinhole leaks at the tabs. You should eventually wrap the threads

of the Schrader valve in teflon tape before screwing it in, but for now hold off on the
Schraders.

""""""""

9

picture of “tab” to serve as base for Schrader
(1/8” mild steel, ca. 1” x 5/8” bent or shaved
with radius to match casing) (silver soldering by

Niekamp Tool)

3) Do the same for the rear strut. Again the placement is just below the flange material, also on

the opposite side of the black cap of the electronic solenoid.