Raise the car and secure it on jack stands. Take the wheel off.
Undo the big bolt holding the bottom of the shock to the suspension arm.
You might need a long breaker bar to wrestle it loose.
Undo the two bottom housing bolts and slacken the two at the top, slide
the shock down.
Lower the shock to the ground, note if it has any shims installed - you
will need to restore those.
There's a good chance that the retainer nut and the top of the shock will
be rusted beyond belief. Start by cleaning the threads with a wire wheel.
If you have a die set, it could be a big help in restoring the threads.
The rod has a metric thread of M10x1.5mm.
Compress the spring and undo the retainer nut. If you don't have an impact
wrench, grab the top of the rod with vice-grips and use a 16 mm spanner
on the nut. Turn the nut back and forth if it gets stuck in midway.
The shock apart (counter-clockwise): damper, spring, dust shield with bump
stop, washer, lower bushing with a sleeve, spring seat, spring bracket,
upper bushing, washer, retainer nut.
New vs old
The Bilstein Sport damper has a slightly shorter rod, but looks similar
to OEM otherwise. The Eibach spring is actually slightly longer, but has a
number of "barrel" coils that are normally compressed together, while
the lower coils provide the springing action. The barrel coils are
thoughfully wrapped in a sleeve to prevent them from scratching each
other and developing rust. See
what happens when they're left unprotected.
Ready to go
The damper comes with an aluminium lower spring support and a new spacer
ring that fit at the base of the rod. When you compress the spring, use
the working coils only, because the upper barrel coils will end up too
close to each other, so your spring compressor would get stuck. All the
other parts of the shock assembly are reused here. Make sure that the upper
coil of the spring matches the recess in the upper spring seat.
Tighten the retainer nut to 15 ft-lbs. Use a 5 mm Allen key on the rod
while holding socket with vice-grips (or use a spanner on the nut).
Slide the housing up as far as it goes until it's propped against the top
bolts (use a jack if necessary). Insert the lower bolts and
tighten all four to 40 ft-lbs.
Insert the lower bolt, rotate the damper if needed to align the holes.
Tighten to 140 ft-lbs.
Install the wheels, tighten the lug bolts to 81 ft-lbs and lower the
car to the ground. Enjoy a 35 mm lower ride height.