Radius Arm

Remove and replace the front radius arm (lower control arm) on a 2007-2013 Mercedes-Benz S65 AMG with 6.0L V12 BiTurbo engine.

Warnings

Tools required

Parts

• Front radius arm (lower control arm) × 1 — Use OEM specification A2213308807 or A2213308907
• Ball joint nut (if damaged) × 1 — Use OEM specification
• Control arm mounting hardware kit × 1 — Use OEM specification

Preparation

1. Park vehicle on level surface and engage parking brake
2. Place wheel chocks behind rear wheels
3. If equipped with Airmatic suspension, enter service mode through instrument cluster or allow system to vent naturally
4. Loosen front wheel lug nuts while vehicle is on ground (do not remove)
5. Raise front of vehicle with floor jack at designated jacking points
6. Support vehicle with jack stands under frame rails, not suspension components
7. Remove front wheel on side being serviced
8. Spray all fasteners with penetrating oil and allow to soak for 15-20 minutes

Procedure

1. 1

   Disconnect sway bar link from radius arm
   Locate the sway bar link connection to the lower control arm. Use an Allen key to hold the shaft from spinning while loosening the lower nut with a socket. Remove the nut completely and separate the link from the control arm. Allow link to hang from sway bar.
   Torque spec

   Sway Bar Link Nuts55 Nm (41 lb-ft)
2. 2

   Support lower control arm
   Position a floor jack under the outer end of the radius arm (near the ball joint). Apply slight upward pressure to support the arm and any suspension load. This prevents the arm from dropping suddenly when disconnected.
   ⚠Keep jack pressure light - just enough to support weight. Excessive pressure will make bolt removal difficult.
3. 3

   Remove ball joint nut
   Locate the ball joint castle nut connecting the radius arm to the steering knuckle. Remove the cotter pin if present. Using an 18mm socket, loosen and remove the ball joint nut completely. Do not reuse this nut if it shows any thread damage.
   Torque spec

   Ball Joint Nut119 Nm (88 lb-ft)
4. 4

   Separate ball joint from knuckle
   Using a ball joint separator tool or pickle fork, carefully separate the ball joint taper from the steering knuckle. Insert the tool between the control arm and knuckle, then apply pressure to break the taper fit. If using a pickle fork, note this may damage the ball joint boot. Apply upward pressure with the jack while separating to ease removal.
   ⚠Do not strike the ball joint stud with a hammer as this can damage the knuckle or ball joint.
5. 5

   Mark control arm position
   Before removing the front mounting bolt, use a paint marker or scribe to mark the position of the radius arm relative to the subframe bracket. This provides a reference point for initial installation alignment, though professional alignment is still required.
6. 6

   Remove front control arm mounting bolt
   Locate the front mounting bolt that secures the radius arm to the subframe. This is typically a large bolt passing through a rubber bushing. Use a 16mm socket and breaker bar to remove the bolt completely. The bolt may be very tight. Support the control arm with the jack as you remove this bolt.
   ℹ️Front mounting bolt is often seized in the bushing. Apply heat carefully if necessary, avoiding rubber components.
   Torque spec

   Control Arm Bolts165 Nm (122 lb-ft)
7. 7

   Remove rear control arm mounting bolt
   Locate the rear mounting bolt securing the radius arm to the subframe or rear mounting bracket. This bolt also passes through a rubber bushing. Remove the bolt completely using appropriate socket and breaker bar. Lower the jack slowly to allow the control arm to drop free from the vehicle.
   Torque spec

   Control Arm Bolts165 Nm (122 lb-ft)
8. 8

   Remove radius arm from vehicle
   With all mounting points disconnected, carefully maneuver the radius arm out from under the vehicle. The arm is heavy and awkward. Note the orientation and position of any washers, spacers, or alignment shims for reinstallation. Clean the mounting areas on the subframe with a wire brush.
9. 9

   Install new radius arm - front mount
   Position the new radius arm into the vehicle, aligning it with the subframe mounting points. Install the front mounting bolt through the bushing and subframe bracket. Thread the bolt by hand first, ensuring proper alignment with your reference marks from step 5. Do not fully tighten yet - snug only to allow movement.
   ℹ️Ensure any alignment shims or spacers are reinstalled in their original positions.
10. 10

    Install new radius arm - rear mount
    Align the rear mounting point of the radius arm with the subframe bracket. Install the rear mounting bolt through the bushing. Thread by hand first to verify alignment. Snug the bolt but do not torque to final specification yet. The control arm should be able to move slightly on both bushings.
11. 11

    Connect ball joint to knuckle
    Raise the floor jack under the control arm to lift the ball joint stud into the steering knuckle taper. Ensure the stud is fully seated in the taper. Install a new ball joint nut and tighten to specification. Install a new cotter pin if the design uses one.
    Torque spec

    Ball Joint Nut119 Nm (88 lb-ft)
12. 12

    Reconnect sway bar link
    Lift the sway bar link back to the mounting point on the radius arm. Install the nut onto the link stud. Hold the shaft with an Allen key and torque the nut to specification.
    Torque spec

    Sway Bar Link Nuts55 Nm (41 lb-ft)
13. 13

    Install wheel and lower vehicle
    Reinstall the wheel and hand-tighten all lug nuts in a star pattern. Lower the vehicle until the tire just touches the ground but the suspension is not fully loaded. Torque the wheel lug nuts to specification in a star pattern. Then lower the vehicle completely so full weight is on the suspension.
    ⚠Do not fully torque control arm bolts until vehicle weight is on wheels.
    Torque spec

    Wheel Lug Nuts183 Nm (135 lb-ft)
14. 14

    Final torque control arm mounting bolts
    With the vehicle at normal ride height and full weight on the wheels, torque both the front and rear control arm mounting bolts to final specification. This must be done with the suspension loaded to prevent binding the bushings. Use a torque wrench and ensure both bolts reach the specified torque value.
    ⚠Failure to torque these bolts at ride height will cause premature bushing failure and poor handling.
    Torque spec

    Control Arm Bolts165 Nm (122 lb-ft)

Reassembly

1. Ensure all fasteners are torqued to specification, particularly control arm bolts which must be torqued at ride height
2. Verify ball joint nut is properly torqued and cotter pin is installed if applicable
3. If Airmatic suspension was put in service mode, exit service mode and allow system to re-pressurize
4. Remove jack stands and lower vehicle completely

Verification

• Test drive vehicle at low speed in a safe area and verify no unusual noises from front suspension
• Check for any clunking or rattling during turns and over bumps
• Verify steering returns to center normally after turns
• Schedule professional 4-wheel alignment immediately - this repair significantly affects alignment angles
• After alignment, verify vehicle tracks straight and steering wheel is centered
• Re-torque wheel lug nuts after 50-100 miles of driving

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