As this old ad indicates, effortless steering without wandering over the road is a combination all drivers appreciate.
However, after 50-plus years of lack of attention, internal bearings and bushings wear, seals leak, and water can enter the steering box, creating rust on major components. When restoring your classic, the steering box is a critical component and careful evaluation is required to determine if it is a candidate for rebuilding.
Diagnosing Loose Steering
In our workshop, we start our initial evaluation at the steering wheel itself. Move the wheel back and forth, checking the “free play.” We look for approximately 25mm, or one inch, of movement. Any more than that and the workshop manual recommends evaluating the following additional components within the steering system prior to checking the steering box itself.
- Check the universal joint at the end of the steering shaft for play. There should be no measurable free play.
- Inspect the drag link assembly for wear and condition.
- Inspect movement in the idler arm pivot. There should be no up- and downward movement at the pivot point as the steering wheel is turned.
- Inner/outer tie-rod ends should have no excessive play due to wear.
- Spindles and kingpins can be evaluated by attempting to create movement in the wheel while the car is on a lift. Hold the wheel at 3 and 10 o’clock and attempt to move the wheel in and out. If movement is detected, watch the spindles/kingpins to evaluate whether that is the area of wear. If not, then suspect the wheel bearings.
Bias-ply tires installed on your vehicle can also significantly contribute to poor steering and road handling. For optimal driving, always install quality radial tires.
Removing the Steering Box
The steering box is easily removed by following the M-B Workshop Manual. On a 190 SL, we follow this simple procedure, which takes approximately 15 minutes to complete.
- Disconnect the steering-column wiring harness from its terminal block. Pull the harness through the box cable tube.
- Disconnect the U-joint at the box.
- Disengage the shaft from the box by pulling the steering wheel partially out.
- Disconnect the attaching drag link and tie-rod ends from the box.
- Remove the three hex-head 10mm bolts that secure the box to the frame.
- Remove the dipstick and lift the box upward.
Rebuilding the Steering Box
The pictures illustrate the sequence we use in our workshop to rebuild steering boxes.
A recirculating-ball steering box before rebuilding
Note that all measurements by micrometer are cross-referenced to the Mercedes-Benz Technical Data Manual (TDM) PN S-2588-000, Section 46. Many models use the DB-recirculating-ball-type steering, so careful reference to these specifications is required, as measurements and specs will vary from model to model.
The box with drag link and tie rod ends disconnected
Complete break-down of all parts of the box
Purchasing your rebuild kit from Mercedes-Benz will ensure that you have all the correct parts. The kit contains all the necessary parts, including a cover gasket, a sector shaft seal and two bearings, two sector shaft replacement bushings, a worm-gear seal, and two tapered bearings.
Todd Prevatte using a micrometer to verify surfaces
After removing the steering box and tearing it down to component parts, inspect and measure the sector shaft for a consistent size, top to bottom. Measure each ball bearing with a micrometer, looking for a 6.5mm consistent size. New balls can be purchased if required. The most common problem with the balls is rust-spotted surfaces. If the steering box leaks over time and 90-weight gear oil seeps out, then water can seep in. This creates a potential rust problem that usually shows up when inspecting the ball bearings. The TDM indicates that the number of balls in the ball guide is 62 +/- 2; however, over many years and hundreds of steering-box rebuilds, we have only seen the 62-ball variety.
All parts cleaned and polished prior to assembly
A common misconception is that this pre-load adjustment, if tightened down, will be a quick repair for a loose steering box. Not so! Bottoming out the spring on the shaft may cause the steering to lock up, preventing the wheels from returning to center, and is not recommended. If the box is loose, the problem is most likely worn sector shaft bushings and a rebuild is recommended.
Installing the worm gear using new tapered bearings
Carefully inspect the cable tube with the threaded end for rust damage as well. Replace if the end is rusted. The gasket surfaces should be resurfaced (decked) by the machine shop before installing the gasket. We use Wurth black RTV gasket adhesive sealer for best results. Tighten the cover hex-head screws evenly at 35 foot-pounds. Do not overtorque the four 8mm cover bolts, as it may stretch the treads and create potential leaking.
The pressure sleeve and compression spring are held in place by the large hexagon nut located on the cover plate. Adjust the set screw completely in until it bottoms out, then release the tension a quarter-turn back. Lock the adjustment by tightening the hexagon nut securely. This is a pre-load adjustment to maintain minimal worm-gear bounce and allows the gear to center itself.
A common misconception is that this pre-load adjustment, if tightened down, will be a quick repair for a loose steering box. Not so! Bottoming out the spring on the shaft may cause the steering to lock up, preventing the wheels from returning to center, and is not recommended. If the box is loose, the problem is most likely worn sector shaft bushings and a rebuild is recommended.
Refill the completed steering box with SAE 90 Hypoid gear oil. The capacity of the box is 0.3 liters. We like to position the box on the workbench for several days before installing to ensure no leaks. The steering worm and nut should now be free from all play and turn true. Also apply Molykote paste to the steering nut ball head before inserting the steering shaft in the box housing.
Refill the completed steering box with SAE 90 Hypoid gear oil. The capacity of the box is 0.3 liters. We like to position the box on the workbench for several days before installing to ensure no leaks. The steering worm and nut should now be free from all play and turn true. Also apply Molykote paste to the steering nut ball head before inserting the steering shaft in the box housing.
Installing the sector shaft (as above and below)
The final step before installing the box is to install the 10mm centering hex-head screw. It temporarily replaces the inspection screw and provides the correct alignment (centering) of the worm gear. The tool we are using can be easily made at your local machine shop. It is a 10mm thread bolt machined with a tapered end. When inserted in the fill screw hole of the box, the taper should fit into the dimple of the worm gear, ensuring a centered gear.
10mm tapered centering tool
Dimple in the worm gear visible through the opening
Centering the box before installation