Steering stops are a matched...
Steering stops are a matched set front (top) and rear (bottom). Shown in matched sets, the thinnest stop (left) has no rear stop and allows the most travel lock-to-lock, while the thickest stop (right) allows the smallest range of travel and could make U-turns a real problem. The mid-thickness stops from our GN gear are shown at center.
Parts That Matter
A few important components can make the difference between a crisp, responsive power-steering system and one that belongs on a piece of highway maintenance equipment. Carefully blueprinting the steering gear is the most critical key to success, but it's necessary to build the right combination of ratio and steering effort into your gear and match it with a properly sized pump.
The piston assembly consists...
The piston assembly consists of a worm (inner) and nut (outer) separated by a trail of ball bearings that create a rolling screw-like thread. Valenzuela installed oversized ball bearings so the worm has a slight preload when it's turned. Note that oversized ball bearings are sized in 0.0001 increments, so it's obvious that precision clearances are important for the gear to work properly.
The steering ratio relates how the output (pitman) shaft turns relative to the input shaft. For example, if the pitman shaft rotates 45 degrees after two full turns of the input shaft (720 degrees), the ratio is 720:45, or 16:1. This ratio is built into the piston assembly (see caption 2). Common ratios for GM passenger-car piston assemblies range from 12:1 to 20:1. A numerically low ratio means you won't saw at the steering wheel around corners, but it may be too sensitive for comfortable high-speed highway driving. In this case, building more road feel into the gear may do the trick.
Road feel, we learned, is the amount of force transmitted from the wheels, through the steering gear, and to the steering wheel when the driver makes a turn. Road feel can be masked with large amounts of power assist, which is responsible for the limp steering common to many American cars of the '60s and '70s. Fortunately, the GM steering gear can be modified for a more modern road feel.
Power assist is initiated by a torsion bar inside the steering gear. Whenever there's resistance between the steering wheel and the front wheels, say when the driver turns a corner at a reasonable speed, the torsion bar is twisted a small amount. If it's twisted enough, it exposes holes for fluid to pass through, which turns on the power assist. A thicker torsion bar gives more road feel because it takes more force to twist it and expose the fluid passages, so the driver must exert more force before the power assist kicks in. GN steering gears are fitted with 30-lb-in torsion bars, and since Lee doesn't recommend a torsion bar stiffer than 35 lb-in for a street gear, we figured we had a good core for a buildup.
Valenzuela checks the torsion...
Valenzuela checks the torsion bar's balance on this machine. Our steering box was fitted with a 30-lb-in torsion bar, so the driver exerts around 30 lb-in of torque turning left or right before the power assist kicks in. Our torsion bar was poorly balanced and needed only 25 lb-in to turn right and 32 in-lb to turn left; Valenzuela adjusted the bar until he saw an equal 28 lb-in in both directions.
Probably the last common concern is the thickness of the steering stops, which limit the range of the steering gear. The stops built into the GN gear seem pretty close but may be a little thicker than stock '70 Chevelle equipment--we haven't experienced any tire rubbing on the sway bar, but we do need a little more road for tight U-turns.
Highlights From the Rebuild Process Although most of our steering gear's internals were reusable, everything needed attention before it could be reassembled. Following the exploded view and captions will help you visualize what's going on. First, the pitman shaft is checked for runout to ensure it isn't bent. The pitman shaft, piston nut, and input are then given a fine micropolish for smooth operation and to ensure a smooth surface for the seals to ride on.
Valenzuela checks the pitman-shaft...
Valenzuela checks the pitman-shaft runout using V-blocks and a dial indicator. Runout is checked where the bearing rides, and must be less than 0.003 inch for the shaft to be reused.
The real blueprinting comes from making sure all the moving parts interact properly. The piston nut is reunited with the worm, and Valenzuela packs the assembly with oversized ball bearings to give proper operating preload. The input-shaft assembly, complete with torsion bar, is tested on a special machine to measure driver effort on left and right turns. It's hard to believe, but our torsion bar was 7 pounds out of balance and required 32 lb-in to turn right and only 25 lb-in turning left. Valenzuela tuned the torsion bar until he saw 28 lb-in turning left and right--this yields exactly 30 lb-in (primarily from the added friction of the rubber seals) when it's installed in the steering gear.
After installing new seals and O-rings, Valenzuela assembled the steering gear and made the final high-center adjustment. Turning the lash adjuster on the side cover changes the mesh of the gear teeth, and the amount of clearance (lash) between the gears is smallest at the center. Valenzuela aims for 12 to 14 lb-in of total drag, measured at the input shaft, while turning across the center of the steering range. The final step is bolting the steering gear to Lee's steering dyno where the torsion bar effort is checked once again, and the gear is tested for proper operation.