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Buick Grand National Steering Box Build - Straight Arrow

Blueprinting a Buick GN Steering Box for a '70s Chevelle

Tony Nausieda Jun 1, 2004 0 Comment(s)
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Don't recognize this as an O.E. SS 396 Chevelle steering box? You're right. It's from an '86 Buick Grand National, and it almost bolts right into the B-body. You'll need Lee Engineering press-fit flare-seat inserts (rather than the modern GM O-ring seals) for the hydraulic lines and a special rag-joint steering coupler to make it work in the older chassis.

Few parts have such a direct effect on driving enjoyment and safety as the power-steering gear. While most folks spring for new tie rods and often a new centerlink, the worn-out stock steering gear is lucky to get a fresh coat of detailing paint. At least that's all we'd done on our '70 Chevelle SS 396. The gear looked great in the engine bay, but the power assist came and went as it pleased, usually delivering a heap of assist partway into a turn. This of course, wasn't good in an otherwise well-planned corner-carver suspension.

The exploded views in the Chevy shop manual humbled our thoughts of reviving the steering gear ourselves, so we entrusted it to the experts at Lee Manufacturing in Sun Valley, California. Owner Tom Lee has the great distinction of developing and bestowing the first race-bred power-steering systems on Sprint and Stock cars. From the late '60s through the present, Lee has enough hours of R&D invested in power-steering technology to rival a team of career engineers, and his involvement in current OEM and racing efforts are a testament to his reputation. Although we spec'd a nearly stock rebuild on our steering gear, we'll show some of the options that you might choose if your Chevy sees the track more often than the street. This story won't guide you through a complete rebuild; a factory service manual is invaluable if you're courageous enough to try.

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The torsion bar senses the force between the wheels and the driver's hands and allows power assist if there's enough force or road feel. Increasing the diameter of the torsion bar makes it harder to twist, so the driver feels more road-going forces in the wheel before the power assist comes in. The 0.210-inch through 0.285-inch bars are shown front to back in the picture and will generate about 30 lb-in to over 80 lb-in, respectively, of steering effort when installed in a gear.

The compact GM power-steering-gear design has been installed in rear-wheel-drive cars and trucks from 1964 through most of the '90s. That means there's a good chance that a later-model gear will bolt into a muscle-era Chevy with minimal modification. We'd already installed a steering gear from an '86 Buick Grand National (GN) in our Chevelle using Lee's own press-fit flare-seat inserts to make our factory Chevelle hydraulic >> lines work (later GM cars like the GN used O-rings instead of 45-degree flares to seal the hose ends). A special steering coupler (rag joint) is also needed and is available through Lee. Such minute details don't change the scope of this story, but it's important to realize that a later-model gear swap isn't necessarily a 100 percent drop-in replacement.

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Here's a sampling of the more common GM piston assemblies. Constant-ratio pistons are on the left, and variable-ratio pistons are on the right. Shown clockwise from lower left, these pistons give ratios of 12:1, 17:1, 20:1, and 16:1. Variable-ratio gears have a numerically higher ratio near center ramping down to a lower ratio near full lock.

With the tired GN gear on the workbench, master tech Julio Valenzuela had it disassembled in a matter of minutes. First impression: This thing was really, really dirty inside, so Valenzuela trash-canned the spent O-rings and seals and bathed the rest in a parts washer. Cleanliness is the name of the game. The cast-iron housing was inspected for cracks and the bore was checked for smoothness; then it was sandblasted and painted. While the paint was drying, Valenzuela laid out the cleaned internals on the bench and began the blueprinting process.

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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.

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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.

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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.

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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.

Volume Check

The power-steering pump is a less complicated animal but still deserves the same level of blueprinting. Tom Lee explained that the factory Chevelle pump is a sound piece that delivers adequate pressure (about 1,250 psi at peak) for low-speed or no-speed turns. Volume, not pressure, is the important factor in matching an older pump with a newer, faster-ratio steering gear. Charlie Hutchinson led us through the teardown and reassembly. The Chevelle uses a vane-style pump that's pretty simple in operation, so it's straightforward to rebuild. After Hutchinson pulled it apart, the shaft was bead-blasted and polished. A new seal was installed in the housing for good measure. Hutchinson checked for excessive clearance between the rotor and the liner and found tolerances to be within spec, so they were lightly sanded and the pump was reassembled with new O-rings and seals.

The size of the flow control valve (FCV) orifice dictates how much fluid volume is pumped through the steering gear. Additionally, the FCV contains a spring and a check-ball to cut off the maximum pressure the pump will generate, for example, when the driver spikes the pressure at full lock during a parking maneuver. Aiming for a 2.75 gpm flow rate, Hutchinson drilled out the orifice in the FCV to 0.148 inch (#25 drill) on a lathe. The 1,250-psi cutoff pressure was verified on an in-house test machine, and while it can be changed with shims or springs, we left it alone. Pumps are bench-tested on Lee's own test rig, and ours flowed exactly 2.75 gpm as expected.


Lee Manufacturing Co.
Sun Valley, CA 91352
(818) 768-0371




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