Parts of a Rack and Pinion – Labeled Steering Gear

All 15 parts on the diagram

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The gear 4

Rack housing The rack slides inside it. On a power gear part of the tube is the cylinder for the power piston.
Steering shaft It turns with the steering wheel. The pinion is on its lower end, inside the housing.
Flexible coupling It joins the steering shaft to the gear's own short input shaft.
Fluid lines They carry power steering fluid from the control valve. It goes to one side of the power piston or the other.

At each end 6

Bellows (boots) It seals the housing's open end round the inner tie rod.
Bellows clamps A large clamp holds the bellows on the housing. A small one holds it on the tie rod.
Inner tie rod Its inner end is a ball socket on the rack's end. The bellows hide it. It threads into the outer tie rod end.
Jam nut It locks the outer tie rod end on the threads. Loosened, the tie rod can be turned to set the wheel's toe.
Outer tie rod end A ball and socket joint. Its stud fastens to the steering arm at the wheel, so the rack's sideways movement turns the wheel.
Breather tube It runs between the two bellows.

Inside the housing 5

Pinion Its teeth mesh with the rack's. When the steering wheel turns, the pinion turns and pushes the rack sideways.
Rack The pinion moves it left and right. A tie rod on each end passes that movement to the wheels.
Control valve Turning the steering wheel moves it. It sends fluid under pressure to the side of the power piston that helps the turn.
Torsion bar It twists a little when you turn the wheel. The amount depends on how hard you turn. That twist is what works the valve.
Power piston It slides in a bore in the housing. Fluid pushed against one side or the other helps move the rack.

How this list was compiled: Part names checked on 2026-09-30; a name is kept only when two independent sources use it. The two books were read as the passages a full-text search returns. (1) US Army TM 9-8000, Principles of Automotive Vehicles, 1985, paragraphs 33-3c, 33-8 and 33-10, https://archive.org/details/tm-9-8000-principles-of-automotive-vehicles-1985 (steering shaft, flexible coupling, pinion, rack, rack housing, breather tube, tie rod and tie rod end, ball socket joint, control valve, power piston). (2) Birch, Automotive Suspension and Steering Systems, https://archive.org/details/automotivesuspen0000birc (pinion gear, rack housing, inner tie rod, outer tie rod end, jam nut, bellows and bellows clamps, flexible coupling, control valve, torsion bar, the piston in a bore in the rack housing, steel tubing). (3) Motor Domestic Front End and Brake Service, 1987-91, https://archive.org/details/motordomesticfro0000unse (tie rod bellows, bellows clamp, rack boot, breather tube, jam nut, inner tie rod, tie rod end, pressure and return lines). (4) Wikipedia, text only: Steering (rack and pinion, tie rods, steering arms); Power steering (torsion bar, valves, hydraulic cylinder and piston). Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry; no maker's gear, nothing traced. Licence: own work

How it works

In rack and pinion steering a small gear on the end of the steering shaft, the pinion, slides a toothed bar, the rack, from side to side. Tie rods pass that on to the front wheels.

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From the shaft out to the wheels

In the whole view, follow the steering shaft down through the flexible coupling to the gear, then along the housing to either end: bellows, inner tie rod, jam nut, outer tie rod end. The cut view shows what the housing hides. The pinion sits on the rack's teeth. Above it are the control valve and torsion bar, and farther along the rack is the power piston.

Both ends are the same, so an end part can be tapped on either side.
A manual gear has no control valve, power piston or fluid lines; the rest is alike.

Questions and answers

What are the parts of a rack and pinion?

The pinion, the rack and the housing they work in; a bellows, inner tie rod, jam nut and outer tie rod end at each end; and on a power gear a control valve with its torsion bar, a power piston and fluid lines.

How does rack and pinion steering work?

Turning the steering wheel turns the pinion. Its teeth mesh with the rack and push it left or right. The tie rods on the ends of the rack then swing the steering arms at the wheels.

What is the difference between inner and outer tie rods?

The inner tie rod joins the end of the rack by a ball socket under the bellows. The outer tie rod end is the ball joint at the wheel. The inner rod threads into the outer end, and a jam nut locks the two.

What do the bellows on a steering rack do?

Each bellows seals an open end of the rack housing round the inner tie rod. A clamp holds each of its ends, one on the housing and one on the tie rod.