Owen Bradley Something on your chair has stopped working and you need to order a replacement, but the product listings all use words you have never had a reason to learn: stem diameter, tilt housing, synchro mechanism, caster grade, seat plate. Without the vocabulary it is genuinely difficult to search for the right part, and ordering the wrong one is easy. This guide names every component of a typical office chair from the floor upwards, explains what each part does, describes how each one fails, and tells you which measurements matter when you order a replacement. By the end you will be able to look under any chair and describe exactly what you are seeing.
The Anatomy in Brief
Working from the floor up, a standard task chair consists of casters, a base, a gas cylinder, a mechanism housing, a seat plate and seat pan, foam and upholstery, a backrest with lumbar support, a back support arm or hinge, and armrests. Everything else is a variation on those parts. Executive and drafting chairs add components such as footrings and headrests, and mesh chairs replace foam and upholstery with a tensioned suspension deck, but the skeleton is identical.
The useful mental model is that the chair splits into two halves. Everything below the mechanism carries load and moves the whole chair, and those parts are highly standardised, meaning replacements are cheap and widely interchangeable. Everything above the mechanism shapes and supports your body, and those parts are model-specific, meaning replacements often have to come from the original manufacturer. That distinction determines whether a repair is a ten-minute job or a reason to consider a new chair, and it is the reason browsing the best office chairs is sometimes the better answer to a broken upper assembly.
Casters
Casters are the five wheels at the bottom. Each has a stem that pushes into a socket in the base, and the overwhelming majority use an 11mm diameter grip-ring stem, which is why aftermarket casters fit almost any chair. Hard nylon casters suit carpet, while soft polyurethane or rubber-tyred casters suit hard floors and protect laminate and timber from scuffing.
Casters fail by seizing when hair and fibre wrap the axle, by cracking, or by the grip ring losing tension so the wheel drops out. All three are trivial to fix: pull the old caster straight out with a firm tug or a lever, push the new one in until it clicks. Before ordering, measure the stem diameter and length, and check whether your base uses a grip ring or a threaded stem, since a small minority of chairs use the latter.
Choosing the Right Wheel for Your Floor
Hard wheels on hard floors roll fast but scratch and rumble. Soft wheels on carpet feel sluggish and can flat-spot. Braked casters, which lock when the chair is unweighted, help on sloped or slippery floors. If you are switching floor types, a caster swap is by far the cheapest chair upgrade available and takes under five minutes.
The Base
The base is the five-armed star that spreads the load and provides stability. Nylon bases are the cheapest and are perfectly adequate for lighter users, though they can crack under sustained heavy load or if the chair is used as a step stool. Glass-reinforced nylon adds strength. Aluminium bases, either painted or polished, are the strongest and appear on contract-grade chairs.
Bases are measured across the diameter, from caster tip to opposite caster tip, and common sizes run roughly from 600mm to 700mm. A wider base is more stable but occupies more floor and is more likely to catch on desk legs. The taper socket at the centre that accepts the gas cylinder is a standardised cone, so bases are largely interchangeable across brands as long as the diameter and caster stem size match.
The Gas Cylinder
The gas cylinder, also called the gas lift or pneumatic lift, raises and lowers the seat. It contains compressed nitrogen and a valve released by a pin at the top of the cylinder, which the height lever presses. Cylinders carry a safety class rating, and a higher class indicates more rigorous testing for cycle life and burst resistance.
The classic failure is a chair that slowly sinks under your weight, meaning the internal seal has failed. This is not repairable and the cylinder must be replaced. When ordering, three numbers matter: the taper diameter at the base end, the stroke length, and the overall extended length. Stroke length is what determines your height range, so a longer-stroke cylinder is the standard fix if your chair simply will not rise high enough for your desk. Chairs with generous stroke ranges are common among the best ergonomic office chairs, which is one reason they suit a wider range of body heights.
Removing a Stuck Cylinder
Cylinders seat into their tapers by friction and can become extremely tight over years. The usual method is to protect the base, grip the cylinder body with a pipe wrench and twist while pulling. Never attempt to cut, drill or heat a cylinder, since it is a pressurised vessel and puncturing it is genuinely dangerous.
The Mechanism Housing
The mechanism is the metal box between the cylinder and the seat, and it is the most complex and expensive component. It contains the tilt pivot, the tension spring, the locking positions and the levers. The simplest type is a knee-tilt or centre-tilt mechanism that rocks the whole seat and back together. A synchro-tilt mechanism reclines the back further than the seat, typically in a two-to-one ratio, keeping your thighs supported as you lean.
Mechanisms fail by developing play at the pivot bushings, by the tension spring losing force, or by the locking pawl wearing so the tilt no longer holds position. Squeaks almost always originate here, usually from a dry pivot or a loose bolt rather than anything broken. Because mechanisms are model-specific in their bolt pattern, replacement usually means sourcing from the original manufacturer. The step-by-step diagnosis in fix a squeaky or wobbly office chair covers exactly which bolts and pivots to check first.
Levers and Paddles
The levers protruding from the mechanism control height, tilt lock and tilt tension. The tension control is usually a knob or crank underneath that compresses the spring, and it should be set so the back pushes against you with roughly your own weight of resistance. A tilt tension that is too loose lets you fall backwards suddenly, which is the most common complaint mistaken for a broken mechanism.
Seat Plate, Seat Pan and Cushion
Above the mechanism sits the seat plate, a metal or moulded panel that bolts to the mechanism and carries the seat pan. The seat pan is the rigid shell, usually plywood or moulded polymer, that the foam is bonded to. Over the foam goes the upholstery, stapled or clipped around the underside. On chairs with seat depth adjustment, a slider assembly sits between the plate and the pan.
Foam failure shows as a permanent hollow that no longer springs back, and it is repairable if you are willing to remove the staples, replace the foam block and re-staple the cover. Cracked seat pans, common on cheaper plywood shells, are effectively terminal because the crack propagates under load. Check the pan before investing in new foam or upholstery.
Backrest, Lumbar and Back Support
The backrest attaches either directly to the seat via a fixed bracket, or through a back support arm, sometimes called a J-bar or S-bar, which curves up from the mechanism. Height-adjustable backs use a ratchet or a locking collar on that arm. Lumbar support may be built into the back shape, provided as a separate pad on vertical rails, or delivered by an adjustable tension band behind a mesh panel.
On mesh backs, the tensioned panel is bonded or clipped into a polymer frame. Frame cracks at the mounting points are the usual failure and cannot be reliably glued. On upholstered backs, the same foam and pan logic applies as for the seat. Headrests, where fitted, mount to the top of the back frame and are usually the easiest upper component to source separately, which is why they appear so often as an option on the best executive chairs.
Armrests
Armrests are the most varied component. Fixed loop arms are a single moulded piece bolted to the seat plate. Height-adjustable arms add a vertical post with a button-release ratchet. Multi-directional arms, described as 3D or 4D, add width adjustment at the base, depth sliding on the pad, and pivot rotation of the pad.
The parts that fail are the pads, which wear through or go sticky as the polyurethane degrades, and the ratchet mechanisms, which lose their detents so the arm sinks. Pads are frequently sold separately and clip or screw on from underneath. Full arm assemblies are model-specific, so measure the bolt pattern on the seat plate before ordering. If you are choosing rather than repairing, the adjustment range on arms is one of the clearest quality markers across the best desk chairs currently available.
Which Parts Are Worth Replacing
- Casters: always worth it, cheap and universal.
- Gas cylinder: almost always worth it, standardised and inexpensive relative to a new chair.
- Base: worth it if the rest of the chair is sound, and largely interchangeable.
- Armrest pads: worth it, usually a simple unscrew and clip.
- Mechanism: worth it only under warranty or on a premium chair, since it is model-specific and costly.
- Seat foam and upholstery: worth it on a good frame, provided the seat pan is not cracked.
- Cracked mesh or back frame: rarely worth it outside warranty, as the part is proprietary and load-bearing.
Frequently Asked Questions
Are office chair parts interchangeable between brands?
Below the mechanism, largely yes. Casters, cylinders and bases follow common standards. Above the mechanism, generally no, because seat plates, back frames and arm mounts use proprietary bolt patterns.
Why does my chair keep sinking?
The gas cylinder seal has failed. There is no repair, but replacement cylinders are inexpensive and fit most chairs once you match the taper, stroke and extended length.
What causes a squeak?
Nearly always the mechanism pivots or a loose fixing. Tighten every bolt you can reach, then apply a light lubricant to the tilt pivot points and the spring contact surfaces before assuming anything is broken.
How do I measure a gas cylinder for replacement?
Record the diameter of the taper that enters the base, the overall length when fully collapsed, and the stroke, which is the difference between collapsed and extended length. Those three figures identify almost any standard cylinder.
Can I fit a longer cylinder to sit higher?
Yes, provided the taper matches. A longer-stroke cylinder is the standard solution for tall users or high desks, and it fits the existing base and mechanism without modification.
Final Thoughts
Knowing the names of office chair parts turns a vague problem into a specific, searchable one. Once you can say the gas cylinder has lost its seal, or the tilt housing pivot is dry, or the caster stem is 11mm, ordering the right replacement becomes straightforward and often surprisingly cheap. Remember the dividing line: everything below the mechanism is standardised and repairable, everything above it is model-specific and worth checking against warranty first. Approach repairs with that map in mind in 2026 and you will keep a good chair working for years rather than replacing an entire assembly over one failed component.
