The gas cylinder is the quiet workhorse of your office chair — the part that lets you float up and down with the touch of a lever and then holds you steady. It's also the part most likely to fail over a chair's life. Understanding its actual dimensions, what the class numbers mean, and why a sinking chair can't be repaired will save you money and stop you buying the wrong replacement.
What the gas cylinder is
The gas cylinder — also sold as a gas lift, gas spring, or pneumatic rod — is the telescopic steel assembly between your seat mechanism and the five-star base. The plastic sleeve you see around it is purely cosmetic; the working part is the sealed steel canister inside. It does two jobs: it lets you adjust seat height, and it holds your weight at whatever height you set.
How it works
Inside the canister is compressed nitrogen and a piston. Lifting the height lever presses a valve pin (typically a 10 mm diameter pin seated in a retaining clip in the mechanism's centre hub), which opens a passage between the chambers on either side of the piston. With your weight on the seat, the gas transfers and the chair descends; unweighted, the pressure pushes it up. Release the lever, the valve closes, and the trapped gas locks the piston in place.
Why a sinking chair can't be "topped up"
Sinking is caused by failure of the internal piston seal or bypass valve, letting pressurised nitrogen migrate between chambers under load. This is a sealed, non-repairable failure — there is no valve to refill and no seal you can replace. Replacement is the only fix.
The dimensions that actually matter
For roughly the last two decades, office chair cylinders have been highly standardised. Three measurements determine whether a replacement will fit:
| Measurement | Typical standard | Notes |
|---|---|---|
| Outer tube diameter | 50 mm (≈2 in) | Tolerances usually 48–50.8 mm to suit different base hubs |
| Inner tube (piston rod) | 28 mm | The polished shaft that extends |
| Outer tube length | 160–240 mm | Measure the black body, not the shiny rod |
| Stroke (travel) | 80–150 mm | Determines your height range; bar stools run ~260 mm |
| Valve pin | 10 mm | Engages the lever mechanism |
Measure the black outer canister, never the silver rod. The rod's exposed length varies with the load on it, so measuring it gives you a number that changes depending on whether anyone is sitting in the chair.
Cylinder classes decoded
Cylinders are graded Class 1 to Class 4, and the grading comes down to one thing: wall thickness of the steel tubing. Thicker walls mean higher load capacity and longer service life.
| Class | Outer tube wall | Inner tube wall | Typical load rating | Common use |
|---|---|---|---|---|
| Class 3 | ~1.5 mm | ~2.3 mm | 100–150 kg | Mainstream office chairs |
| Class 4 | ~1.8–2.0 mm | ~2.5 mm | 150–250 kg | Heavy-duty, gaming, premium |
Class 3 is the mainstream choice in most office chairs sold today. Class 4 uses thicker seamless steel and frequently a QPQ (quench–polish–quench) surface treatment, which hardens the surface against wear and gives the cylinder its characteristic black finish rather than bright plating. In well-built Class 4 designs, service life can exceed ten years.
Certification marks worth looking for
Check the cylinder body for stamped markings. Look for BIFMA X5.1 (the North American office seating standard), EN 1335-2 (the European office chair standard), or third-party marks such as TÜV or SGS. Their presence generally indicates a cylinder built to at least Class 3 standards. An unmarked cylinder tells you nothing reassuring.
A construction detail that reveals quality
The bottom cover is made three ways, and it's a quick quality tell. Welded bottoms fully seal the canister. Curled (crimped) bottoms look neat but are only possible on tubing 1.2 mm thick or less — meaning a crimped bottom indicates Class 3 or lower, since thicker Class 4 walls can't be crimped. Double-cone bodies are stepped, narrower at the base so the cylinder seats deeper into the five-star base for extra stability.
The compatibility trap
Roughly 90% of office chairs use the universal 50 mm body with a tapered friction fit, so a generic replacement usually works. But several major brands — IKEA and Herman Miller among them — use proprietary diameters or cable-actuated valve systems that are not interchangeable with universal parts. Before ordering, measure your existing cylinder and check whether your chair's manufacturer uses a standard fitting. This is the single most common reason a replacement cylinder arrives and won't fit.
Choosing a replacement
- Match the outer diameter — 50 mm for the vast majority of chairs.
- Measure outer tube length and stroke to keep (or deliberately change) your seat height range.
- Choose Class 4 over Class 3 if you're near the upper end of your chair's weight capacity, adjust the height frequently, or simply want longer life — the price difference is usually small.
- Look for stamped certification rather than an unmarked bargain cylinder.
Spotting a cylinder problem early
The first sign is a slow sink: you set your height and drift downward over several minutes, needing to pump the lever again. Initially it may only happen under full load or after a while, then more readily as the seal degrades. Other clues include the seat rising sluggishly when unweighted, or an audible hiss during adjustment. None of these is dangerous — the cylinder is safe to keep using — but they tell you a replacement is due, so you can order the part before the sinking becomes constant.
A note on safety
The canister holds compressed nitrogen. It is entirely safe in normal use and when removed or refitted as a whole unit, but you must never puncture, drill, saw, open, or heat one. Treat it as a sealed component, and dispose of an old one through appropriate scrap-metal or recycling channels rather than tampering with it. This is covered step by step in our replacement guide.
How the cylinder fits with other parts
The cylinder seats into the five-star base below and the tilt mechanism above using tapered friction joints — a Morse-style taper wedged tight by your body weight rather than held by bolts. That's why no fasteners are involved, why the chair holds firmly, and why a well-seated old cylinder can be stubborn to remove.
Conclusion
The gas cylinder makes your chair height-adjustable using compressed nitrogen and a sealed piston. Its class reflects steel wall thickness — Class 3 handles 100–150 kg, Class 4 handles 150–250 kg with a longer service life. Nearly all office chairs use a 50 mm outer tube with a 28 mm rod, but measure before you buy and watch for proprietary brands. And when a cylinder starts sinking, remember it's a sealed failure: replacement is the only genuine fix, and at the price of a cylinder versus a chair, it's an easy decision.