Office Chair Sinking? How to Fix It With a Replacement Gas Lift Cylinder (UK Guide)

An office chair that slowly sinks while you're sitting in it is almost always down to one part: the gas lift cylinder. It's a cheap, straightforward fix once you know what to look for — no need to replace the whole chair. This guide covers why it happens and how to choose the right replacement cylinder.

Why chairs sink

The gas lift cylinder is a sealed gas strut that supports your weight and lets the chair height adjust with the lever. Over years of daily use, the internal seals wear and the gas charge gradually leaks past them — the cylinder slowly loses its ability to hold position under load, so the seat sinks, often faster the longer you sit in it. This is normal wear and tear on a part that's designed to be replaced, not a fault with the rest of the chair.

How to check it's the cylinder (and not something else)

  • The seat sinks gradually while seated, but holds its height fine when empty — classic worn cylinder.
  • The seat drops suddenly or wobbles side to side — check the seat plate bolts and mechanism first, as this can be a looseness issue rather than the cylinder.
  • The height lever doesn't do anything at all — the lever linkage or the cylinder's valve pin may be the issue rather than gas loss.

If the chair sinks slowly under your weight but the rest of the chair feels solid, it's almost always the cylinder.

What to measure before ordering a replacement

  • Overall length: most standard office chair cylinders fall around 260–320mm, but measure yours rather than assuming.
  • Top stem diameter: where the cylinder connects into the seat mechanism, typically 28mm — this needs to match for the new cylinder to seat correctly.
  • Base diameter: where the cylinder meets the wheelbase, standard sizing across the vast majority of office chairs.
  • Class rating: look for "Class 3" or "Class 4" printed on the cylinder body. Class 4 is BIFMA/TÜV safety-tested for typical office use and is the rating we'd recommend for daily use — avoid uncertified cylinders regardless of price.

Fitting a replacement

  1. Tip the chair on its side and, using a rubber mallet or firm pressure, knock/pull the wheelbase off the bottom of the old cylinder.
  2. Remove the seat mechanism from the top of the old cylinder the same way — it's a friction fit, no tools or bolts involved on most chairs.
  3. Fit the new cylinder into the wheelbase, then press the seat mechanism onto the top stem until it seats fully.
  4. Sit down carefully and test the height adjustment lever before returning the chair to normal use.

While you're in there — check the tilt mechanism too

If your chair also feels loose, doesn't hold its recline angle, or rocks unevenly, that's a separate part — the tilt/recline mechanism under the seat — rather than the gas lift cylinder. It's worth checking both at the same time if the chair has been in daily use for several years, since they wear on a similar timescale.

All SpringFix UK gas lift cylinders are UK stock, dispatched the same day for orders placed Monday to Sunday before 16:00, via Royal Mail or Evri. If you're not sure which size your chair needs, send us the measurements or a photo and we'll help you find the right match.

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