Mistake-proof stem selection - set includes five wheels and 10 stem fasteners (5 each: 3/8"D x 1"L and 7/16"D x 7/8"L) for all metal-base chairs
Full hood design - 1-1/2"D oversizedneck, weight rated 100 lb./wheel
Determine correct stem diameter by removing a caster from the leg of your chair base. Match diameter of existing stem and insert into caster body- Discard the other five fasteners
Comments about Office Star High-Back Multi Function Ergonomic Task Chair, Black:
I'd tried the Staples Vocazo and other chairs at a similar price point, before finally settling on this Office Star model. The price is a bit of a leap but so too is the comfort. Here is a truly ergonomic chair, with independent, locking back and seat tilt, and a unique "ratchet back" design that permits vertical adjustment of the entire chair back. The back and seat padding are medium-density foam, and the above-the-shoulder-blades-high back is somewhat cradled along the vertical axis, lending very nice support. The arms are sufficiently low (with ~5 inches of vertical adjustment possible) to allow for a comfortable position at the keyboard, and the armrests are amply padded and ergonomic.
The only complaints I can raise are that the chair didn't come with assembly instructions (I found them on Office Star's website; the chair shipped directly from the mfr rather than from Staples), and that assembly - while straightforward - requires a phillip's head screwdriver (not included), which is very difficult to torque adequately when setting these screws. Allen screws throughout would have made the job much easier.
Bottom line - This chair is a computer user's dream come true, infinitely adjustable to match the user to whatever height and posture a particular environment requires.