Gas cylinder mechanisms are often installed with the pivot pin about 60% back from the front seat edge. After installing, sit in a normal user position and adjust the height using the height adjustment lever. Correct height is usually when thighs are parallel to the floor or if prefered, an inch or two higher.
Both Manual and Gas installation problem solving:
Problem: Chair "falls" back too easily: The Pivot Pin has been placed too far "forward". Remove the mechanism and re-mount further rearward. Usually only 2" of movement is necessary. Also, check to see if the spring tension adjustment wheel is at mid-range.
Problem: Too much "push" is required to tilt back comfortably: The Pivot Pin is too far "rearward". Remove the mechanism and re-mount further forward. Usually only 2" of movement is necessary. Also, see if the spring tension adjustment wheel is at mid-range.
Either type mechanism
Most mechanism existing screw holes may not fit replacement mechanism holing. See Mechanism-Positioning and Pivot point locating.
The mechanism Pivot (point) Pin location, shown on Pivot pin, determines the proper balance of any chair whether gas or manual. Virtually every chair manufacturer has determined a different pivot pin and mechanism placement depending upon which mechanism was used and the weight and size of the chair back.
Your replacement mechanism pivot pin placement is common to many but not all Office swivel chairs. You may find after installation and testing, that your chair is too easy or too difficult to tilt back properly. The "fix" is not difficult. See "Problem:" above.
The replacement mechanism pivot pin location should be in the same general area of the original, that is, assuming that the original was comfortable for you. See Retrofitting/adapting.
After installation is completed, caution should be exercised when first "trying or testing the chair". Incorrect installation may "dump" occupant onto floor and cause bodily injury. Proceed SLOWLY to test, holding firmly on to a solid object to prevent any harm in case of sudden movement.
Before starting, insure that the mechanism has not been installed backwards. The "hand wheel" (handle or knob) on the mechanism should face the front of the chair, making it convenient (more or less) to adjust the spring tension while sitting upright in the chair.
Manual mechanism, adjust the chair to a normal comfortable height by turning the Bell either clockwise or counter clockwise. This is easiest accomplished if the chair is pushed over onto it's front edge of the seat cushion.