Riflespeed Spare Control Knob RS87 3.660in. These install in place of the current Control Knob on your Gas Control Assembly. These are the same Control Knobs that are included with each Gas Control assembly. If you want to switch do a different length Gas Control Assembly you can swap one of these with your current Control Knob. These Control Knobs are ONLY for the Gas Controls. Specifications for Riflespeed RS87 Spare Control Knob: Manufacturer: Riflespeed Color: Black Coating: QPQ Fabric/Material: Steel Additional Features: RS87 Condition: New Finish: Nitride Package Contents: Riflespeed RS87 Spare Control Knob