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This blog is a quick guide on how to effectively remove and reinstall the knobs that are used in a multitude of Cessna aircraft up to the Caravan series. The knob is used to control 2 different circuits and typically has an outer black plastic knob with an aluminum center knob. I have fought with this set up many times and eventually have won. You don’t want to destroy the knobs, as they are relatively expensive. The only tool you will need is a .035 Allen wrench. These are usually not included in a typical Allen wrench set but they can be found online and at some specialty tool stores. Without the .035 Allen wrench don’t even try this task. If you totally destroy the set screw you may have to replace your rheostat stack and this could increase labor times significantly.
The way this arrangement works is the center brass shaft of the rheostat stack is threaded all the way through and this is where the .035 set screw lives. The outer and inner knobs have a hole on one side to allow access with your Allen wrench. The center knob is secured by the set screw and captures the outer knob and holds it in place. The set screw is pointed and drives into the inside of the center aluminum knob. Remember this is not a lug nut. It only needs to be tight enough to hold the center knob in place.
To remove the knob stack you have to align the two holes in both knobs so you can see the set screw. Use your .035 Allen wrench and completely remove the set screw. Once the set screw is out both knobs will come right off. You may have to find a “sweet spot” where you can easily insert the Allen wrench. Remember the sweet spot because this will make reinstalling much easier.
To reinstall the knobs
1: Install the set screw into the brass shaft. The trick here is to make sure the set screw does not protrude through either side of the shaft. I use the center knob to make sure the set screw is positioned correctly. The knob should slide on and off easily. If it doesn’t adjust the set screw in or out as required.
2: Install the outer knob and then the center knob making sure both holes are lined up and you can see the end of the set screw.
3: Tighten set screw as required.
4: Go fly.