The cylinders on the CO₂ motor shown are threaded into the crankcase. Motor speed is normally adjusted by rotating the cylinders in the crankcase and is then fixed by tightening the thin blue knurled lockrings. The settings on both cylinders have to be adjusted to match.

Mario loosened the lockrings so that the cylinders can be rotated without too much effort. He then CA'ed a thin rod sticking straight up between the cooling fins of each cylinder.

A small block of balsa is glued to the base of the crankcase and serves as a base for the center pivot rod. The crosswise piece is made from aluminum and the slots have to be just large enough to not bind against the rods, but small enough not to be sloppy. You have to work carefully when you make it. It rests on a small thrust bearing rivet epoxied to the rod on the crankcase.

The cross-piece is driven by the Hitec HS-50 servo and as it rotates about the central rod it changes the cylinders' settings to control the RPM over a surprisingly large range. The RPM change is very repeatable and linear.