When the helical gears are rotating fast, the cam wants to move forward. Henry Ford used a spring and plunger device to push back on the cam. H&H's solution is a small aluminum threaded plunger for the lower drive gear with a trunnion bearing at the hub. The plunger provides resistance to cam thrust, and is held in place by an outer cap built by Max Sr. The nut on the outside of the cap adjusts tension. Turn the nut until you feel resistance, back it off a touch, and tighten down the cap with Allen bolts. Trick piece.