Cervelo's integrated cockpit design on the Caledonia-5, Aspero-5, R5, and R5-CX looks clean from a distance, but up close there's still a hex bolt recess in the stem top cap where the headset preload screw sits. The PL-338 is the small rubber plug that fills that hole, completing the finished appearance and keeping water and road grit from pooling in the bolt head. If yours popped out during a stem adjustment or went missing somewhere along the way, this is the replacement.

The plug press-fits into the top cap bolt recess on both the ST31 carbon stem and ST32 alloy stem—Cervelo's proprietary stems designed for their D-shaped steerer tube. Installation takes about two seconds: push the plug into the hex recess until it seats flush with the top cap surface. Removal for headset adjustment is equally simple—pry it out with a fingernail or small flathead, access the bolt, make your adjustment, and press the plug back in when finished.

This is a cosmetic and protective part, not a structural component. The headset functions identically whether the plug is installed or not. What you're getting is the finished look Cervelo intended for these integrated cockpits, plus a barrier that prevents water from sitting in the bolt recess and accelerating corrosion on the preload screw. For riders who detail their bikes regularly or simply appreciate complete aesthetics, these small finishing touches matter. The plug is specific to the 338-series top cap geometry used on the ST31 and ST32 stems—other Cervelo stems use different plug configurations.