This question refers to the device which transfers the thrust force from the engine to the vehicle while allowing the engine to swivel - the "gimbal joint".
Both the F-1 and SSME have a bolt- or shaft-like device inserted (or able to be inserted) into the side of the gimbal joint. Obviously these engines share a common design heritage. (Curiously, the "male" and "female" sides of the joint are inverted between the two engines. In both cases the "shaft" goes into the "male" side.)
Is it used to lock the gimbal? To expand the mating parts so they fit properly? Neither of these "feels" right.
Answers with solid references only please!
F-1 (from "The Saturn V F-1 Engine" by Anthony Young):
SSME (from Rocketdyne SSME Pocket Data Book)