What is the purpose of a cross head?

• A. To convert the up-and-down motion of the connecting rod into pure reciprocating motion.
• B. To seal the piston in the cylinder.
• C. To cool the combustion chamber.
• D. To measure gas temperature.

Answer: (A)

The crosshead guides and connects the piston rod to the rest of the linkage so that the piston's motion stays a straight, reciprocating stroke along the cylinder axis. By sliding in guides and allowing the connecting rod to pivot only at a joint, it prevents angular motion from the crank from forcing the piston rod to move off line. This arrangement converts the connecting-rod motion into a clean linear motion that drives the crankshaft smoothly, while also reducing side thrust and wear on the cylinder walls. It doesn’t seal the piston, cool the combustion chamber, or measure gas temperature—those functions belong to pistons seals, cooling systems, and temperature sensors, respectively.