This is because each engine has different specifications when it comes to bolt size

this BMW N55 piston set features:

5150 AutoSport Pro-Xtreme Forged Pistons for BMW S58 Engines

part numbers 11217616597 and 11217616596

50mm Exhaust   Rally / Race  322° Intake / 314° Exhaust   12

BMW M50 M52 S50 S52 Connecting Rod Set, Manley H-Beam Fury x This is because each engineManley H Beam Connecting Rods for BMW M50, M52, S50, and S52 Engines Available at 5150 AutoSport The Manley Performance H Beam Connecting Rod Set is engineered for exceptional strength and reliability, designed specifically for high performance applications in BMW M50, M52, S50, and S52 engines. Crafted from 4340 forged steel, these rods offer a lightweight yet durable solution for engines running forced induction, high RPM builds, or demanding track