The engine bottom end is the rotating assembly — pistons, connecting rods, bearings, crankshafts, piston rings, sleeves and stroker kits. It is what sets the power ceiling of a build, and it is the part of the engine you only want to open once.
Almost every European performance engine in this catalogue follows the same pattern: the factory bottom end is strong enough for the power it left the showroom with, and becomes the limiting factor the moment boost or RPM goes up. On the BMW S54 and S65 the rod bearings are the known wear item and are worth replacing on schedule regardless of power. On the N54, N55 and B58, and on Audi's EA888 and 2.5 TFSI, forged pistons and rods are the standard step before a larger turbo.
Order to your block's finished bore rather than the stock bore, and check compression height against rod length before committing to a piston set. Bearings are sized — measure, do not assume.
Browse by part: forged pistons, connecting rods, pins and bushings, engine gaskets and seals and ARP rod bolts and main studs. For the other half of the engine see engine top end.