The document describes an assembly of a capsule (5) and brew chamber (3) for a beverage preparation machine. The brew chamber (3) has a first brew chamber part (7) including a cavity (10) for the capsule (5), and knives (11) for piercing the capsule (5). The aluminum capsule body (15) is frusto conical shaped and has a bottom (20), a side wall (21), a flange (22) and a lid (17) for closing the capsule (5). The flange (22) is configured to be held between the first and a second brew chamber parts (7, 8), to provide a seal (30) cooperating with a rim of the brew chamber parts (7, 8). The bottom (20) includes a slanted wall portion (35) under an angle with a longitudinal axis through the capsule (5), and configured for cooperating with the knives (11) which engage therewith upon closing of the brew chamber (3). This enables centering of the capsule (5) relative to the central axis of the brew chamber (3) prior to piercing of the capsule.