Abstract: The document describes an assembly of a capsule and brew chamber for a beverage preparation machine. The brew chamber has a first brew chamber part including a cavity for the capsule, and knives for piercing the capsule. The aluminum capsule body is frusto conical shaped and has a bottom, a side wall, a flange and a lid for closing the capsule. The flange is configured to be held between the first and a second brew chamber part, to provide a seal cooperating with a rim of the brew chamber parts. The bottom includes a slanted wall portion under an angle with a longitudinal axis through the capsule, and configured for cooperating with the knives which engage therewith upon closing of the brew chamber. This enables centering of the capsule relative to the central axis of the brew chamber prior to piercing thereof.