| 摘要 |
A cylindrical can having a longitudinal seam presenting higher and lower inwardly projecting portions, and a piston within the can having at least three external ribs. The spacing between each two successive ribs is no greater than the length of one lower seam portion, and the spacing between two alternate ribs is greater than the length of one higher seam portion. The ribs may extend around the entire periphery of the piston or only a portion thereof. The top and side walls of the piston may be provided with inwardly projecting ridges.
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