Hitachi 998428 — O‑Ring (C), genuine Hitachi replacement for demolition hammers. This small rubber sealing ring fits between the damper and front cover to restore proper air/oil sealing and smooth hammer action; the parts diagram lists a single O‑ring (part 998428) per tool.

Compatible examples: H65SB / H65SB2 / H65SB3, the H65SD /H65SD2 /  H65SD3 and H70SA / H70SD models all show part 998428 (O‑RING (C)) in their exploded parts lists.

Details you should know: genuine Hitachi part number 998428 is sold individually and is made of a flexible rubber compound sized to the tool’s factory sealing groove; it’s the exact replacement shown in the Hitachi / Hikoki / Metabo HPT / HiKoki parts diagrams. If your hammer shows leaks or reduced impact performance around the front cover/damper area, replacing this O‑ring restores the original sealing that keeps the tool operating correctly.