Volvo Penta HS1A Clutch Piston O-Ring 977891 | Inboard Transmission Seal
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Part Details
- Part Number: 977891
- Brand: Volvo Penta
- Type: O-ring — clutch piston seal
- System: HS1A reverse gear / inboard transmission — clutch piston assembly
- Diagram Position: Item 20 in HS1A parts manual — clutch piston seal position (both ratio variants)
- Installed With: Piston 872518 / 3861194
- Qty Per Assembly: 2 (one per clutch piston)
- Included In: Piston Kit 876589 (HS1A clutch overhaul kit)
- Weight: Approx. 0.05 kg
- Important: Replace as a pair — both piston O-rings should be replaced together during clutch service
About This Part
Genuine Volvo Penta 977891 is an OEM O-ring used on the clutch piston assembly inside the Volvo Penta HS1A marine reverse gear and inboard transmission. Volvo Penta parts manuals list 977891 at item 20 in the HS1A transmission, with two O-rings required per transmission — one per clutch piston. The HS1A contains several different O-rings and sealing rings in close proximity; 977891 is specifically the clutch piston seal and should not be confused with 977890, which occupies a different internal position and performs a different sealing function. For complete clutch overhaul work, Volvo Penta supplies these O-rings as part of Piston Kit 876589.
Compatible Applications
- Volvo Penta HS1A — clutch piston O-ring position, item 20 (2 required)
- Volvo Penta HS1A 1.96:1 — clutch piston O-ring position (2 required)
- Volvo Penta HS1A 2.62:1 — clutch piston O-ring position (2 required)
Confirm transmission model, gear ratio, and exact diagram position before ordering. Several different O-rings and sealing rings are used within the HS1A — position matching from the exploded parts diagram is essential.
When to Replace
Replace the piston O-rings during clutch piston service or whenever either seal shows:
- Flattening or permanent compression set
- Cuts, nicks, or tearing
- Hardening or brittleness from age or heat
- Swelling or distortion
- Loss of hydraulic clutch pressure or clutch engagement issues
Both O-rings should be replaced together as a set — do not replace only one piston seal. For a complete HS1A clutch overhaul, Piston Kit 876589 includes these O-rings along with the other clutch components required for a full rebuild.
Installation Notes
Inspect both clutch pistons, piston groove surfaces, clutch discs, and surrounding hydraulic components during service. Lubricate the new O-rings with clean transmission fluid and install them according to the applicable Volvo Penta HS1A workshop procedure. Take care not to twist or nick the O-rings during piston installation. Confirm correct clutch function and hydraulic engagement after reassembly before returning the transmission to service.
Related Parts in This Assembly
- Clutch piston: 872518 / 3861194 — the piston this O-ring seals
- Piston Kit: 876589 — HS1A clutch overhaul kit that includes 977891 (2 required)
- Volvo Penta HS1A Reverse Gear O-Ring 977890 — different O-ring in the same transmission (valve/housing position)
Cross-Reference Part Numbers
- OEM: 977891
- Included in Piston Kit: 876589
- Note: No confirmed Volvo Penta OEM superseding number identified. Do not substitute with 977890 — different position, different sealing function.
Parts Desk Insight
977891 is the clutch piston seal in the HS1A, and two are required per transmission. The most common issue is ordering a single O-ring when both piston seals need replacement, or confusing 977891 with the nearby 977890. Both are small HS1A O-rings that appear similar, but they are not interchangeable — they seal different components at different positions in the transmission. If a full HS1A clutch overhaul is being performed, ordering Piston Kit 876589 rather than individual O-rings ensures the complete set of clutch components is replaced together.
Shipping information
In-stock and OEM parts: ship same or next business day.
Supplier-stock items are usually ready for pickup or shipping within 24 hours of ordering.
Special-order timing may vary.
Returns & warranty
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