{"product_id":"volvo-1387","title":"Volvo Penta HS1A Clutch Piston O-Ring 977891 | Inboard Transmission Seal","description":"\u003ch2\u003ePart Details\u003c\/h2\u003e\n\u003cul\u003e\n\u003cli\u003e\n\u003cstrong\u003ePart Number:\u003c\/strong\u003e 977891\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eBrand:\u003c\/strong\u003e Volvo Penta\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eType:\u003c\/strong\u003e O-ring — clutch piston seal\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eSystem:\u003c\/strong\u003e HS1A reverse gear \/ inboard transmission — clutch piston assembly\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eDiagram Position:\u003c\/strong\u003e Item 20 in HS1A parts manual — clutch piston seal position (both ratio variants)\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eInstalled With:\u003c\/strong\u003e Piston 872518 \/ 3861194\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eQty Per Assembly:\u003c\/strong\u003e 2 (one per clutch piston)\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eIncluded In:\u003c\/strong\u003e Piston Kit 876589 (HS1A clutch overhaul kit)\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eWeight:\u003c\/strong\u003e Approx. 0.05 kg\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eImportant:\u003c\/strong\u003e Replace as a pair — both piston O-rings should be replaced together during clutch service\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch2\u003eAbout This Part\u003c\/h2\u003e\n\u003cp\u003eGenuine 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 \u003ca href=\"\/products\/8131530686500\"\u003e977890\u003c\/a\u003e, 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.\u003c\/p\u003e\n\n\u003ch2\u003eCompatible Applications\u003c\/h2\u003e\n\u003cul\u003e\n\u003cli\u003eVolvo Penta HS1A — clutch piston O-ring position, item 20 (2 required)\u003c\/li\u003e\n\u003cli\u003eVolvo Penta HS1A 1.96:1 — clutch piston O-ring position (2 required)\u003c\/li\u003e\n\u003cli\u003eVolvo Penta HS1A 2.62:1 — clutch piston O-ring position (2 required)\u003c\/li\u003e\n\u003c\/ul\u003e\n\u003cp\u003e\u003cem\u003eConfirm 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.\u003c\/em\u003e\u003c\/p\u003e\n\n\u003ch2\u003eWhen to Replace\u003c\/h2\u003e\n\u003cp\u003eReplace the piston O-rings during clutch piston service or whenever either seal shows:\u003c\/p\u003e\n\u003cul\u003e\n\u003cli\u003eFlattening or permanent compression set\u003c\/li\u003e\n\u003cli\u003eCuts, nicks, or tearing\u003c\/li\u003e\n\u003cli\u003eHardening or brittleness from age or heat\u003c\/li\u003e\n\u003cli\u003eSwelling or distortion\u003c\/li\u003e\n\u003cli\u003eLoss of hydraulic clutch pressure or clutch engagement issues\u003c\/li\u003e\n\u003c\/ul\u003e\n\u003cp\u003eBoth 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.\u003c\/p\u003e\n\n\u003ch2\u003eInstallation Notes\u003c\/h2\u003e\n\u003cp\u003eInspect 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.\u003c\/p\u003e\n\n\u003ch2\u003eRelated Parts in This Assembly\u003c\/h2\u003e\n\u003cul\u003e\n\u003cli\u003e\n\u003cstrong\u003eClutch piston:\u003c\/strong\u003e 872518 \/ 3861194 — the piston this O-ring seals\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003ePiston Kit:\u003c\/strong\u003e 876589 — HS1A clutch overhaul kit that includes 977891 (2 required)\u003c\/li\u003e\n\u003cli\u003e\u003ca href=\"\/products\/8131530686500\"\u003eVolvo Penta HS1A Reverse Gear O-Ring 977890 — different O-ring in the same transmission (valve\/housing position)\u003c\/a\u003e\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch2\u003eCross-Reference Part Numbers\u003c\/h2\u003e\n\u003cul\u003e\n\u003cli\u003e\n\u003cstrong\u003eOEM:\u003c\/strong\u003e 977891\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eIncluded in Piston Kit:\u003c\/strong\u003e 876589\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eNote:\u003c\/strong\u003e No confirmed Volvo Penta OEM superseding number identified. Do not substitute with 977890 — different position, different sealing function.\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch2\u003eParts Desk Insight\u003c\/h2\u003e\n\u003cp\u003e977891 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 \u003ca href=\"\/products\/8131530686500\"\u003e977890\u003c\/a\u003e. 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.\u003c\/p\u003e","brand":"VOL","offers":[{"title":"Default Title","offer_id":43564069191716,"sku":"977891","price":8.2,"currency_code":"CAD","in_stock":false}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0743\/5799\/7604\/files\/images-2026-09-05T161444.433.jpg?v=1788651529","url":"https:\/\/marisolmarine.com\/products\/volvo-1387","provider":"Marisol Marine Company","version":"1.0","type":"link"}