Mercury MerCruiser Quicksilver Water Tube O-Ring 8M0204645
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Part Details
- Part Number: 8M0204645
- Service Format: 25-8M0204645
- Brand: Mercury / Quicksilver
- Type: O-ring — water tube / water-pump coupling seal
- System: Cooling — lower unit water tube and water-pump coupling assembly
- Dimensions: Approx. 0.796 in. ID × 0.139 in. cross-section
- GTIN: 745061130579
- Replaces: 21836 (historical cross-references: 20679, F53795, F53795-1)
- Qty Per Assembly: 1 or 2 depending on application — confirm by diagram position
- Important: Mercury uses several similarly sized O-rings in the same lower unit. Do not select by diameter alone — confirm the exact diagram position and part number.
About This Part
Genuine Mercury / Quicksilver 8M0204645 is an OEM O-ring used at the water tube, water tube extension, and water-pump coupling area of selected Mercury outboard and MerCruiser gearcase assemblies. Mercury parts diagrams identify 8M0204645 as the direct replacement for earlier O-ring 21836, and list it in several cooling-water and lower-unit gearcase arrangements where it seals the water passage at the tube or coupling joint. Because Mercury uses several nearly identical O-rings at different positions within the same lower unit, the correct part must be confirmed by diagram position — not by diameter or appearance alone.
Compatible Applications
- Mercury 75 HP EFI FourStroke — standard-rotation gearcase, water-pump coupling area (2 required in this application)
- Selected Mercury two-stroke outboards — water tube extension position
- Selected Mercury / MerCruiser gear housing and water-pump rebuild assemblies using the original 21836 / 8M0204645 seal
Confirm compatibility by engine or drive model, serial number, and the exact exploded-view diagram position. Some assemblies use one O-ring; others use two at the water-pump coupling — do not assume quantity without checking the diagram.
O-Ring Position Reference — Same Lower Unit
Mercury uses several different O-rings in the same gearcase. Do not substitute by appearance or diameter:
- 8M0204645 — 0.796 × 0.139 in. — water tube / water-pump coupling (this part)
- 32509 — 0.489 × 0.070 in. — water tube (different position)
- 859774 — 0.412 × 0.210 in. — water tube (different position)
When to Replace
Replace this O-ring whenever the water tube, coupling, or gearcase cooling assembly is disassembled, or when inspection reveals any of the following:
- Flattening or permanent compression set
- Cuts, nicks, or tearing
- Hardening or brittleness
- Swelling or distortion
- Cracking
- Visible leakage or evidence of poor sealing around the water-tube connection
A damaged seal at this location can reduce cooling-water flow to the engine, potentially leading to overheating. Replace O-rings as a set whenever the assembly is apart rather than reusing old seals.
Installation Notes
Clean the water-tube or coupling groove and all mating surfaces before installation. Lightly lubricate the O-ring with clean water or the recommended assembly lubricant and seat it evenly so it is not twisted or pinched during reassembly. Where the applicable Mercury diagram specifies two 8M0204645 O-rings (such as the 75 HP FourStroke gearcase), replace both and confirm their individual positions before reinstalling the coupling or gear housing.
Related Service Parts
- Mercury MerCruiser Gasket Inlet Seat 25033
- Mercury MerCruiser Quicksilver Lockwasher 8M0204647
Cross-Reference Part Numbers
- Current OEM: 8M0204645 / 25-8M0204645
- Replaces: 21836
- Historical cross-references: 20679, F53795, F53795-1
- GTIN: 745061130579
Parts Desk Insight
8M0204645 is specifically a cooling-water / water-tube O-ring, not a generic gearcase seal. The most common issue is selecting the wrong O-ring by diameter or appearance — Mercury uses several similarly sized seals at different positions within the same lower unit, and they are not interchangeable. Matching by the service part number 25-8M0204645 or the superseded number 21836, and confirming against the exact exploded-view diagram position, is the most reliable approach. When servicing the lower unit cooling system, replace all O-rings in the water-tube assembly at the same time rather than replacing only the one that shows obvious damage.
Shipping information
In-stock and OEM parts: ship same or next business day.
Locally warehoused items: ship within 1-3 business days.
Special-order items: timeline confirmed after order.
Returns & warranty
Returns & Satisfaction Guarantee
We stand firmly behind the quality of every product we sell. Your confidence matters to us — and we back it with a straightforward 30-Day Satisfaction Guarantee.
If you are not fully satisfied, you may request a return within 30 days of delivery. All returns must receive pre-approval prior to being sent back. Unauthorized returns will not be accepted.
To qualify for a return:
Item must be in new, unused condition
Product must be in its original packaging
All parts, components, hardware, manuals, and accessories must be included
Packaging must be undamaged and suitable for resale
Important:
Electronics are strictly inspected. No returns on open-box electronics. Exceptions may apply only with prior written approval. Any electronic unit that has been installed, powered, activated, or altered may not be eligible for return.
We are serious about product integrity — and equally serious about taking care of our customers. If there is ever a product issue, contact us immediately so we can make it right.
For return authorization, please contact our support team before shipping anything back.