Volvo DuoProp Outdrive Gear Lube Fill Adapter | Sierra 18-99529

Sierra Volvo DuoProp Gear Lube Fill Adapter | 18-99529 | 3/8"-16 | Replaces CDI 551-334V

The Sierra #18-99529 is the gear lube fill adapter for Volvo Penta DuoProp and OMC joint venture DuoProp outdrive units. The DuoProp's counter-rotating dual propeller drive uses a different fill port thread than standard Volvo Penta sterndrives, requiring this dedicated adapter to connect a standard 3/8"-16 gear lube pump fitting to the DuoProp gearcase. Includes O-ring for a leak-free seal at the fill port. Replaces CDI 551-334V. If you need adapters for multiple outdrive brands, the full 5-adapter kit is also available.

Part Details:

• Part Number: 18-99529 (Sierra)
• SKU: SIERRA-18-99529
• Manufacturer: Sierra
• Type: Gear Lube Fill Adapter — Volvo Penta DuoProp
• Fits: Volvo Penta DuoProp and OMC joint venture DuoProp outdrive units
• Pump Connection: 3/8"-16 standard lube pump fitting
• Includes: O-ring
• Replaces CDI: 551-334V
• Condition: New

Additional Information

Why DuoProp Requires a Dedicated Adapter:

Volvo Penta's DuoProp drive system uses counter-rotating dual propellers on a shared shaft — a design that requires a different lower unit gearcase configuration than a single-propeller sterndrive. The DuoProp fill port uses a thread specification that differs from standard Volvo Penta sterndrive lower units and from the common M8 × 1.25P or M10 × 1.5P threads used on other outboard and sterndrive brands. The Sierra 18-99529 adapts the DuoProp's fill port thread to accept a standard 3/8"-16 gear lube pump nozzle, allowing clean pump-assisted gear lube fill rather than a manual pour.

Individual Adapter vs. Full Kit:

If you only service Volvo Penta DuoProp drives, the 18-99529 individual adapter is the right choice — no need to buy a full multi-brand kit. If you service multiple outdrive brands (Mercury, Suzuki, Yamaha, standard Volvo Penta, etc.), the full 5-adapter kit covers all major applications in one purchase. Also see:

• Sierra Lower Unit Gear Oil Filler Adapter Kit (18-99525) — 5-adapter kit including M8 × 1.25P, Suzuki M10 × 1.25P, Volvo Penta M10 × 1.5P, 3/8"-16, and unthreaded adapters; replaces CDI 551-334K
• Sierra Gear Lube Pump Hose Assembly (18-99526) — replacement hose for CDI 551-33 pump kits
• Sierra Replacement Check Valve (18-99528) — replacement check valve for CDI 551-33-1 gear lube pump

Frequently Asked Questions

1. How do I confirm this adapter fits my DuoProp drive?
The Sierra 18-99529 fits Volvo Penta DuoProp and OMC joint venture DuoProp outdrive units. Confirm your drive is a DuoProp model (counter-rotating dual propellers) before ordering. Standard Volvo Penta single-propeller sterndrives use a different thread — the M10 × 1.5P adapter (551-334V in the full kit) — which is a different adapter than this DuoProp-specific unit. Contact us at MacombMarineParts.com with your outdrive model if you need confirmation.

2. Does this adapter replace CDI 551-334V?
Yes — the Sierra 18-99529 replaces CDI 551-334V. Note that CDI also used the 551-334V designation for the standard Volvo Penta M10 × 1.5P adapter in the full 551-334K kit — confirm your application is DuoProp-specific before ordering the 18-99529 vs. sourcing the M10 × 1.5P adapter from the full 18-99525 kit.

3. What gear lube pump is compatible with this adapter?
The 18-99529 connects to standard 3/8"-16 gear lube pump fittings. It is compatible with CDI/Sierra fill pump assemblies 551-33-1, 34PV, and 551-34HA. Confirm your pump fitting is 3/8"-16 before ordering.