Lower Unit Gear Oil Filler Adapter Kit | Sierra 18-99525

Sierra Lower Unit Gear Oil Filler Adapter Kit | 18-99525 | 5-Adapter Kit | Multi-Brand Outboard

The Sierra #18-99525 is a 5-piece lower unit gear oil filler adapter kit that makes the CDI 551-33-1 gear lube pump compatible with virtually every major outboard and sterndrive brand. Each adapter in the kit threads onto the pump nozzle and into a specific lower unit drain plug thread, allowing clean and controlled gear lube delivery without removing the pump from the bottle. Replaces CDI 551-334K. Compatible with Sierra fill pump assemblies 551-33-1, 34PV, and 551-34HA.

Part Details:

• Part Number: 18-99525 (Sierra)
• Manufacturer: Sierra
• Type: Lower Unit Gear Oil Filler Adapter Kit — 5 Adapters
• Replaces: CDI 551-334K
• Compatible Pumps: CDI/Sierra 551-33-1, 34PV, 551-34HA
• Note: 551-334B Bravo & Verado adapter hose sold separately
• Condition: New

Kit Contents — 5 Adapters:

• 551-334M — Metric M8 × 1.25P thread adapter (Mercury, Mariner, MerCruiser, Johnson, Evinrude, Yamaha, Tohatsu, Chrysler, Force)
• 551-334S — Suzuki M10 × 1.25P thread adapter
• 551-334V — Volvo Penta M10 × 1.5P thread adapter
• 551-33TN — Threaded 3/8 × 16P nozzle adapter
• 551-33UN — Unthreaded nozzle adapter (push-in fit for non-threaded drain ports)

Additional Information

Which Adapter Do I Use for My Engine?

Match your lower unit drain plug thread to the correct adapter in the kit:

• Mercury, Mariner, MerCruiser, Johnson, Evinrude, Yamaha, Tohatsu, Chrysler, Force — use the 551-334M (M8 × 1.25P metric adapter); this covers the majority of outboard and sterndrive lower units across these brands
• Suzuki outboards — use the 551-334S (M10 × 1.25P adapter); Suzuki uses a larger drain plug thread than most other brands
• Volvo Penta sterndrives — use the 551-334V (M10 × 1.5P adapter); Volvo Penta uses M10 but with a different pitch than Suzuki
• Mercury Bravo and Verado — requires the 551-334B adapter hose, sold separately; not included in this kit
• Non-threaded or push-in drain ports — use the 551-33UN unthreaded nozzle adapter

Using This Kit for Correct Bottom-Up Lower Unit Fill:

Thread the correct adapter onto the pump nozzle and into the lower drain plug hole. Remove the upper vent/fill plug. Pump gear lube upward until it flows from the upper hole without bubbles — indicating the lower unit is full. Install the upper plug first, then quickly remove the pump nozzle from the lower hole and install the lower drain plug. Always replace drain plug sealing rings or washers at each service — reused sealing rings are a leading cause of lower unit gear lube leaks.

For other gear lube pump service components, also see:

• Sierra Gear Lube Pump Hose Assembly (18-99526) — replacement hose for CDI 551-33 pump kits; also replaces CDI 551-33HB
• Sierra Replacement Check Valve (18-99528) — replacement check valve for CDI 551-33-1 pump; restores pump prime and flow
• Suzuki OEM Gear Oil Pump (990C0-99002) — OEM Suzuki bottle-mount pump for Ecstar quart bottles

Frequently Asked Questions

1. Does this kit work with my pump if I have a Mercury Bravo or Verado?
No — Mercury Bravo and Verado lower units require the 551-334B adapter hose, which is sold separately and not included in the 18-99525 kit. If you have a Bravo or Verado, order the 551-334B in addition to this kit, or confirm whether your pump setup already includes the Bravo/Verado adapter.

2. What pump does this adapter kit work with?
The Sierra 18-99525 adapter kit is compatible with CDI/Sierra fill pump assemblies 551-33-1, 34PV, and 551-34HA. It replaces CDI 551-334K. If you have a different gear lube pump brand or model, verify adapter thread compatibility before ordering.

3. Do I need all five adapters or just one?
You only use one adapter at a time — the one that matches your lower unit's drain plug thread. The remaining adapters store in the kit for use on other engines or future service on a different boat. The 5-adapter kit is particularly useful for technicians servicing multiple engine brands, or for boaters with multi-engine setups combining different brands.

4. Can I use this kit for both outboard and sterndrive lower units?
Yes — the 18-99525 includes adapters covering both outboard lower units (Mercury, Yamaha, Suzuki, Johnson/Evinrude, Tohatsu, etc.) and sterndrive applications (MerCruiser, Volvo Penta). Match the adapter to your specific lower unit's drain plug thread specification.