Sierra Fuel Filter 18-7844 (Short) 21‑Micron Spin‑On — 90 GPH Flow, Mercury Cross‑Reference

This compact spin-on fuel filter from Sierra is built as a direct-fit replacement for Mercury Marine part numbers 35-807172 and 35-802893Q. With a 21-micron media rating and a design that supports a steady 90 gallons-per-hour flow, it balances filtration efficiency with the fuel delivery needs of larger or high-revving marine engines. The element also helps manage contaminants introduced by ethanol-blended fuels that can loosen deposits and absorb moisture.

How does this filter protect my outboard engine?

The 21-micron element traps fine particles that could abrade internal fuel system components and clog injectors or carburetors. It also reduces the amount of water and debris reaching the pump and motor, helping maintain consistent combustion and performance.

• 21-micron media captures small particulates without starving fuel flow
• Designed to handle ethanol-blended gasoline contaminants
• Maintains clean fuel to protect pumps, injectors, and carburetors
• Spin-on design simplifies replacement during routine maintenance

Will it fit my Mercury engine and how do I identify compatibility?

Sierra 18-7844 is made to replace Mercury OEM 35-807172 and 35-802893Q, matching thread and physical dimensions for a direct swap in compatible models. Check your current filter’s part number or consult your engine’s parts manual to confirm fit before purchasing.

• OEM cross-reference: Mercury Marine 35-807172
• Thread pitch specification included for fit verification (42690)
• Short size form factor: listed as Short - 3875" / 984mm for space-limited mounts
• Designed to meet factory flow and filtration needs for applicable engines

How does it perform with ethanol-blended fuels?

Ethanol in modern fuels tends to absorb water and loosen varnish and deposits; this filter’s media is sized to trap those loosened particles and separate some free water before it reaches the engine. Regular replacement helps avoid filter clogging and fuel delivery interruptions caused by ethanol-related contaminants.

• Media captures varnish and debris released by ethanol blends
• Helps limit water carry-over into the fuel system
• Recommended regular change intervals prevent flow restriction

What flow and service expectations should I plan for?

The filter is rated to flow at 90 GPH, which supports even high-demand motors while providing fine filtration. Service interval depends on fuel quality and engine hours; inspect during scheduled maintenance and replace when flow or contamination indicates reduced effectiveness.

• Rated flow: 90 gallons per hour to support high-demand engines
• Filter efficiency: 21-micron rating for fine particulate removal
• Inspect at regular intervals; replace when clogged or per manufacturer schedule

Who is this for: The Sierra 18-7844 is aimed at boat owners and marine technicians who operate Mercury-compatible outboards and need a compact, high-flow fuel filter that balances fine-particle filtration with steady fuel delivery. It’s especially beneficial for vessels that run on ethanol-blended gasoline or that require a short-style filter to fit tight mounting locations. Use it when you need a direct-replacement element to maintain factory-level performance and protect fuel-system components.