Mercury – Quicksilver Gear Lube Drain & Fill Screw with Seal | Lower Unit Component | 79953Q04

Overview

Keeping your lower unit sealed tight is one of the easiest — and most important — ways to protect your engine’s gears. The Mercury Marine 79953Q04 Quicksilver Drain and Fill Screw with Seal is designed for all MerCruiser sterndrives and Mercury/Mariner outboards (except Verado models). This non-magnetic plug and seal combination ensures your gear lube stays in and water stays out. It’s a simple part that prevents major damage by keeping your gearcase clean, dry, and properly lubricated.

Key Features

• Purpose: Seals the gear lube drain/fill hole to prevent leaks and water intrusion.
• Compatibility: Works with 3/8-16 drain/fill hole screws on MerCruiser sterndrives and Mercury/Mariner outboards (excluding Verado).
• Design: Includes a non-magnetic plug and sealing washer for a tight, reliable fit.
• Performance: Helps maintain correct gear lube levels for smooth, quiet gear operation.
• Durability: Built from corrosion-resistant materials that withstand saltwater and long-term exposure.
• Ease of Use: Installs quickly with a standard screwdriver or wrench during routine maintenance.

Engine & Boat Compatibility

• MerCruiser sterndrives (all models)
• Mercury outboards (excluding Verado)
• Mariner outboards (excluding Verado)

Performance & Real-World Benefits

The 79953Q04 drain and fill screw is a small part with a big job. By sealing the lower unit properly, it prevents water from mixing with gear lube — a leading cause of internal corrosion and bearing wear. Using the included washer ensures the screw seats flush, maintaining proper case pressure and keeping contaminants out.

This OEM replacement helps your gearcase last longer and operate more efficiently. It’s an easy maintenance task that pays off every time you shift into gear.

Why Choose Mercury OEM

The Quicksilver 79953Q04 is a genuine Mercury Marine component, designed for an exact fit and tested for sealing reliability. Aftermarket plugs or seals may not match Mercury’s tolerances, leading to minor leaks or poor seating that can cause expensive damage over time.

Mercury OEM hardware is precision-machined, corrosion-tested, and made from the right materials for marine use. With OEM parts, you know your gearcase is sealed the way Mercury intended — tight, clean, and dependable.

FAQ

Q1: What engines does this fit?
It fits all MerCruiser sterndrives and Mercury/Mariner outboards, except Verado models.

Q2: What size is the screw?
It’s made for 3/8-16 drain/fill holes, standard on most Mercury and MerCruiser gearcases.

Q3: Is the plug magnetic?
No — this is the non-magnetic version of the drain and fill plug.

Q4: Why is the seal so important?
Without the proper seal, gear lube can leak out or water can seep in, leading to gear and bearing damage.

Q5: Why OEM instead of aftermarket?
Mercury OEM plugs and seals guarantee perfect fit, strong corrosion resistance, and reliable sealing performance over time.