Upgrade your Mercury 9.9–20 HP outboard to a long-shaft configuration with this Bay Manufacturing 5-inch Extension Kit. Designed specifically for newer 11-spline models, this kit enhances performance by improving propeller bite, reducing cavitation, and optimizing overall handling.
All Mercury extension kits include marine-grade cast housings and stainless-steel driveshafts and hardware, ensuring long-lasting durability and OEM-level reliability.
Product Features
- Adds a full 5 inches of shaft length
- Compatible with 11-spline driveshaft models only
- Designed for Mercury:
- 9.9 HP (2021 & Newer)
- 15 HP (2018 & Newer)
- 20 HP (2018 & Newer; EFI & Pro Kicker compatible)
- Motors must have cap bolts and dowel pins
- Includes all necessary components for conversion
- Durable black-painted finish
- 1-Year Manufacturer Warranty
Motor Compatibility — MUST Match All Specifications
Option 1 — Mercury 9.9 HP
| HP | Stroke | Year | Splines | Notes |
|---|---|---|---|---|
| 9.9 | 4 | 2021 & Newer | 11 splines | Motor will have dowel pins; 0R serial range |
Option 2 — Mercury 15 & 20 HP
| HP | Stroke | Year | Splines | Notes |
|---|---|---|---|---|
| 15 | 4 | 2018 | – | Transition year |
| 15 | 4 | 2019 & Newer | 11 splines | Must have cap bolts & dowel pins |
| 20 | 4 | 2018 | – | Transition year |
| 20 | 4 | 2019 & Newer | 11 splines | EFI & Pro Kicker compatible |
Important Fit Notes
- If your motor has a 10-spline driveshaft, you need Kit 304A instead.
- Motors must have dowel pins and cap bolts.
- Compatible with 0R-series serial numbers.
Parts Included
- (1) 5" Extension Housing
- (1) 5" Driveshaft
- (1) 5" Shift Shaft
- (1) 5" Water Tube
- (3) Studs & Nuts
-
(1) Bushing
Additional Information
- Some newer kits ship painted black, others unpainted sandblasted, depending on production batch.
- Adds a true 5 inches to shaft length for increased stability and propulsion performance.
- Built by Bay Manufacturing, the industry leader in outboard extension kits.
- Not returnable except for warranty issues.
Shipping Information
Additional shipping fees to Alaska, Hawaii, U.S. Territories, or international destinations will be calculated after checkout.
A PayPal request will be sent for any additional charges.
(Some items may be excluded from international shipping.)
Customer Support
For questions or to confirm compatibility, please email customerservice@marinepartswarehouse.com
Please include your engine model or serial number to help confirm proper fitment before ordering.
