Extend the shaft length of your Mercury or Johnson outboard by a full 5 inches with this high-quality Bay Manufacturing Extension Kit. Engineered for long-term durability, this kit includes all components required for a proper long-shaft conversion, including stainless-steel shafts and studs, precision-cast spacer housing, and all necessary hardware.
Designed for Mercury 9.9–20 HP 4-Stroke outboards (specific years and spline counts required) as well as Johnson 15 HP Tohatsu-built models. This kit provides a reliable solution for improving handling, propeller bite, and overall boat performance.
Product Features
- Adds 5 inches to your outboard’s shaft length
- Fits select Mercury 8–20 HP 4-Stroke and Johnson 15 HP (Tohatsu-built) engines
- Stainless-steel shafts, studs, and high-strength hardware
- Spacer housing cast from marine-grade alloy
- Ideal for correcting cavitation, performance loss, and shallow propeller draft
- Includes 1-year manufacturer warranty
- Some units ship painted black; others ship sandblasted and unpainted
Motor Compatibility — Must Match All Specs
Mercury 4-Stroke (11-Spline Driveshaft):
- 9.9 HP — 2005 & Newer (0R052328 and up), 11 splines, 8mm cap bolt thread
- 15 & 20 HP — 2017 & Older (11 splines)
- 2018 models — verify splines
- If your driveshaft has 10 splines → use Kit 302A
- If your motor is 2019 or newer → use Kit 304
Mercury Command Thrust:
- 8 HP
- 9.9 HP
Johnson (Tohatsu-Built):
- 15 HP — 2012 & Newer
Parts Included
- (1) 5" Extension Housing
- (1) 5" Driveshaft
- (1) 5" Shift Shaft
- (1) 5" Water Tube
- (3) Studs & Nuts
- (1) Bushing
Additional Information
This kit is commonly used to convert short-shaft engines to long-shaft configuration, improving:
- Propeller performance
- Control and steering stability
- Cavitation prevention
Not eligible for return except for warranty claims.
Shipping Information
Additional shipping fees apply to Alaska, Hawaii, U.S. Territories, and international destinations.
After checkout, a PayPal request will be emailed for any additional required 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.
