Sierra Heavy-Duty U-Joint 18-2104 | MerCruiser / OMC / Volvo Penta
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Sierra Heavy-Duty U-Joint 18-2104
Part Number: 18-2104 (Sierra) | UPC: 030999310293
The Sierra 18-2104 is a heavy-duty, greasable Rockwell-style universal joint for selected MerCruiser, OMC Cobra, and Volvo Penta AQ-series sterndrive drivelines. It transmits torque between the engine coupler and the drive while allowing angular movement through the transom. The kit includes both a straight grease fitting and a 90° grease fitting for flexible service access, plus retaining clips.
Specifications
- Construction: Rockwell style, heavy-duty steel
- Greasable: Yes
- Includes: Straight grease fitting, 90° grease fitting, retaining clips
Compatible Applications
MerCruiser
- MerCruiser #1
- MR / Early Alpha One (Serial: 1684188 through B418818)
- TR
- TRS
- II SSM
OMC
- OMC Cobra — approx. 1986–1989
- OEM references: 912214, 915716
Volvo Penta
- AQ200
- AQ250
- AQ270
- AQ280
- AQ290
- OEM references: 853255, 897707
Fitment Notice
Later Alpha One, OMC/Volvo, SX, and DP-S drivetrains can use different universal joints. Confirm before ordering:
- Drive model and generation
- Drive serial number
- Existing U-joint part number
- Retaining-clip style
- Grease-fitting access requirement
- Yoke condition
OEM & Cross-Reference Part Numbers
Mercury / MerCruiser
- 36312
- 41431
- 58749
- 75832
- 758321
- 75832A1
- 75832A2
- 75832A3
- 75832T
- 75832T1
OMC
- 912214
- 915716
Volvo Penta
- 853255
- 897707
Aftermarket
- Mallory 9-72404
- GLM 28020
When to Replace the U-Joint
- Play or looseness felt in the cross bearing
- Needle bearings feel rough or bind during rotation
- Rust, corrosion, or water contamination is visible
- Grease loss or bearing-cup damage is found
- Clunking, vibration, or driveline shudder under load
- Joint operated with inadequate lubrication
Installation Notes
- Remove the sterndrive following the manufacturer's service procedure
- Inspect both yokes for wear, distortion, or bore damage before installing the new joint
- Seat retaining clips fully into their grooves — a clip not fully seated is a common cause of joint failure
- Choose the straight or 90° grease fitting based on which provides better access in the installed position
- Lubricate with the specified marine grease before and after installation
- While the drive is off, inspect the U-joint bellows, gimbal bearing, and drive alignment
Related Parts
- Sierra Heavy-Duty U-Joint (related SKU)
- U-Joint AP-1821
- U-Joint Bellows
- OMC U-Joint Bellows
- U-Joint Bellows — King Cobra
- Sierra Gimbal Bearing 18-2100 | MerCruiser / OMC / Volvo Penta
- Sierra 18-21006 Gimbal Bearing – MerCruiser Bravo & Gen II Sterndrive
- Mercury Quicksilver Gimbal Housing Lock Nut 11-34087
- OMC Bellows Kit
- OMC Sterndrive / Cobra Exhaust Bellows
- Sierra Cobra Outdrive Mounting Gasket Set (47-2613)
Parts Desk Insights
The Sierra 18-2104 is the correct heavy-duty U-joint for older MerCruiser drivelines through the early Alpha One era (serial through B418818) and for OMC Cobra and Volvo Penta AQ-series drives. The dual grease-fitting option — straight and 90° — makes in-service greasing practical even in tight transom areas.
The most common fitment error is assuming this joint fits all Alpha One drives. It covers the early Alpha One / MR serial range; later Alpha One, Bravo, and Alpha Gen II applications use different U-joints. Always cross-check the drive serial number and existing OEM joint number. On installation, the retaining clips are the single most important detail: a clip that isn’t fully seated in its groove will cause the bearing cup to walk out under load, leading to rapid joint failure and potential driveline damage.
Shipping information
In-stock and OEM parts: ship same or next business day.
Locally warehoused items: ship within 1-3 business days.
Special-order items: timeline confirmed after order.
Returns & warranty
Returns & Satisfaction Guarantee
We stand firmly behind the quality of every product we sell. Your confidence matters to us — and we back it with a straightforward 30-Day Satisfaction Guarantee.
If you are not fully satisfied, you may request a return within 30 days of delivery. All returns must receive pre-approval prior to being sent back. Unauthorized returns will not be accepted.
To qualify for a return:
Item must be in new, unused condition
Product must be in its original packaging
All parts, components, hardware, manuals, and accessories must be included
Packaging must be undamaged and suitable for resale
Important:
Electronics are strictly inspected. No returns on open-box electronics. Exceptions may apply only with prior written approval. Any electronic unit that has been installed, powered, activated, or altered may not be eligible for return.
We are serious about product integrity — and equally serious about taking care of our customers. If there is ever a product issue, contact us immediately so we can make it right.
For return authorization, please contact our support team before shipping anything back.