Mercury MerCruiser Idler Pulley 864625T – 6-Groove Serpentine, 4.25" OD
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Mercury MerCruiser Idler Pulley – 864625T
Keep your MerCruiser’s accessory drive system running smoothly with this genuine OEM idler pulley. The idler pulley guides the serpentine belt across the accessory drive, maintaining proper belt tracking and tension. When the sealed bearing wears out, the belt can squeal, track off-center, or fail prematurely — taking the alternator, power steering pump, and seawater pump with it. This is a direct OEM replacement that restores proper belt routing and protects the full accessory drive system.
Compatible Applications
- MerCruiser 4.3L V6 stern drive and inboard engines (1998 and newer)
- MerCruiser 5.0L V8 stern drive and inboard engines (1998 and newer)
- MerCruiser 5.7L / 350 Mag V8 stern drive and inboard engines (1998 and newer)
- MerCruiser 6.2L / MX 6.2L V8 stern drive and inboard engines (1998 and newer)
- Compatible with Alpha and Bravo drive packages on applicable engine families
- Verify fitment by engine serial number and belt routing diagram before ordering — pulley fitment varies by accessory bracket layout and production range
Part Details
- OEM Part Number: 864625T
- Supersedes / Also Referenced As: 849296T, 57-8638764, 57-862086
- Aftermarket Interchange: Sierra Marine 18-15101
- Brand: Mercury / MerCruiser / Quicksilver
- Product Type: Idler pulley — serpentine accessory drive
- System: Accessory drive / serpentine belt system
- Outside Diameter: 4.25” (4-1/4”)
- Belt Grooves: 6-groove serpentine
- Bearing: Sealed bearing construction
- Condition: New OEM
Key Features
- Genuine OEM replacement — exact fit for compatible MerCruiser V6 and V8 accessory drive systems
- Sealed bearing construction — pre-lubricated, maintenance-free bearing for long service life
- 6-groove serpentine design — matches OEM belt profile for proper tracking and load distribution
- 4.25” outside diameter — precision-matched to OEM belt routing geometry
Service Interval
Inspect the idler pulley bearing at every serpentine belt service. Replace the pulley any time the bearing feels rough, loose, or noisy, or when the belt shows abnormal wear patterns, squealing, or tracking issues. Mercury recommends replacing the belt and inspecting all pulleys at the 300-hour or 3-year service interval — always follow your engine’s service manual.
Technical FAQ
- What does the idler pulley do? It guides the serpentine belt across the accessory drive system, maintaining proper belt tracking and tension between the driven accessories (alternator, power steering pump, seawater pump).
- How do I know if the idler pulley bearing is failing? Common signs include a squealing or grinding noise from the accessory drive area, belt tracking off-center, unusual belt wear on one edge, or a rough/wobbling feel when spinning the pulley by hand with the engine off.
- Should I replace the belt at the same time? Yes — if the pulley bearing has failed, the belt has likely been running under abnormal load. Replace both together for a complete service.
- What’s the difference between 864625T and 849296T? 849296T is a superseded part number — 864625T is the current OEM replacement that covers both references.
- Does this fit all MerCruiser V8 engines? No — fitment is specific to 1998-and-newer V6/V8 engines using the 6-groove serpentine accessory drive layout. Confirm by engine serial number and belt routing diagram.
Related Parts
- Mercury MerCruiser Serpentine Belt & Pulley Kit – 8M0150824
- Mercury MerCruiser Power Steering Pump Pulley – 8M0150723
- Mercury MerCruiser Seawater Pump Pulley – 8M0150724
- Mercury MerCruiser Idler Pulley Support Bushing – 865193
Parts Desk Insights
The idler pulley is one of the most overlooked parts in a serpentine belt service — most customers replace the belt and ignore the pulley, then come back with a failed belt a few months later. If the bearing feels rough or gritty when you spin it by hand, replace the pulley at the same time as the belt. The most common mistake is ordering by horsepower alone: MerCruiser used different accessory drive layouts across production years, so the same 5.7L engine can take different pulleys depending on the serial number range and bracket configuration. Always confirm 864625T against your engine serial number and the Mercury parts diagram for your specific drive before ordering.
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
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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.
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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:
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