Mercury / Quicksilver 807757T Black Idler Pulley | MerCruiser 4.3L–8.2L V6 / V8 Serpentine Belt
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Part Details
- Part Number: 807757T
- Brand: Mercury Marine / Quicksilver (Genuine OEM)
- Official Product Type: Serpentine-Belt Idler Pulley Assembly — Black
- Diameter (approx.): 3 in. / 76 mm
- Groove Profile: 6-groove (poly-V / serpentine)
- Construction: Thermal-resin composite with integrated bearing
- System: Accessory drive / serpentine-belt system
- Sierra Aftermarket Cross-Reference: 18-6457
- GTIN / UPC: 745061288324
- Weight (approx.): 250 g
What This Part Is
Mercury / Quicksilver 807757T is a genuine OEM black serpentine-belt idler pulley assembly used on a wide range of GM-based MerCruiser sterndrive and inboard engines. The pulley guides and tensions the accessory drive belt through the alternator and other driven accessories. The bearing is integrated into the pulley assembly. In some MerCruiser accessory-drive arrangements, two 807757T pulleys are used — confirm the quantity required for your specific engine before ordering.
Compatible Applications
Fitment must be confirmed by engine model and serial number against the MerCruiser accessory-drive or alternator-bracket parts diagram. Documented GM-based MerCruiser applications include:
- MerCruiser 4.3L V6 sterndrive and inboard
- MerCruiser 5.0L V8 sterndrive and inboard
- MerCruiser 5.7L V8 sterndrive and inboard
- MerCruiser 350 MAG
- MerCruiser 377 MAG
- MerCruiser 6.2L MPI
- MerCruiser 7.4L / 454
- MerCruiser 8.1L / 496
- MerCruiser 8.2L
- Mercury Racing 520, 525 EFI, 540, 565 EFI, 600 SCi, 662 SCi, and 700 SCi applications
807757T is used across both sterndrive and inboard configurations. Always verify part number 807757T in the accessory-drive / alternator-bracket parts diagram for your specific engine model and serial number before ordering.
Diagram Position
807757T appears in the MerCruiser Accessory Drive / Alternator Bracket parts group, used with related components including idler bracket 807753T1, idler adjustment bracket 807767T, and support bushing 818380T. Some assemblies call for two 807757T pulleys — confirm quantity in the diagram for your engine.
When to Replace
Replace the idler pulley if any of the following are present:
- Audible bearing noise, grinding, or chirping from the accessory drive
- Roughness when the pulley is rotated by hand
- Visible pulley wobble or radial play in the bearing
- Cracking, damage, or distortion to the pulley body
- Belt tracking off-centre or walking off the drive
- Repeated or accelerated serpentine-belt wear
A worn idler accelerates belt wear and can allow the belt to misalign or exit the accessory drive under load.
Service Interval
Mercury does not publish a fixed calendar or hour interval for idler pulley replacement. The pulley should be inspected at every serpentine-belt service and whenever the belt is removed. Replace any time bearing roughness, noise, or pulley wobble is detected — do not reinstall a noisy or loose idler.
Installation Notes
- Inspect the serpentine belt for cracks, glazing, fraying, or rib damage at the same time — replace the belt if it is due or shows any wear
- Inspect the idler support bushing 818380T for wear before reassembly
- Check the idler bracket 807753T1 and adjustment bracket 807767T for corrosion, cracks, or stripped fasteners
- Verify pulley alignment with all other accessory-drive components after installation
- Ensure the serpentine belt is properly seated in all pulley grooves before starting the engine
- If two 807757T pulleys are installed on this engine, replace both at the same time
Technical FAQ
Does 807757T fit both sterndrive and inboard engines?
Yes. 807757T is used across MerCruiser GM-based engines in both sterndrive and inboard configurations. Always confirm by engine model, serial number, and the accessory-drive parts diagram.
Do I need one or two pulleys?
Some MerCruiser accessory-drive arrangements use two 807757T pulleys. Check the parts diagram for your specific engine before ordering to confirm the quantity required.
What aftermarket cross-reference applies to 807757T?
Sierra 18-6457 is a documented aftermarket equivalent. Always verify the cross-reference against your engine’s diagram and current OEM fitment before substituting.
What causes idler pulleys to fail?
Marine environments expose pulleys to moisture, heat cycling, and salt air, which accelerates bearing corrosion and wear. Belt misalignment or incorrect belt tension can also overload the bearing and shorten pulley life. Replacing the belt, bushing, and pulley together at the first sign of noise is the most cost-effective approach.
Cross-Reference Part Numbers
- Mercury / Quicksilver OEM: 807757T
- Sierra aftermarket: 18-6457
- GTIN / UPC: 745061288324
Related Parts
- Idler bracket 807753T1 — mounting bracket for the idler pulley assembly
- Idler adjustment bracket 807767T — belt tension adjustment bracket
- Support bushing 818380T — idler bushing; inspect and replace with pulley
- Browse MerCruiser Marine Engine Parts for all accessory-drive components
Parts Desk Insights
Best use case: 807757T is the go-to replacement when a MerCruiser GM-based engine develops serpentine-belt noise, belt-tracking problems, or visible pulley wobble. It covers a very wide range of 4.3L through 8.2L applications, making it one of the more commonly stocked MerCruiser accessory-drive items for service shops and boat owners doing their own maintenance.
Common issue: The most frequent mistake is replacing only the pulley while leaving the worn idler bushing (818380T) in place. The bushing is the mounting interface for the pulley — a worn bushing allows the replacement pulley to run off-axis, which causes rapid re-failure and continued belt-tracking problems. Always replace the bushing at the same time as the pulley. A second common issue is ordering one pulley when the specific engine configuration requires two — check the diagram for your engine 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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