Mercury Marine — Black Idler Pulley — 807757T

Mercury MerCruiser Black Idler Pulley 807757T

The Mercury MerCruiser 807757T is a genuine OEM black serpentine-belt idler pulley 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, with a bearing integrated directly into the thermal-resin pulley body. Some MerCruiser accessory-drive arrangements use two 807757T pulleys, so confirm the quantity your engine needs before ordering.

Compatible Applications

Fitment should be confirmed by engine model and serial number against the MerCruiser accessory-drive or alternator-bracket parts diagram. Documented GM-based MerCruiser applications include:

• 4.3L V6 sterndrive and inboard
• 5.0L V8 sterndrive and inboard
• 5.7L / 350 V8 sterndrive and inboard
• 350 MAG
• 377 MAG
• 6.2L V8 / MPI
• 7.4L / 454 V8
• 8.1L / 496
• 8.2L / 502 V8
• Selected 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. Confirm the part number against the accessory-drive / alternator-bracket parts diagram for your specific engine model and serial number before ordering. Do not confuse this grooved pulley with flat or larger-diameter idlers used in similar accessory-drive systems.

Part Details

• Part number: 807757T
• Brand: Mercury Marine
• Official product type: Serpentine-belt idler pulley
• Diameter: approximately 3 in. / 76 mm
• Groove profile: 6-groove (poly-V / serpentine)
• Construction: Thermal-resin composite with integrated bearing
• System: Accessory drive / serpentine-belt system
• GTIN / UPC: 745061288324

Diagram Position

807757T appears in the MerCruiser accessory drive / alternator bracket parts group, used alongside related components including idler bracket 807753T1, idler adjustment bracket 807767T, and support bushing 818380T.

When This Part Needs Replacement

• 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. Inspect the pulley at every serpentine-belt service and whenever the belt is removed. Replace at the first sign of bearing roughness, noise, or wobble rather than reinstalling a worn idler.

Installation Notes

• Record the serpentine-belt routing before removal
• Inspect the serpentine belt for cracks, glazing, fraying, or rib damage, and replace it if due or worn
• 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 the rest of the accessory-drive components after installation
• Ensure the belt is properly seated in all pulley grooves before starting the engine
• If your engine uses two 807757T pulleys, replace both at the same time

Alternate/Superseded Part Numbers

• 807757T — OEM Mercury / MerCruiser part number
• No verified later superseding number

Related Parts

• Idler bracket 807753T1 — mounting bracket for the idler pulley
• Idler adjustment bracket 807767T — belt tension adjustment bracket
• Support bushing 818380T — idler bushing; inspect and replace with the pulley

Technical FAQ

Does 807757T fit both sterndrive and inboard engines? Yes, it's used across MerCruiser GM-based engines in both configurations. Confirm by engine model, serial number, and the accessory-drive parts diagram.

Do I need one or two pulleys? If you are servicing an engine with two idler positions, some MerCruiser accessory-drive arrangements use two 807757T pulleys. Check the parts diagram for your engine to confirm the quantity.

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 tension can also overload the bearing and shorten pulley life.

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, covering a wide range of 4.3L through 8.2L applications.

Common mistake: 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, so a worn bushing can cause a new pulley to run off-axis and fail again quickly. Replacing the bushing alongside the pulley avoids a repeat failure.

Sizing and fitment: If you are unsure whether your engine needs one or two idler pulleys, confirm against your accessory-drive parts diagram before ordering, and don't confuse this 3 in. grooved pulley with flat or larger-diameter idlers used elsewhere in the drive system.