2021 GMC Sierra 2500HD: Serpentine Belt

Quick Answer

• Belt Length & Rib Count: 103.5 inches, 6-rib (Gas V8) / 88.0 inches, 10-rib (Diesel V8)
• OEM Belt PNs: 12686036 (Gas V8) / 12686035 (Diesel V8)
• Aftermarket Cross-Refs (Gas V8): Gates K061035, Dayco 5061035, Continental 4061035
• Aftermarket Cross-Refs (Diesel V8): Gates K100880HD, Dayco 5100880, Continental 4100880
• Auto-Tensioner PN: 12670921 (Gas V8) / 12693241 (Diesel V8)
• Idler Pulley PN: 12629232 (Gas V8) / 12693242 (Diesel V8)

Belt Routing

Before removing the old belt, we strongly recommend taking a clear photograph of the existing serpentine belt routing. This will be an invaluable reference. The belt snakes around a number of pulleys in a specific order to power all of the engine's accessories.

Gas V8 (6.6L)

Starting from the crankshaft pulley at the bottom of the engine, the belt travels up and to the right to the A/C compressor, then down and around the idler pulley. From the idler, it goes up to the alternator, then across to the power steering pump. Next, it routes down to the water pump, and finally to the tensioner before returning to the crank pulley.

Diesel V8 (6.6L)

The diesel engine has a slightly different accessory layout. The belt starts at the crankshaft pulley, goes up to the A/C compressor, then to the idler pulley. From there it wraps around the alternator, down to the power steering pump, across to the water pump, and finally around the tensioner before returning to the crankshaft.

How to Replace the Serpentine Belt

Replacing the serpentine belt is a manageable DIY job for many owners. You will need a serpentine belt tool or a 3/8" breaker bar. Locate the automatic belt tensioner; it has a 3/8" square hole for your tool. Rotate the tensioner to relieve pressure on the belt, then slide the old belt off the pulleys. When installing the new belt, refer to your photo or the routing description to ensure it is correctly seated on every pulley. A misaligned belt can be damaged or thrown from the pulleys.

Replacement Interval

While there is no universally mandated replacement interval, it is wise to inspect the serpentine belt at every oil change. A general guideline for replacement is every 60,000 to 100,000 miles, or when signs of wear appear.

Signs of a Failing Belt or Tensioner

• Squealing or Chirping Noises: This is often the first sign of a worn or slipping belt.
• Visible Wear: Look for cracking, fraying, or glazing on the belt surface.
• Tensioner Pulley Wobble: A wobbling tensioner pulley indicates a failing bearing.
• Reduced Accessory Performance: Dimming headlights or a stiff steering wheel can be a sign of a slipping belt.

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