2025 Subaru Forester — Serpentine Belt Routing & Length

The 2025 Subaru Forester carries on Subaru’s tradition of reliability, utilizing the proven 2.5L FB25 Boxer four-cylinder engine. While the 2025 model introduces a new generation for the Forester, the accessory drive system remains a critical maintenance item that every owner or technician needs to understand.

As an ASE-certified master technician, I’ve replaced countless belts on Subaru Boxer engines. Below is your comprehensive guide to the serpentine belt routing, part numbers, and replacement procedures for the 2025 Subaru Forester.

Quick Answer: Specs & Part Numbers

Before diving under the hood, here are the essential specifications and part numbers for the 2025 Subaru Forester (2.5L engine). Note: Always verify exact part numbers with your VIN, as mid-year production changes can occur.

• Belt Length: Approximately 66.5 inches (1688 mm)
• Rib Count: 6-rib (6PK)
• OEM Subaru Belt PN: 23780AA140 (or superseding number)
• Gates Cross-Reference: K060664
• Continental Elite Cross-Reference: 4060664
• Dayco Cross-Reference: 5060665
• OEM Auto-Tensioner PN: 23769AA050
• OEM Idler Pulley PN: 23770AA070

Belt Routing

Because modern Subarus utilize Electric Power Steering (EPS), you will not find a power steering pump on the accessory drive. The serpentine belt on the 2.5L FB25 engine is responsible for driving the alternator, A/C compressor, and the water pump.

Pro-Tip: Always take a clear photograph of your existing belt with your smartphone before removing it. It will save you a headache if you get confused during reassembly.

Text-Based Routing Path:

1. Start by looping the ribbed side of the belt around the bottom of the Crankshaft Pulley.
2. Route the belt straight up and over the ribbed Water Pump Pulley (located above and slightly to the left of the crank when facing the engine).
3. Bring the belt down and underneath the smooth Auto-Tensioner Pulley.
4. Pull the belt up and over the ribbed Alternator Pulley at the top center/right.
5. Route the belt down from the alternator, under the smooth Idler Pulley.
6. Finally, wrap it around the ribbed A/C Compressor Pulley on the lower right, bringing it back to the crankshaft.

How to Replace the Serpentine Belt

Replacing the serpentine belt on a 2025 Forester is a relatively straightforward job thanks to the top-mount alternator and accessible tensioner.

Tools Needed:

• Serpentine belt tool or a long 3/8" drive breaker bar / 14mm socket (depending on the tensioner bolt head).
• Safety glasses and gloves.

Step-by-Step Procedure:

1. Locate the Tensioner: Find the automatic belt tensioner assembly located on the front of the engine, just below the water pump and alternator.
2. Release Tension: Place your 14mm socket and long-handled ratchet or breaker bar onto the tensioner pulley bolt. Rotate the tensioner counter-clockwise (towards the driver's side) to compress the internal spring and relieve belt tension.
3. Remove the Belt: While holding the tensioner open, slip the old belt off the alternator pulley (the easiest one to reach). Slowly release the tensioner back to its resting position. Remove the belt completely.
4. Inspect Pulleys: Spin all the pulleys by hand. The idler, tensioner, alternator, and water pump pulleys should spin smoothly without grinding, wobbling, or excessive noise.
5. Install the New Belt: Route the new belt following the path described above (or using your reference photo), leaving the alternator pulley for last.
6. Apply Tension: Rotate the tensioner counter-clockwise again, slip the belt over the alternator pulley, and gently release the tensioner.
7. Verify Alignment: Double-check that all belt ribs are perfectly seated in the grooves of the ribbed pulleys. Start the engine and observe the belt to ensure it is running smoothly.

Replacement Interval

Subaru typically recommends inspecting the serpentine belt every 30,000 miles or 30 months. However, the standard replacement interval for the serpentine belt on the 2.5L FB25 engine is usually between 60,000 and 90,000 miles. If you live in a region with extreme temperatures, harsh winters, or dusty conditions, you may need to replace it closer to the 60,000-mile mark.

Signs of a Failing Belt or Tensioner

Don't wait for the belt to snap on the highway. Look out for these common symptoms of accessory drive failure:

• Squealing or Chirping Noises: A high-pitched squeal on startup or during acceleration usually indicates a stretched or glazed belt, or a weak tensioner spring.
• Visible Wear: Look for cracks, missing chunks of ribs, fraying edges, or a shiny "glazed" appearance on the belt's surface.
• Accessory Malfunctions: A slipping belt can cause the alternator to undercharge the battery (battery light on the dash), or cause the A/C to blow warm air. If the belt snaps, the water pump will stop turning, leading to rapid engine overheating.
• Tensioner Bounce: While the engine is idling, look at the auto-tensioner. A slight vibration is normal, but excessive bouncing or a rattling noise indicates the hydraulic/spring mechanism inside the tensioner has failed.

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