2021 Subaru Forester Serpentine Belt Guide

Quick Answer

Here are the key specifications and part numbers for the serpentine belt system on a 2021 Subaru Forester with the 2.5L FB25D engine. As always, it is best practice to confirm part numbers with your local Subaru dealer or preferred parts supplier before purchasing.

• Serpentine Belt Length: Approximately 2100mm
• Rib Count: 6-rib
• OEM Serpentine Belt Part Number: Subaru 809221020
• Aftermarket Belt Cross-References:
  • Gates: K060827
  • Dayco: 5060825
  • Continental Elite: 4060825
• Automatic Tensioner Assembly Part Number: Subaru 23769AA05A
• Idler Pulley Part Number: Subaru 23770AA07A

Belt Routing

The serpentine belt on the 2021 Subaru Forester follows a specific path to drive all the engine's accessories. Before you even think about removing the old belt, take a clear photograph of the current belt routing with your phone. This simple step can save you a lot of headache when it's time to install the new one.

Here is the step-by-step routing for the 2021 Forester's 2.5L engine:

1. The belt starts at the crankshaft pulley at the bottom of the engine.
2. From the crankshaft pulley, it goes up to the A/C compressor on the passenger side of the engine.
3. Next, it routes to the idler pulley, which is located just above the A/C compressor.
4. From the idler pulley, the belt moves across the top of the engine to the alternator on the driver's side.
5. It then goes down to the power steering pump (though on modern electric power steering systems, this may be an idler in the same position).
6. From there, it wraps around the water pump pulley.
7. Finally, the belt goes to the automatic tensioner, which is typically located between the crankshaft pulley and the A/C compressor, before returning to the crankshaft pulley. The tensioner is the component you will articulate to get the belt on and off.

How to Replace the Serpentine Belt

Replacing the serpentine belt on a 2021 Forester is a manageable task for a DIYer with the right tools. You will need a serpentine belt tool, which is a long, thin ratchet designed for this job, or a standard 3/8" drive breaker bar.

1. Safety First: Park the vehicle on a level surface and engage the parking brake. For added safety, disconnect the negative terminal of the battery.
2. Locate the Tensioner: Find the automatic tensioner assembly. It will have a 3/8" or 1/2" square hole in it to accept your breaker bar or serpentine belt tool.
3. Relieve Tension: Insert the tool into the tensioner and rotate it to release the tension on the belt. The tensioner is spring-loaded, so you will need to apply steady pressure.
4. Remove the Old Belt: While holding the tensioner in the released position, carefully slide the old belt off one of the pulleys. An easily accessible pulley like the alternator is a good choice. Once the belt is off one pulley, you can release the tensioner and remove the belt completely.
5. Inspect Pulleys: With the belt removed, now is the perfect time to inspect the tensioner and idler pulleys. Spin them by hand. They should spin smoothly and quietly. If you feel any roughness, wobbling, or hear any grinding noises, the pulley is bad and should be replaced.
6. Install the New Belt: Route the new belt around the pulleys according to the diagram you photographed earlier. Make sure the ribbed side of the belt is seated correctly in the grooves of each pulley. The last pulley to loop the belt over should be a smooth one, like the water pump or idler, as this is usually the easiest.
7. Apply Tension: Once the belt is routed correctly, use your serpentine belt tool to move the tensioner and create enough slack to slide the belt over the final pulley.
8. Final Check: Slowly release the tensioner, making sure the belt is properly seated in all the pulley grooves. Give the belt a final inspection to ensure it is routed correctly and not twisted.
9. Reconnect and Start: Reconnect the battery terminal, start the engine, and let it run for a minute to make sure the belt is tracking correctly.

Replacement Interval

Subaru recommends inspecting the serpentine belt at regular service intervals. While there is no hard-and-fast replacement mileage, a good rule of thumb is to replace the serpentine belt every 60,000 to 100,000 miles, or sooner if it shows signs of wear.

Signs of a Failing Belt or Tensioner

Your car will usually give you some warning signs that the serpentine belt or tensioner is on its way out.

• Squealing or Chirping Noises: This is the most common symptom of a worn or loose belt. The noise may be more noticeable on cold starts or when the A/C is running.
• Visible Wear: Inspect the belt for cracks, fraying, glazing (a shiny appearance), or missing ribs. Any of these are signs that the belt needs to be replaced immediately.
• Chattering or Grinding from the Tensioner: A failing tensioner can make a chattering or grinding noise. You may also see the tensioner pulley vibrating excessively while the engine is running.
• Accessory Malfunctions: Since the serpentine belt drives multiple accessories, a failing belt can cause issues with the charging system (alternator), power steering, or air conditioning.

A worn serpentine belt is not something to ignore. If it breaks, your engine will lose power to critical components and you will be stranded. Regular inspection and timely replacement are key to keeping your Forester reliable.

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