Choosing the Correct Parts to Rebuild Your Subaru EJ25 Engine

The Subaru WRX STI with its EJ25 engine is beloved by enthusiasts for its robust performance and rally-inspired pedigree. However, like any high-performance engine, it’s not immune to issues over time, such as a blown head gasket or a spun rod bearing. When it’s time to rebuild, knowing the right parts to select is crucial to restoring performance and reliability. This guide will walk you through the essential parts you may need based on the type of failure and considerations for a comprehensive rebuild.

Understanding the EJ25 Engine

The EJ25 engine features a 2.5L horizontally opposed (Boxer) 4-cylinder design with a turbocharger. It’s known for its high power output, symmetrical all-wheel-drive pairing, and tunability. However, it’s also known for specific vulnerabilities, such as head gasket failures and rod bearing issues, especially under high-stress conditions or poor maintenance.

Scenario 1: Rebuilding After a Blown Head Gasket

A blown head gasket is a common issue in EJ25 engines, often caused by overheating or age-related wear. Here’s what you’ll need for this type of rebuild:

Essential Parts:

1. Head Gaskets:

   • Use multi-layer steel (MLS) gaskets, such as OEM Subaru or aftermarket brands like Cometic or Fel-Pro.

   • Ensure the gasket matches your engine’s bore size and compression requirements.

2. Cylinder Head Bolts or Studs:

   • Replace the factory head bolts with OEM or ARP head studs for improved clamping force and durability.

3. Timing Belt Kit:

   • If your engine has not had the timing belt replaced recently, this would be a good time to change it. Include a new timing belt, tensioner, idlers, and water pump to ensure reliable operation.

4. Intake and Exhaust Gaskets:

   • Replace these to ensure no leaks after reassembly.

5. Coolant and Thermostat:

   • Flush the cooling system and replace the thermostat to avoid future overheating issues.

6. Oil and Filters:

   • Change the oil and filters to remove contaminants that may have entered during the failure.

7. Cylinder Head Resurfacing:

   • Resurface the heads may be required to ensure a perfect seal with the new gaskets.

Optional Upgrades:

• High-Performance Radiator: To help prevent future overheating.

• Oil Cooler: To manage engine temperatures during spirited driving.

 

Scenario 2: Rebuilding After a Spun Rod Bearing

A spun rod bearing is more severe and usually results from oil starvation or degradation. This failure often necessitates a complete bottom-end rebuild. Here’s what you’ll need:

Essential Parts:

1. Crankshaft (if damaged) :

   • Inspect for damage. Replace or machine the crankshaft as necessary.

2. Cylinder Head Bolts or Studs:

   • Replace the factory head bolts with OEM or ARP head studs for improved clamping force and durability.

3. Connecting Rods (if damaged) :

   • Replace damaged rods with OEM or aftermarket forged options, such as Manley or Eagle.

4. Rod Bearings and Main Bearings:

   • Use high-quality bearings like OEM or ACL Race Bearings/King Engine Bearings for durability under stress.

5. Pistons and Piston Rings:

   • Inspect for scoring or wear. Replace with OEM or forged pistons if increasing boost levels.

6. Oil Pump:

   • Replace with OEM or Upgrade to a high-flow oil pump to ensure proper lubrication.

7. Seals and Gaskets:

   • Use a complete engine gasket kit to ensure no leaks after assembly.

8. Cam Shafts and Timing Components:

   • Inspect and replace as necessary.

9. Engine Block Inspection:

   • Check for damage to the block. A spun bearing may require line boring or even block replacement.

Additional Considerations:

• Replace Oil-Contaminated Components:

  • A spun rod bearing often results in metal shavings contaminating the oil. Replace any components that come into contact with engine oil, including:

    • Oil pump (must replace)

    • Oil pan (can be cleaned in some cases but we recommend replacing)

    • Oil pickup (must replace)

    • AVCS solenoids (can be cleaned)

    • AVCS cam gears (can be cleaned)

    • Turbocharger (can be cleaned in some cases)

• Short Block Replacement:

  • In some cases, the best option is to replace the damaged engine with a new short block. Upgraded options like IAG short blocks provide improved durability and performance.

Optional Upgrades:

• Forged Internals: For increased durability under higher power output.

• Performance Oil Pan: To reduce the risk of oil starvation. (IAG or Killer B)

• ARP Fasteners: For additional strength in high-performance builds.

 

General Recommendations for Any EJ25 Rebuild

• Replace Wear Items: Always replace wear items like belts, hoses, and filters during a rebuild.

• Inspect All Components: Have a machine shop inspect the block, heads, and rotating assembly to ensure they’re within spec.

• Consider Upgrades: If you’re rebuilding, consider upgrading components to prevent future issues, especially if you plan to increase power output.

• Use Quality Parts: Stick to trusted brands, whether OEM or aftermarket, to ensure reliability.

• Follow Torque Specs: Always use Subaru’s torque specifications and procedures during reassembly.

Conclusion

Rebuilding the EJ25 in your Subaru WRX STI can restore it to its former glory or even take it to the next level of performance. Whether you’re dealing with a blown head gasket or a spun rod bearing, or somthing else, selecting the right parts is essential to ensure a successful rebuild. By investing in quality components and addressing all potential failure points, you’ll enjoy a reliable, high-performance engine for years to come.