Subaru EJ25 Engine Build Tutorial - Step by Step Guide

Introduction

This tutorial provides a comprehensive step-by-step guide for building a Subaru EJ25 engine, based on a detailed YouTube tutorial. Whether you're a seasoned mechanic or a DIY enthusiast, this guide will help you understand the necessary tools, measurements, and assembly techniques to successfully build your engine.

Chapter 1: Tools Needed

Before beginning your engine build, gather the following tools and components:

Essential Parts

• Case Halves: Use a set of 703 castings; send them to a trusted machine shop like Outfront Motorsports for machining.
• Crankshaft: Use a nitrated crankshaft from a 2018+ STI.
• Case Bolts: Brand new zinc-coated bolts or ARP bolts (requires line boring).
• Pistons: CP Pistons (9975 in diameter).
• Connecting Rods: Manley H tough rods for strength.
• Bearings: King coated bearings.
• Piston Rings: Essential for sealing.

Required Tools

• Measuring Tools: Calipers, micrometers, and a dial bore gauge.
• Torque Wrenches: 3/8 inch and half inch.
• Assembly Tools: Rubber mallet, piston ring compressor, and an ARP lubricant.
• Cleaning Supplies: Parts cleaner, brake cleaner, paper towels, and compressed air.

Specialty Tools

• Dial Bore Gauge: For measuring bearing clearances.
• Rod Bolt Stretch Gauge: Optional for high-power builds.

Chapter 2: Crankshaft Measurements

1. Calibrate Measuring Tools: Zero out your gauge to ensure accurate measurements.
2. Clean Crankshaft: Wipe down the crankshaft journals with parts cleaner.
3. Measure Main Bearings:
   • Use micrometers to measure each journal.
   • Record measurements carefully to ensure consistency.
4. Verify Measurements: Repeat measurements to confirm accuracy.

Chapter 3: Case Assembly

1. Prepare Case Halves: Remove any shipping bolts and lightly tap to separate.
2. Install Bearings:
   • Clean journal areas and install the main bearings.
   • Ensure the thrust bearing is properly positioned.
3. Torque Case Halves Together:
   • Follow the specified torque sequence and specs, starting at 7.4 lb-ft and working up to 14.8 lb-ft.
   • Complete multiple rounds of torqueing, including 90-degree increments.

Chapter 4: Main Bearing Clearance

1. Measure Bearing Clearances:
   • Use a dial bore gauge to check clearances against crankshaft measurements.
   • Target a clearance of 0.002 to 0.004 inches.
2. Adjust Bearings if Necessary: Mix and match standard and oversized bearings as needed to achieve desired clearances.

Chapter 5: Connecting Rod Install

1. Prepare Connecting Rods:
   • Ensure tangs on bearings are aligned (tang to tang).
   • Clean the bearing surfaces.
2. Torque Connecting Rods:
   • Use ARP lube and torque to specified values (start at 30 lb-ft, then 55 lb-ft, and finally 90 lb-ft).

Chapter 6: Case Prep & Crank Install

1. Clean and Lubricate: Wipe down bearings and crankshaft with assembly lube.
2. Install Crankshaft: Position the crankshaft into the case and align the connecting rods.
3. Seal and Bolt Case Halves: Apply sealant and bolt both halves together, ensuring a proper seal.

Chapter 7: Piston Ring Cutting

1. Measure and Cut Rings:
   • Measure top and oil scraper rings for proper gap (0.014" for the top ring).
   • Use a piston ring cutter and check gaps with feeler gauges.
2. Install Rings: Ensure proper orientation and fit of all rings on the pistons.

Chapter 8: Piston Install

1. Lubricate and Insert Pistons:
   • Apply oil to piston skirts.
   • Carefully slide the piston into the cylinder, aligning with the connecting rod.
2. Secure Wrist Pin: Insert the wrist pin and secure with clips.

Conclusion

By following this tutorial, you are now equipped with the knowledge to build a Subaru EJ25 engine. Remember to double-check all measurements and torque specifications throughout the process. For your next steps, consider installing additional components and preparing the engine for final assembly into your vehicle. Happy building!