FA20 Engine Assembly Guide

The FA20 engine, found in popular models like the Subaru BRZ and Toyota GT86, is a marvel of modern engineering. While not overly complex, its assembly requires precision, patience, and a thorough understanding of its unique characteristics. This guide will walk you through the essential stages of building an FA20 engine, drawing on the experience of a recent FR-S engine build.

Preparing Your FA20 Engine Components

Before diving into the assembly process, the first and most crucial step is meticulous preparation. All components must be thoroughly cleaned and inspected. This isn't just about aesthetics; it's about ensuring that no debris or foreign material compromises the integrity of your freshly built engine. Laying out all your parts methodically will help you stay organised and prevent any mix-ups during the build. Pay close attention to the crankshaft journals. You might notice a bluish discolouration; this is a result of induction hardening, a factory process designed to increase the surface hardness of the journals, thereby improving their resistance to wear. The light can catch this blue hue, but rest assured, it's a sign of durability, not a defect. The internal sections of the crank remain softer and more ductile, providing essential strength.

Measuring and Setting Bearing Clearances

Accurate bearing clearance is paramount for engine longevity and optimal performance. This is where precision measurement tools and careful calculation come into play. In the FA20 build, the process began with assembling the case halves, ensuring the King XP main bearings were correctly seated. Once torqued, the bore diameters of the main bearing housings were meticulously measured. Following this, the crank journals themselves were measured. The critical calculation involves subtracting the crank main journal diameter from the main bearing bore diameter. This difference gives you the total bearing clearance. For aluminium blocks like the FA20, these clearances are typically quite small. This is due to the inherent expansion of aluminium with heat; the clearances are designed to accommodate this expansion without compromising lubrication or creating excessive play.

Adjusting Bearing Clearances with King Bearings

Should the initial measurements indicate that the bearing clearances are outside the acceptable range, adjustments can be made. The King bearing range offers various shell thicknesses. By selecting different sized shells, or even mixing and matching shells within the same set, minor adjustments to the clearance can be achieved. This flexibility allows for fine-tuning the engine's internal tolerances to meet the manufacturer's specifications or your specific build requirements.

Crankshaft Preparation: The Foundation of Smooth Operation

The crankshaft is the heart of the engine, converting reciprocating motion into rotational motion. Its preparation is therefore critical for smooth operation and power delivery. For the FA20 engine featured in this build, the crankshaft preparation was relatively straightforward. After undergoing a Magnaflux crack check to ensure there were no hidden fractures, the crankshaft was sent for balancing. A balanced crankshaft minimises vibrations, leading to a smoother-running engine and reduced stress on other components. Finally, the journals received a light polish. This polishing process ensures a smooth surface for the bearings to run on, further reducing friction and wear.

Key Stages of FA20 Engine Assembly

Once all components are prepped and measured, the actual assembly can commence. This is a methodical process, and it's vital to follow the correct sequence:

1. Installing the Crankshaft and Bearings

With the main bearing clearances confirmed, the crankshaft can be carefully placed into the lower crankcase. Ensure the correct King main bearings are in their respective positions, lubricated with assembly lube. The upper crankcase half is then fitted, and the main bearing caps are torqued to the manufacturer's specifications. It's crucial to use a torque wrench for this step and to follow the specified torque sequence to ensure even clamping force.

2. Piston and Connecting Rod Assembly

The pistons and connecting rods are typically assembled before being fitted to the crankshaft. Piston rings are installed onto the pistons, ensuring they are oriented correctly and gapped appropriately. The connecting rod big-end bearings are then installed onto the connecting rod journals, again with assembly lube. Each piston and connecting rod assembly is then carefully inserted into its corresponding cylinder bore. The connecting rod caps are then torqued down onto the crankshaft journals, completing the installation for each cylinder.

3. Cylinder Head Installation

The cylinder heads house the valves, camshafts, and spark plugs. They are installed onto the engine block, with new head gaskets in place. The head bolts are then torqued down in a specific sequence and to specific torque values, often involving a multi-stage tightening process (e.g., torque to yield). This ensures a proper seal between the cylinder head and the engine block, preventing coolant and oil leaks and maintaining compression.

4. Valvetrain Assembly

With the cylinder heads in place, the valvetrain components can be installed. This includes the camshafts, lifters, valves, and springs. Proper timing is absolutely essential here. The timing system, whether it's a timing belt or a timing chain, must be installed and tensioned precisely according to the manufacturer's service manual. Incorrect timing can lead to severe engine damage, including piston-to-valve contact.

5. Ancillary Components and Final Assembly

The final stages involve fitting all the ancillary components: the oil pump, water pump, intake manifold, exhaust manifold, sensors, wiring harness, and any other external parts. Once all these are in place and connected, the engine is essentially assembled.

Common FA20 Engine Assembly Challenges and Tips

While the FA20 is generally considered manageable for experienced DIY mechanics, there are a few points to be aware of:

• Torque Specifications: Always adhere strictly to the manufacturer's torque specifications and sequences. Overtightening or undertightening can lead to component failure.
• Cleanliness: As stressed earlier, absolute cleanliness is non-negotiable. Even a tiny piece of grit can cause significant damage.
• Lubrication: Use appropriate assembly lubricant on all bearing surfaces, camshaft lobes, and cylinder walls before initial startup.
• Timing: Double and triple-check the engine timing. This is one of the most critical steps.
• Piston Ring Gapping: Ensure piston rings are gapped correctly for the specific application, especially if you're using forged pistons.

Frequently Asked Questions

Is the FA20 engine difficult to assemble?

The FA20 engine is not excessively difficult to assemble, but it requires precision, attention to detail, and a good understanding of engine mechanics. It's more complex than some older, simpler engines, but manageable for those with prior engine building experience or who follow a detailed guide meticulously.

What are the key considerations for FA20 bearing clearances?

The key consideration is maintaining the correct, typically small, clearances specified by Subaru/Toyota. These are crucial for proper lubrication and to account for the thermal expansion of the aluminium block. Using the correct bearing shell thicknesses from reputable brands like King is essential for achieving these clearances.

What is induction hardening on the FA20 crankshaft?

Induction hardening is a heat treatment process that uses electrical resistance to locally heat the crankshaft journals. This makes the journal surfaces much harder and more resistant to wear, significantly extending the life of the crankshaft and bearings. The bluish tint is a visual indicator of this process.

Can I mix and match different bearing shells on the FA20?

Yes, it is acceptable to mix and match different King bearing shell thicknesses for minor adjustments to achieve the precise bearing clearance required. However, ensure you understand the implications of each thickness and maintain consistency where necessary.

What is the most critical step in assembling an FA20 engine?

While many steps are critical, engine timing is arguably the most critical. Incorrect timing can lead to catastrophic engine failure, such as pistons hitting valves. Always follow the service manual precisely for this step.

Building an FA20 engine is a rewarding experience that can breathe new life into your vehicle. By following these guidelines, paying close attention to detail, and utilising quality components, you can achieve a successful and robust engine build.