Understanding Valve Gaps in Subaru Ex21 Engines: A Comprehensive Guide

Subaru ex21 engines are renowned for their high-quality engineering and durability. However, to keep these engines running smoothly, proper maintenance is essential. One often-overlooked aspect of Subaru ex21 engine maintenance is valve gaps. In this article, we will provide a comprehensive guide to help you understand what valve gaps are, why they matter, and how to measure them in your Subaru ex21 engine.

What is a Valve Gap?

A valve gap is the space or clearance between the valve stem and rocker arm. This space is necessary to allow for thermal expansion of the engine and prevent the valve from staying open when the engine is hot. When the engine is cold, the valve gap is wider than when it’s hot. A valve gap that’s too wide or too narrow can affect the engine’s performance and cause significant damage over time.

Why is a Valve Gap Important for Subaru Ex21 Engine?

The Subaru ex21 engine is a high-performance engine that requires regular maintenance to continue functioning at optimal levels. A valve gap that’s too large or too small can cause fuel inefficiency, loss of power, and even engine damage. A valve that stays open for too long can result in burned exhaust valves, which can cause costly engine damage.

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How to Measure Valve Gap in Subaru Ex21 Engine?

Measuring valve gap in a Subaru ex21 engine is a simple process that involves a feeler gauge and a wrench. Here’s how to check the valve gap:

  1. Remove the spark plug cover and spark plug from the cylinder head.
  2. Rotate the engine’s flywheel to bring the piston up to Top Dead Centre (TDC) on the compression stroke. You can use a piston stopper tool to help locate TDC.
  3. Once you have found TDC, use a feeler gauge to check the clearance between the valve stem and rocker arm. The correct valve clearance specifications can be found in the engine’s manual.
  4. Adjust the valve clearance using the appropriate wrench if the clearance is not within the specified range.
  5. Reinstall the spark plug and spark plug cover.

What is the Ideal Valve Gap for Subaru Ex21 Engine?

The ideal valve gap for a Subaru ex21 engine varies depending on the engine’s specification. Generally, the intake valve gap should be between 0.17mm to 0.20mm (.006" to .008") and the exhaust valve gap should be between 0.25mm to 0.28mm (.010" to .011"). It is essential to check the engine’s manual for specific valve clearance specifications.

What Can Happen if the Valve Gap is Incorrect?

A valve gap that’s too wide can cause the engine to lose power, while a gap that’s too narrow can cause the engine to misfire. If left unaddressed, these issues can lead to a decrease in fuel efficiency, decreased engine performance, and even engine damage. A valve that stays open for too long due to incorrect valve gap adjustment can cause the exhaust valve to overheat and warp, requiring costly engine repairs.

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Q: Can I use a different feeler gauge size to adjust valve gaps?
A: No, the engine’s manual specifies the correct valve clearance and the specific feeler gauge must be used.

Q: How often should I check valve gaps in my Subaru ex21 engine?
A: Valve gaps should be inspected and adjusted every 300 hours of operation or every two years, whichever comes first.

Q: Can’t I just leave the valve gaps as they are?
A: Ignoring valve gaps can lead to decreased engine performance, inefficient fuel consumption, misfires, and engine damage.


Proper engine maintenance is essential to keep a high-performance engine, such as the Subaru ex21, running smoothly. Valve gaps are a crucial aspect of engine maintenance that should not be overlooked. Checking and adjusting valve gaps at regular intervals will ensure optimal engine performance, increase fuel efficiency, and prevent costly engine repairs. By following the steps outlined in this comprehensive guide, you can keep your Subaru ex21 engine running smoothly for years to come.

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Peter Banks

With years of experience as a professional mechanic and Subaru specialist, Peter is one of the most respected members of our team. He's written several articles on Subaru maintenance and repair, and his advice and tips are always practical and helpful. When he's not working on cars, he enjoys cooking and trying out new recipes.

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