Understanding Valve Shims and Clearances
Valve shims are small, precisely machined discs or pellets used in overhead cam engines to set the correct clearance between the camshaft lobe and the valve stem. This clearance, often referred to as valve lash, is critical for the proper operation and longevity of your engine. Without the correct clearance, valves can either not close fully or open too far, leading to a host of problems.
The correct valve clearance allows for thermal expansion of engine components. As the engine heats up, metal parts expand. If the clearance is too tight, the valve may remain slightly open when hot, leading to burnt valves and loss of compression. If the clearance is too loose, the valve train will be noisy, and the valve may not open fully, reducing engine performance and causing excessive wear on components.
Why Adjust Valve Clearances?
Regular valve clearance adjustment is a vital part of engine maintenance, particularly for motorcycles, ATVs, and older car engines that use mechanical tappets or shims. Over time, wear and tear on valve train components, such as valve seats, valve faces, and cam lobes, can alter the initial factory settings.
- Too Tight Clearance: This is the more dangerous scenario. A valve that doesn't fully close when the engine is hot can lead to hot exhaust gases escaping past the valve seat, burning the valve face and seat. This results in permanent damage, loss of compression, and ultimately, a need for costly cylinder head repair. It also reduces engine power and efficiency.
- Too Loose Clearance: While generally less damaging than a tight clearance, a loose clearance causes excessive noise (often described as a ticking or clattering sound). It also means the valve doesn't open as much or for as long as it should, reducing engine breathing, power output, and fuel efficiency. Furthermore, the impact from the cam lobe on the tappet/shim can be more severe, accelerating wear.
The Valve Shim Calculator
Our valve shim calculator simplifies the process of determining the correct replacement shim thickness. Instead of manual calculations that can be prone to error, this tool provides an accurate result based on your measurements and desired specifications.
How to Use the Calculator
Follow these simple steps to get your new shim thickness:
- Current Shim Thickness (mm): Measure the thickness of the shim currently installed in the valve bucket. Use a micrometer for the most accurate reading.
- Measured Valve Clearance (mm): With the engine cold, use a feeler gauge to measure the actual clearance between the camshaft lobe and the valve bucket (or tappet).
- Desired Valve Clearance (mm): Refer to your engine's service manual for the manufacturer's specified ideal valve clearance. This value is crucial for optimal engine performance and longevity.
- Click "Calculate New Shim" to instantly get the recommended new shim thickness.
The Calculation Formula Explained
The calculator uses a straightforward but essential formula to determine the required shim change:
New Shim Thickness = Current Shim Thickness + Measured Valve Clearance - Desired Valve Clearance
Let's break it down:
- If your measured clearance is larger than the desired clearance, it means the current shim is too thin. The formula will add the difference (Measured - Desired) to your current shim, resulting in a thicker new shim.
- If your measured clearance is smaller than the desired clearance, it means the current shim is too thick. The formula will subtract the difference (Desired - Measured) from your current shim, resulting in a thinner new shim.
The "Shim Change Needed" value tells you how much thicker or thinner the new shim needs to be compared to the old one. A positive value means you need a thicker shim, a negative value means a thinner shim.
Step-by-Step Guide to Valve Adjustment (Beyond the Calculator)
While the calculator handles the math, the physical process requires precision and patience. Here's a general overview:
Preparation
- Tools: Gather a micrometer, feeler gauges, camshaft holding tool (if necessary), torque wrench, and a set of valve shims if you have them.
- Engine Cold: Always perform valve clearance measurements on a completely cold engine (preferably overnight).
- Access: Remove the fuel tank, fairings, valve cover, and any other components obstructing access to the camshafts and valve buckets.
Measuring Current Clearance
- Rotate the crankshaft to bring each cylinder to Top Dead Center (TDC) on its compression stroke for accurate measurement of its valves.
- Carefully insert a feeler gauge between the camshaft lobe and the valve bucket. The feeler gauge should slide in with a slight drag. Note down the tightest gauge that fits and the loosest that doesn't.
- Repeat for all valves, ensuring you keep track of which measurement belongs to which valve (e.g., Cylinder 1 Intake Left).
Identifying Current Shim
- Once all clearances are measured, remove the camshaft(s). This usually involves loosening bearing caps in a specific sequence.
- Use a magnetic pickup tool to remove the valve buckets, revealing the shims underneath.
- Measure the thickness of each removed shim with a micrometer. Record this alongside your measured clearance for that specific valve.
Installing New Shim
- Use the calculator with your recorded 'Current Shim Thickness', 'Measured Valve Clearance', and the 'Desired Valve Clearance' from your service manual to find the 'New Shim Thickness'.
- Select a new shim that matches the calculated thickness as closely as possible. Shims are typically available in 0.05mm increments. Always aim for a shim that brings you within the manufacturer's specified range.
- Install the new shim, reassemble the valve bucket, camshaft(s), and camshaft bearing caps, torquing them to the manufacturer's specifications.
- Crucially, re-measure the valve clearance after reassembly to verify the adjustment is correct. It's not uncommon for clearances to change slightly upon re-torquing.
Important Considerations and Tips
- Engine Temperature: Always measure clearances on a stone-cold engine.
- Precision is Key: Use a high-quality micrometer and feeler gauges. Even a 0.01mm error can lead to an incorrect shim choice.
- Shim Availability: Shims come in specific increments (e.g., 2.00, 2.05, 2.10 mm). You may need to choose the closest available shim that keeps the clearance within the acceptable range.
- Multiple Checks: Measure, calculate, install, and then re-measure. Never skip the re-measurement step.
- Documentation: Keep a detailed log of your measurements, calculations, and the shims you installed for future reference.
- Manufacturer's Specifications: Always consult your engine's service manual for the exact desired valve clearances and torque specifications.
Common Mistakes to Avoid
- Rushing the Process: Valve adjustments require patience and attention to detail. Rushing can lead to incorrect measurements or assembly errors.
- Incorrect TDC: Not ensuring the piston is at TDC on the compression stroke for the cylinder you're working on will lead to inaccurate clearance measurements.
- Using Worn Feeler Gauges: Feeler gauges can wear down over time. Ensure yours are in good condition.
- Ignoring Torque Specs: Over or under-tightening cam bearing caps can distort the camshaft or cylinder head, affecting clearances and causing damage.
- Not Re-checking: Failing to re-measure clearances after installing new shims is a critical oversight.
Conclusion
Maintaining proper valve clearance is fundamental to the health and performance of your engine. While the process can seem daunting, with the right tools, careful measurement, and the aid of this valve shim calculator, you can confidently perform this essential maintenance task. Regular adjustment ensures your engine runs smoothly, efficiently, and avoids costly damage, extending its lifespan and keeping you on the road or trail.