Valve Shim Calculator
Your expert tool for precise engine valve clearance adjustments.
Calculate Required Shim Size
Enter the current clearance you measured with a feeler gauge.
Please enter a valid positive number.
Enter the thickness of the shim currently installed in the engine.
Please enter a valid positive number.
Enter the manufacturer’s specified clearance from your service manual.
Please enter a valid positive number.
Clearance Comparison Chart
Common Shim Size Availability (7.48mm Diameter)
| Size (mm) | Size (mm) | Size (mm) | Size (mm) | Size (mm) |
|---|---|---|---|---|
| 1.200 | 1.600 | 2.000 | 2.400 | 2.800 |
| 1.250 | 1.650 | 2.050 | 2.450 | 2.850 |
| 1.300 | 1.700 | 2.100 | 2.500 | 2.900 |
| 1.350 | 1.750 | 2.150 | 2.550 | 2.950 |
| 1.400 | 1.800 | 2.200 | 2.600 | 3.000 |
| 1.450 | 1.850 | 2.250 | 2.650 | 3.050 |
| 1.500 | 1.900 | 2.300 | 2.700 | 3.100 |
| 1.550 | 1.950 | 2.350 | 2.750 | 3.150 |
What is a valve shim calculator?
A valve shim calculator is an essential tool for mechanics and DIY enthusiasts who perform engine maintenance, specifically valve clearance adjustments. This type of calculator simplifies the process of determining the correct replacement shim thickness required to bring an engine’s valve clearances back into the manufacturer’s specified range. Instead of complex manual calculations, a valve shim calculator allows you to input your measured values and instantly get the precise shim size needed for the job. This is crucial for ensuring optimal engine performance, longevity, and efficiency.
This tool is primarily used for engines with a “shim-under-bucket” or “shim-over-bucket” valvetrain design, common in high-performance and motorcycle engines. A common misconception is that all engines require this adjustment. However, many passenger cars use hydraulic lifters that automatically adjust for wear, making a valve shim calculator unnecessary for those vehicles.
Valve Shim Calculator Formula and Mathematical Explanation
The calculation to find the new shim size is straightforward. The core idea is to find the difference between your measured clearance and the ideal target clearance, and then apply that difference to the thickness of the shim you currently have installed. The formula is as follows:
New Shim Size = (Measured Clearance – Target Clearance) + Current Shim Size
This formula ensures your engine runs smoothly. For a more detailed walkthrough, consider an engine tuning guide. The valve shim calculator automates this process to prevent mathematical errors. Here is a breakdown of the variables involved:
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| New Shim Size | The thickness of the replacement shim you need to install. | mm | 1.200 – 3.500 |
| Measured Clearance | The current gap you measured between the cam lobe and the valve bucket. | mm | 0.05 – 0.50 |
| Target Clearance | The manufacturer’s specified ideal gap (found in the service manual). | mm | 0.10 – 0.30 |
| Current Shim Size | The thickness of the shim you removed from the engine. | mm | 1.200 – 3.500 |
Practical Examples (Real-World Use Cases)
Example 1: Intake Valve Clearance is Too Tight
A tight valve clearance (too small a gap) is dangerous as it can prevent the valve from fully closing, leading to a loss of compression and potentially a burnt valve. Let’s use the valve shim calculator for this scenario.
- Inputs:
- Measured Clearance: 0.08mm
- Current Shim Size: 2.70mm
- Target Clearance (Intake): 0.15mm
- Calculation: New Shim = (0.08mm – 0.15mm) + 2.70mm = -0.07mm + 2.70mm = 2.63mm
- Interpretation: The calculator recommends a 2.63mm shim. Since shims are often sold in 0.05mm increments, you would purchase a 2.65mm or 2.60mm shim, depending on your preference for being on the tighter or looser side of the spec. A looser valve is generally safer than a tighter one.
Example 2: Exhaust Valve Clearance is Too Loose
A loose valve clearance (too large a gap) can cause a “ticking” or “tapping” noise from the engine and can lead to increased wear on valvetrain components. This is a common issue covered in troubleshooting engine noises guides.
- Inputs:
- Measured Clearance: 0.35mm
- Current Shim Size: 2.40mm
- Target Clearance (Exhaust): 0.25mm
- Calculation: New Shim = (0.35mm – 0.25mm) + 2.40mm = 0.10mm + 2.40mm = 2.50mm
- Interpretation: The valve shim calculator indicates a 2.50mm shim is needed. This is a common size and should be readily available, perfectly correcting the excessive clearance.
How to Use This valve shim calculator
Our valve shim calculator is designed for simplicity and accuracy. Follow these steps for a flawless experience:
- Measure Your Clearance: First, following your service manual’s procedure, use a feeler gauge to measure the existing gap for a specific valve. Enter this value into the “Measured Valve Clearance” field. Learn more by reading about how to measure valve clearance.
- Identify Current Shim: Carefully remove the valve bucket and the shim underneath. Use a digital caliper or micrometer to measure its thickness. Enter this number into the “Current Shim Size” field.
- Enter Target Clearance: Consult your vehicle’s service manual for the specified valve clearance range. It’s best to aim for the middle of that range. Enter this value into the “Target Valve Clearance” field.
- Read the Results: The valve shim calculator will instantly display the “Required New Shim Size.” This is the size you need to purchase and install to achieve the target clearance. The intermediate values and chart provide additional context for your adjustment.
- Reset and Repeat: Use the “Reset” button to clear the fields and begin the process for the next valve.
Key Factors That Affect Valve Shim Calculator Results
Accurate results from a valve shim calculator depend on precise inputs. Several factors can influence your measurements and the final outcome:
- Engine Temperature: Valve clearance must be measured at the temperature specified by the manufacturer (usually a cold engine). Metal expands when hot, so a measurement on a warm engine will be inaccurate.
- Feeler Gauge Technique: The feeler gauge should slide in with a slight drag. If it’s too loose or forced, the measurement will be incorrect. This is a critical step in any motorcycle maintenance routine.
- Measurement of Old Shim: The size printed on an old shim may not be accurate due to wear. Always measure the old shim with a precise tool like a micrometer.
- Component Wear: The valve face, seat, and stem tip all wear over time, which typically causes clearances to tighten. This is the primary reason adjustments are needed.
- Correct Target Specification: Always use the specification from the factory service manual for your exact make, model, and year. Using the wrong spec will result in incorrect clearance. For deep engine work, always consult an engine rebuild checklist.
- Camshaft Position: Ensure the cam lobe is pointing directly away from the valve bucket when you take the measurement (typically at Top Dead Center on the compression stroke). Misalignment can lead to false readings. Understanding camshaft specifications is key.
Frequently Asked Questions (FAQ)
1. What happens if my valve clearance is incorrect?
If the clearance is too tight, valves may not fully close, causing low compression, power loss, poor starting, and potentially burnt valves. If it’s too loose, it can cause a tapping noise, reduced performance, and accelerated wear of valvetrain components. Using a valve shim calculator helps prevent this.
2. How often should I check my valve clearances?
This varies greatly by engine type and manufacturer. High-performance motorcycle engines might require checks every 8,000-15,000 miles, while some touring bikes can go much longer. Always refer to your owner’s service manual for the correct interval.
3. Can I stack valve shims?
No, you should never stack shims. Stacking creates a risk of a shim dislodging at high RPM, which would cause catastrophic engine failure. Always use a single, correct-sized shim as determined by the valve shim calculator.
4. Where do I buy new shims?
Shims can be purchased from motorcycle dealerships, specialized engine parts suppliers, or online motorsport stores. They are often sold individually or in kits containing a wide range of sizes.
5. Why is my valve clearance always getting tighter?
As the engine runs, the valve face and valve seat slowly wear away. This process, known as “valve recession,” causes the valve to sit slightly higher in the cylinder head, reducing the clearance between the stem and the cam. A valve shim calculator is the tool to correct this.
6. What tools do I need to adjust my valves?
You will need a set of feeler gauges, a micrometer or digital caliper, a socket set to turn the engine over and remove the valve cover, and potentially special tools to depress the valve bucket, depending on the engine design.
7. Is a noisy valve (tappet) a happy tappet?
This is an old saying that has some truth to it. A slightly loose valve (making a light ticking noise) is far safer than a silent, tight valve. However, excessive noise indicates the clearance is too large and should be corrected. The goal is to be within the manufacturer’s specified range, which our valve shim calculator helps you achieve.
8. Does this calculator work for both intake and exhaust valves?
Yes, the formula is the same for both. However, intake and exhaust valves typically have different target clearance specifications. Be sure to use the correct target value in the valve shim calculator for the specific valve you are adjusting.