Valve Shim Calculator
Calculate Required Valve Shim
Enter the measured clearance, desired clearance, and the current shim size to find the required shim. Ensure all measurements are in the same unit (e.g., mm).
Clearance Difference: — mm
Measured Clearance: 0.20 mm
Desired Clearance: 0.15 mm
Current Shim: 2.50 mm
If the measured clearance is greater than desired, you need a thicker shim. If it’s less, you need a thinner shim.
Calculation Summary
| Parameter | Value (mm) |
|---|---|
| Measured Clearance | 0.20 |
| Desired Clearance | 0.15 |
| Current Shim | 2.50 |
| Clearance Difference | 0.05 |
| Required Shim | 2.55 |
What is a Valve Shim Calculator?
A valve shim calculator is a tool used primarily by mechanics and engine builders to determine the correct size of shim needed to adjust the valve clearance in an engine, particularly those with shim-over-bucket or shim-under-bucket valve actuation systems. Valve clearance is the small gap between the camshaft lobe (or rocker arm) and the valve stem (or lifter/bucket). This gap is crucial for proper engine operation, allowing for thermal expansion of engine components.
Over time, valve train components wear, and the clearance can change. If the clearance is too small, valves may not fully close, leading to loss of compression and burnt valves. If it’s too large, it can cause noisy operation, reduced performance, and increased wear. The valve shim calculator helps correct these clearances by calculating the thickness of the shim required to bring the clearance back within the manufacturer’s specified range.
Anyone working on engines with shim-adjusted valves, from DIY motorcycle enthusiasts to professional automotive technicians, should use a valve shim calculator after measuring the existing clearances. Common misconceptions are that it’s a very complex process (it’s methodical but manageable with the right tools) or that small deviations don’t matter (they do, for engine health and performance).
Valve Shim Calculator Formula and Mathematical Explanation
The calculation for the required valve shim size is straightforward:
Required Shim Size = Current Shim Size + (Measured Clearance - Desired Clearance)
Here’s a step-by-step breakdown:
- Measure the Current Clearance: Using a feeler gauge, measure the gap between the cam lobe and the lifter bucket (or rocker arm and valve stem, depending on the design) when the valve is fully closed.
- Determine the Desired Clearance: Consult the vehicle’s service manual for the specified valve clearance range. You’ll typically aim for the middle of this range.
- Identify the Current Shim Size: Remove the existing shim and measure its thickness with a micrometer or read the size marked on it.
- Calculate the Difference: Subtract the desired clearance from the measured clearance. A positive difference means the gap is too large; a negative difference means it’s too small.
- Calculate the New Shim Size: Add the difference to the current shim size. If the difference was positive (gap too large), you’ll add to the current shim size, resulting in a thicker shim. If the difference was negative (gap too small), adding the negative number effectively subtracts, resulting in a thinner shim.
For example, if you measure 0.25mm, desire 0.15mm, and have a 2.50mm shim:
Difference = 0.25mm – 0.15mm = 0.10mm
New Shim = 2.50mm + 0.10mm = 2.60mm
Variables Used
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Measured Clearance | The clearance measured with a feeler gauge | mm (or inches) | 0.05 – 0.50 mm |
| Desired Clearance | The target clearance from the manufacturer’s specs | mm (or inches) | 0.08 – 0.30 mm |
| Current Shim | The thickness of the shim currently installed | mm (or inches) | 1.20 – 3.50 mm |
| Required Shim | The calculated thickness of the new shim needed | mm (or inches) | 1.20 – 3.50+ mm |
Practical Examples (Real-World Use Cases)
Example 1: Intake Valve Adjustment
A motorcycle mechanic is working on an engine where the specified intake valve clearance is 0.10mm – 0.15mm. They measure the clearance of one intake valve and find it to be 0.22mm. The current shim installed is 2.75mm.
- Measured Clearance: 0.22 mm
- Desired Clearance (aiming for middle): 0.13 mm
- Current Shim: 2.75 mm
Using the valve shim calculator formula:
Required Shim = 2.75 + (0.22 – 0.13) = 2.75 + 0.09 = 2.84 mm
Shims are often available in 0.05mm increments, so the mechanic would look for a 2.85mm shim as the closest available size to slightly tighten the clearance towards the desired value.
Example 2: Exhaust Valve Adjustment
On the same engine, the exhaust valve clearance spec is 0.20mm – 0.25mm. The mechanic measures 0.17mm on an exhaust valve with a 2.60mm shim installed.
- Measured Clearance: 0.17 mm
- Desired Clearance (aiming for middle): 0.23 mm
- Current Shim: 2.60 mm
Using the valve shim calculator formula:
Required Shim = 2.60 + (0.17 – 0.23) = 2.60 – 0.06 = 2.54 mm
The mechanic would look for a 2.55mm or 2.50mm shim to increase the clearance.
How to Use This Valve Shim Calculator
- Gather Information: Before using the valve shim calculator, you need your engine’s service manual for the correct valve clearance specifications (intake and exhaust, cold or hot), a feeler gauge set, and tools to remove and measure the existing shims (like a micrometer).
- Measure Clearance: Ensure the engine is at the correct temperature (usually cold) and position (e.g., TDC on compression stroke) as specified in the manual. Carefully measure the clearance for each valve.
- Enter Measured Clearance: Input the value you measured into the “Measured Clearance” field of the valve shim calculator.
- Enter Desired Clearance: Input the target clearance (from the manual, typically mid-range) into the “Desired Clearance” field.
- Enter Current Shim Size: Remove and measure (or read the size) of the existing shim for that valve and enter it into the “Current Shim Size” field.
- Calculate: The valve shim calculator will instantly show the “Required New Shim Size”.
- Select New Shim: Shims come in specific size increments (e.g., 0.05mm or 0.025mm). Choose the available shim size closest to the calculated value to achieve a clearance within the specified range.
- Install and Re-check: Install the new shim and re-measure the clearance to confirm it’s within spec.
Understanding the results: The calculator tells you the ideal shim size. If the exact size isn’t available, you’ll choose the closest one that keeps the clearance within the acceptable range provided by your manufacturer.
Key Factors That Affect Valve Shim Calculator Results
- Measurement Accuracy: The precision of your feeler gauge measurement is critical. Inaccurate measurement leads to incorrect shim calculation. Using a quality feeler gauge correctly is vital.
- Engine Temperature: Valve clearances are specified at a certain engine temperature (usually cold). Measuring at the wrong temperature will give false readings.
- Tool Quality: The accuracy of your micrometer for measuring shim thickness also affects the final result.
- Manufacturer Specifications: Always refer to the service manual for the correct desired clearance range. Using generic values for your specific engine displacement or type can be wrong.
- Shim Availability: Shims come in discrete sizes. You may need to choose the closest available size to the calculated value.
- Component Wear: Wear on the valve seat, valve face, or cam lobe can affect clearance over time and may require more frequent checks even after using the valve shim calculator. Understanding valve clearance is key.
Frequently Asked Questions (FAQ)
- Q: What happens if valve clearance is too tight?
- A: If the clearance is too small (tight), the valve may not fully close when the engine is hot, leading to a loss of compression, poor running, and potentially burnt valves and valve seats.
- Q: What happens if valve clearance is too loose?
- A: If the clearance is too large (loose), it can cause a ticking noise from the valve train, reduced engine performance, and increased wear on valve train components like the cam lobes and lifters. You might hear valve train noise.
- Q: How often should I check valve clearances?
- A: Refer to your vehicle’s service manual. It typically ranges from every 8,000 to 30,000 miles, depending on the engine design and manufacturer recommendations. Our motorcycle maintenance schedule guide has more.
- Q: Do I need a special tool to use the valve shim calculator?
- A: The valve shim calculator itself is just for the math. To get the input values, you need feeler gauges, a micrometer (or the ability to read shim sizes), and tools to access the valves and shims.
- Q: Can I reuse old shims?
- A: Yes, if they are in good condition and the correct size needed for another valve. Always measure them before reusing.
- Q: What units does the valve shim calculator use?
- A: This calculator assumes all inputs are in millimeters (mm). If your measurements are in inches, you’ll need to convert them to mm before using the calculator or use a calculator that handles inches.
- Q: Why is desired clearance a range?
- A: Manufacturers specify a range to account for manufacturing tolerances and slight variations. Aiming for the middle of the range is usually best.
- Q: What if the calculated shim size is not available?
- A: Choose the closest available shim size that will result in a clearance within the manufacturer’s specified range. If you are between two sizes, consider whether it’s better to be slightly tighter or looser within the range, based on the engine and general advice (often slightly looser is preferred over too tight).
Related Tools and Internal Resources
- Engine Oil Calculator: Calculate the right amount of oil for your engine.
- Motorcycle Maintenance Schedule: Understand when to check valve clearances and perform other maintenance.
- Compression Ratio Calculator: Learn about another important engine parameter.
- How to Use Feeler Gauges: A guide on accurately measuring small gaps.
- Understanding Valve Clearance: An in-depth look at why valve clearance is important.
- Engine Displacement Calculator: Calculate your engine’s displacement.