Valve Shim Calculator
Your essential tool for precise engine valve clearance adjustments.
Select the unit used for all measurements.
The current gap measured with a feeler gauge. (mm)
The ideal clearance from your service manual. (mm)
The thickness of the shim currently installed. (mm)
What is a Valve Shim Calculator?
A valve shim calculator is a specialized tool used by mechanics and engine builders to determine the correct size of a valve shim needed to adjust the valve clearance in an internal combustion engine. Valve clearance, or valve lash, is the small gap between the camshaft lobe (or rocker arm) and the valve stem. This gap is critical for engine performance, efficiency, and longevity. If the gap is too small, valves may not close properly, leading to a loss of compression and potential damage. If it’s too large, the valvetrain will be noisy and performance will suffer.
This calculator is essential for anyone performing an engine valve adjustment on engines that use a “shim-over-bucket” or “shim-under-bucket” design, common in high-performance motorcycle and automotive engines.
Valve Shim Calculator Formula and Explanation
The calculation is based on a simple formula that compares the measured clearance to the specified clearance and adjusts the current shim size accordingly.
Formula:
New Shim Size = Current Shim Size - Measured Clearance + Specified Clearance
Or, put another way:
New Shim Size = Current Shim Size + (Specified Clearance - Measured Clearance)
| Variable | Meaning | Unit (Auto-inferred) | Typical Range |
|---|---|---|---|
| New Shim Size | The thickness of the replacement shim required to achieve the target clearance. | mm or thou | 1.20 – 3.50 |
| Current Shim Size | The measured thickness of the shim that is currently in the engine. | mm or thou | 1.20 – 3.50 |
| Measured Clearance | The actual valve gap you measured with a feeler gauge. | mm or thou | 0.00 – 0.50 |
| Specified Clearance | The target valve gap recommended by the engine manufacturer’s service manual. | mm or thou | 0.10 – 0.30 |
Practical Examples
Example 1: Tight Intake Valve (mm)
A mechanic is working on a motorcycle and finds the intake valve clearance is too tight.
- Inputs:
- Measured Valve Clearance: 0.08 mm
- Specified Valve Clearance: 0.15 mm
- Current Shim Size: 2.60 mm
- Calculation:
New Shim Size = 2.60 mm - 0.08 mm + 0.15 mm = 2.67 mm - Result:
The mechanic needs a 2.67 mm shim. Since shims usually come in 0.05mm increments, they would choose the closest available size, likely a 2.65 mm or 2.70 mm shim, to bring the clearance within the acceptable range.
Example 2: Loose Exhaust Valve (thou)
An engine builder is checking an exhaust valve and finds the clearance is too large, causing a ticking noise.
- Inputs:
- Measured Valve Clearance: 12 thou (0.012″)
- Specified Valve Clearance: 8 thou (0.008″)
- Current Shim Size: 110 thou (0.110″)
- Calculation:
New Shim Size = 110 thou - 12 thou + 8 thou = 106 thou - Result:
The required new shim is 106 thou (0.106″). This is a crucial step in any quality engine building tools workflow.
How to Use This Valve Shim Calculator
Follow these steps to accurately determine your required shim size:
- Select Units: First, choose whether you are working in Millimeters (mm) or Thousandths of an Inch (thou). Ensure all your inputs use this same unit.
- Enter Measured Clearance: Using feeler gauges as per your service manual, measure the current gap for the valve. Input this value into the “Measured Valve Clearance” field. Our guide on feeler gauge techniques can help.
- Enter Specified Clearance: Look up the target valve clearance in your engine’s service manual. This might be a range (e.g., 0.15mm – 0.20mm); if so, it’s often best to aim for the middle of that range. Enter the target value.
- Enter Current Shim Size: Remove the existing valve shim and measure its thickness with a micrometer. Even if it’s marked, wear can alter its size. Input this value.
- Calculate: Click the “Calculate New Shim Size” button.
- Interpret Results: The calculator will display the ideal new shim size. It also shows intermediate values like the clearance difference to help you understand the adjustment needed.
Key Factors That Affect Valve Clearance
Several factors can influence valve clearance, making regular checks important.
- Engine Wear: Over time, the valve face and seat wear down, which typically causes the valve to sit higher, reducing clearance.
- Engine Temperature: Valve clearance is always measured when the engine is cold. As the engine heats up, metal components expand, which changes the clearance. The specified cold clearance is designed to result in the correct clearance at operating temperature.
- Camshaft Lobe Wear: Wear on the camshaft lobes can increase the valve clearance. This is less common than valve seat wear.
- Shim and Bucket Wear: The shim itself or the valve bucket can wear over time, affecting the clearance. This highlights the importance of using a micrometer to measure the existing shim instead of relying on its printed size.
- Carbon Buildup: Deposits on the valve face or seat can prevent the valve from closing completely, effectively reducing the clearance.
- Improper Installation: Errors during a previous valve adjustment, like misreading a feeler gauge or using the wrong shim, will lead to incorrect clearances. Understanding how to measure valve clearance properly is vital.
Frequently Asked Questions (FAQ)
What happens if my valve clearance is too tight?
If the clearance is too small (or zero), the valve may not fully seat against the cylinder head. This can cause a loss of compression, poor idling, difficulty starting, and, in severe cases, can lead to burnt valves and expensive engine damage.
What happens if my valve clearance is too loose?
Excessive clearance results in a noisy valvetrain (a “ticking” or “tapping” sound), reduced performance due to altered valve timing and lift, and increased wear on valvetrain components like the cam lobes and valve stem tips.
Why are exhaust valve clearances usually larger than intake clearances?
Exhaust valves run much hotter than intake valves. They expand more significantly at operating temperature. The larger cold clearance ensures that they still have adequate clearance when hot to close properly.
How often should I check my valve clearances?
This varies greatly by engine model. Consult your owner’s or service manual. High-performance motorcycle engines may require checks every 8,000 to 15,000 miles, while some car engines can go 100,000 miles or more. A comprehensive motorcycle maintenance guide will have specific intervals.
Can I swap shims between different valves?
Yes! This is a common practice. If you are adjusting multiple valves, measure and record all current shims and calculated required shims. You may find that a shim you removed from one valve is the exact size needed for another, saving you time and money.
My calculated shim size isn’t available. What should I do?
Valve shims are typically sold in increments of 0.025mm or 0.050mm. If your calculated size is 2.47mm, for example, you must choose between a 2.45mm or 2.50mm shim. Check your service manual for the acceptable clearance range. Choose the shim that puts your final clearance within that range, ideally towards the looser end to account for future valve seat wear.
Do I really need a micrometer to measure the old shim?
Yes. While shims are marked with their size, this number can wear off, and the shim itself can wear down over thousands of miles. Using a micrometer is the only way to get an accurate measurement for the `Current Shim Size` input in the valve shim calculator.
Which is better: ‘mm’ or ‘thou’ for measurements?
Neither is better; it depends on your service manual and personal preference. Most modern (especially Japanese and European) vehicle manuals use millimeters (mm). Some American manuals may use thousandths of an inch (thou). The most important thing is to be consistent and not mix units.