🔧 Valve Shim Calculator
Calculate the replacement shim thickness for bucket, shim-over-bucket, and lash-cap valve clearance checks using measured lash, target spec, and available shim increments.
📌 Presets
⚙ Calculator Setup
🎯 Results
🧪 Material and Spec Comparison
📊 Reference Tables
| Shim Formula Case | Measured Lash | Shim Change | Expected Result |
|---|---|---|---|
| Clearance too tight | Below spec | Use thinner shim | Lash increases |
| Clearance too loose | Above spec | Use thicker shim | Lash decreases |
| Clearance in range | Within spec | Often leave installed | Confirm trend |
| Unknown stamped size | Any reading | Mic the shim first | Avoid stack error |
| Common Shim Family | Typical Diameter | Stock Step | Use Note |
|---|---|---|---|
| Small motorcycle under-bucket | 7.48 mm | 0.05 mm | Compact DOHC heads |
| Mid motorcycle under-bucket | 9.48 mm | 0.05 mm | Many ATV and bike heads |
| Automotive bucket shim | 28 to 35 mm | 0.02 to 0.04 mm | Confirm by engine code |
| Shim-over-bucket disc | 25 to 29 mm | 0.05 mm | Secure during high rpm work |
| Lash cap | Stem matched | 0.001 in | Often race-service parts |
| Valve Type | Typical Cold Range | Target Habit | Reason |
|---|---|---|---|
| Motorcycle intake | 0.10 to 0.20 mm | Middle range | Stable cold sealing |
| Motorcycle exhaust | 0.20 to 0.30 mm | Middle to loose | More heat growth |
| Auto intake bucket | 0.15 to 0.25 mm | Middle range | Quiet operation |
| Auto exhaust bucket | 0.25 to 0.35 mm | Middle to loose | Protects hot seat margin |
| Measurement Check | Preferred Tool | Resolution | Shop Note |
|---|---|---|---|
| Lash clearance | Feeler gauge | 0.01 mm or 0.001 in | Use light drag |
| Shim thickness | Micrometer | 0.001 mm or 0.0001 in | Measure clean center |
| Cam position | Timing mark | Base circle contact | Lobe away from follower |
| Final verification | Same feeler set | Same unit system | Check after torque sequence |
💡 Tips
When adjusting engine valves, it is essential to calculate an correct shim thickness. Valve lash refer to the gap between the camshaft and the valve. It is imperative that the mechanic measures the valve lash correctly so that the engine will work correct.
If the valve lash is too small, the valves will not fully seat when the engine heat up. This can lead to burning of the valves. If the valve lash is too large, then the valvetrain will become noisy due to the components of the engine hammering against each other.
How to Measure Valve Gap and Choose the Right Shim
Thus, there must be some balance between the gap that is too small and the gap that is too large. Engines use metal components that will expand when the engine is hot and contract when it is cold. Intake engine valves will remain cooler than exhaust engine valves because the exhaust valves are in contact with hot engine exhaust.
Because of the way that engine valves expand when hot, exhaust valves require a larger gap or valve lash than intake valves. Many mechanics will aim for the middle of the valve lash specification that the engine manufacturer provides. This is to ensure that there is some margin for error in the case that the engine overheat and the valves expand.
If the specification set for the valve lash is too close to the minimum, the valves may become fully seated and the engine may fail. When measuring the valve lash, it is imperative to make sure that the mechanic measures the engine at the base circle of the camshaft. The base circle of the camshaft is the area where the cam lobe point away from the valve.
If the mechanic measures the valve lash in any other part of the engine, the measurement will be incorrect. Additionally, you should consider the temperature of the engine when measuring the valve lash. The manufacturer indicates the measurements for when the engine is cold.
Thus, if the engine is warm, the measurement will not be accurate if these specification are used. Once the valve lash and shim thickness are measured, it is necessary to calculate the thickness of the new shim. Using a calculator can help to avoid making arithmetic errors in calculating the thickness of the new shim.
The calculated thickness might not match with the thickness of the shim that the engine manufacturer sells. These shims are often sold in specific size increment, such as 0.05 mm increments. Thus, it is necessary to round the calculated thickness of the shim to one that the manufacturer sells.
Rounding to a thicker shim will result in a valve lash that is too small. Rounding to a thinner shim will result in a valve lash that is too large. The design of the engine will impact the way that the shims are adjusted.
For instance, the shims could be located under the bucket. In this case, you will have to remove the camshaft altogether to access the shim. Alternatively, the shims could be located over the bucket.
In this case, it will be possible to remove the clip that hold the bucket to the camshaft and simply replace the shim. Regardless of the design of the engine, the logic behind adjusting the shims is the same: a larger shim will result in a smaller valve lash and a smaller shim will result in a larger valve lash. It is not necessary to use the number that is stamped on a used shim to determine the thickness of that shim.
The thickness of the shim may have worn down to that thickness while the engine was working. Additionally, the number on the shim may be incorrectly stamped. Thus, you should use a micrometer to measure the thickness of the used shim.
By ensuring that the mechanic measures the thickness of the used shim correctly, the other step in this process are guaranteed to be accurate. If the engine has multiple cylinder, then it is essential to create a log that details which shim thickness is used for which valve. Valves may have different specification for thickness of shim; some valves might require a 2.40 mm shim while others require a 2.45 mm shim.
Without a log of the specifications for each valve, it is possible that the mechanic may place a thin shim where a thick shim is required. Finally, once all of the shims have been replaced, it is essential to measure the valve lash once more. As the mechanic torqued the camshaft caps to the appropriate specification, the valve lash could change.
Thus, it is essential to ensure that the valve lash remains within the specification that the engine manufacturer provided.
