Keeping a dirt bike running smoothly requires consistent maintenance, and one critical yet often misunderstood task is shimming the valves. When a dirt bike’s engine starts to lose performance, becomes harder to start, or makes unusual ticking noises, the valve clearances may be out of specification. Valves that are out of adjustment can lead to engine damage or poor efficiency, especially in high-performance off-road bikes. Knowing how and why to shim the valves on a dirt bike is key to maintaining long-term reliability and engine health.
Understanding Valve Shimming
Valve shimming is a process used to adjust the clearance between the camshaft and the valve lifters or buckets. In many modern dirt bikes, especially four-stroke models, this clearance is maintained with the help of small metal discs called shims. These shims sit between the valve and the cam lobe, and by changing their thickness, mechanics can fine-tune the spacing to the manufacturer’s specifications.
Why Valve Clearance Matters
The proper valve clearance ensures that the valves open and close at the right time and to the correct extent. If the clearance is too tight, the valves may not fully close, leading to compression loss and potential burning of the valve seats. If the clearance is too loose, it can result in noisy operation and accelerated wear on the valve train components.
- Too tight: valves won’t close properly, risking engine damage
- Too loose: noisy engine, increased mechanical wear
- Proper clearance: optimal engine performance and longevity
How to Know When Your Dirt Bike Needs Valve Shimming
Recognizing the signs of a dirt bike needing valve adjustment is crucial. These symptoms often develop over time and can affect both performance and rideability. If your dirt bike exhibits any of the following behaviors, it may be time to check the valve clearances and potentially shim the valves:
- Difficult or inconsistent starting, especially when cold
- Loss of engine power during acceleration
- Excessive engine noise, such as ticking or clattering sounds
- Reduced fuel efficiency
- Backfiring or misfiring under throttle
Shim Sizes and Valve Clearance Specifications
Each dirt bike model will have specific valve clearance tolerances provided by the manufacturer, usually given in millimeters. Shims come in a range of thicknesses, often increasing in increments of 0.05 mm. Before you begin the shimming process, it’s important to have the correct service manual and a feeler gauge set for measuring the gap.
Example Valve Clearances (Typical Ranges)
- Intake valves: 0.10 mm 0.15 mm
- Exhaust valves: 0.20 mm 0.25 mm
These numbers can vary based on engine design, so always refer to the factory specification for your dirt bike model.
The Valve Shim Adjustment Process
Shimming the valves on a dirt bike requires patience, mechanical knowledge, and the right tools. While it may seem intimidating at first, it becomes a straightforward task once understood. Here is a step-by-step overview of the process:
1. Remove Necessary Components
You’ll need access to the cylinder head, so parts like the fuel tank, seat, and valve cover must be removed. Carefully label and set aside each part as you work.
2. Rotate the Engine to Top Dead Center (TDC)
Use a socket and ratchet on the crankshaft bolt to rotate the engine until the timing marks align. This ensures the valves are in the correct position for accurate measurement.
3. Measure the Valve Clearances
Using a feeler gauge, measure the gap between the cam lobes and the lifters. Record each valve’s measurement for comparison to the factory specifications.
4. Remove the Camshafts
If the clearances are out of spec, remove the camshafts to access the shims. Be sure to mark the position of each cam and keep all parts organized to ensure correct reassembly.
5. Replace the Shims
Calculate the correct shim thickness needed by using the formula:
New Shim = Old Shim + (Measured Clearance – Desired Clearance)
Install the new shims, then reinstall the camshafts and recheck the clearances after torquing everything back to spec.
6. Reassemble the Engine
Once all valves are within the correct range, reassemble the valve cover, reconnect hoses and electrical connectors, and reinstall the tank and seat. Start the engine and listen for any unusual sounds.
Tools and Equipment Needed
Shimming valves on a dirt bike requires specific tools. Having the right equipment not only makes the process easier but also helps avoid mistakes that can damage the engine.
- Feeler gauge set
- Micrometer or digital caliper
- Torque wrench
- Shim kit (specific to your bike’s valve train)
- Service manual for valve specs and torque settings
- Ratchet and socket set
- Magnet tool for shim removal
How Often Should You Shim the Valves?
Valve clearance should be checked at regular service intervals, which can vary based on how aggressively the bike is ridden. For example, a race bike might need inspection every 1520 hours of ride time, while a trail bike may go 4050 hours before requiring adjustment.
Typical Valve Check Intervals
- Race use: every 1520 hours
- Recreational trail riding: every 4060 hours
- Initial break-in: usually recommended after the first 10 hours
Riding in extreme conditions, high RPM use, and aggressive throttle input will accelerate valve wear, making frequent checks more necessary.
Importance of Accurate Valve Maintenance
Neglecting to check and shim your valves can lead to catastrophic engine failure. Over time, valves wear and their clearances change, potentially causing them to hang open or not open enough. This not only reduces performance but also risks damage to the pistons, valve seats, and cylinder head components. Proper shimming ensures your dirt bike runs efficiently and helps extend the lifespan of the engine.
Should You Do It Yourself or Hire a Mechanic?
For riders who are mechanically inclined, shimming valves is a manageable DIY task. However, precision is crucial. If you’re unsure about taking on the job, it’s better to have a certified mechanic handle it. Labor costs can range between $150 and $400 depending on your location and the make of the dirt bike.
Ultimately, learning to shim your own valves can save money in the long term and deepen your understanding of how your dirt bike operates. Many riders find it rewarding and empowering to handle their own engine maintenance.
Shimming the valves on a dirt bike is an essential maintenance procedure that ensures your engine continues to perform at its best. While it requires attention to detail and the right tools, it is a worthwhile task for any serious rider. By understanding the signs of worn valve clearances, following a careful process, and using the correct shims, you can maintain engine health and avoid costly repairs. Whether you race, trail ride, or enjoy weekend adventures, keeping your valves in check is a smart investment in your dirt bike’s performance and longevity.