CHECKING & ADJUSTING VALVE CLEARANCE: MR KNOW-IT-ALL

Mr. Know-it-all,
I’m a converted two-stroke guy and hate to admit it, but I’ve never adjusted the valves on my YZ250F. I look at the O.E. manual and get intimidated. Is there a more plain English procedure on-line that you can recommend?
Buster Morgan
via [email protected]

Checking and adjusting valves is no more difficult than changing a two-stroke top end. The following steps were from a story in the print edition of Dirt Bike, as described the the guys at JE Pistons. No matter what type of off-road four-stroke you own, checking clearances at the specified intervals is incredibly important to ensure the engine continues to run optimally and lasts a long time. Also, as a rule of thumb, any time the top end of the engine is disassembled, it is best practice to check valve clearances.

1. Any time you have the top end apart to replace the piston, you should check your valve clearance and adjust as necessary.
Before servicing your engine, you will need your machine’s factory service manual. The service manual is crucial because it specifies the required clearances, torque specs and other information imperative to performing the task. The outline we’re providing should be considered supplemental
to the information in your service manual and is in
no way a comprehensive substitute.
To tackle this job,
you’ll typically need
the following tools
and supplies:

  • —Lash/feeler gauges
  • —Metric wrenches
  • —Metric sockets
  • —Clean rags or towels
  • —Screwdrivers
  • —Caliper

In most cases, specialty tools aren’t necessary; however, if they are, you’ll find that information in your service manual.

2. A critical tool to measuring valve clearance is a set of feeler gauges. Since the engine is going to be partially opened up and exposed, it is best to work on a clean machine. If your machine is dirty, take the time to clean it thoroughly so the risk of contaminating the engine with debris is lessened. Prioritize cleaning the cylinder head cover and surrounding area.

3. Chances are you’re not working on a new bike, so be sure the area around the cam cover is clean to avoid unwanted debris. We’ll begin outlining the procedure with the removal of the cylinder head cover. You’ll likely need to remove your seat, fuel tank and various other components before this. These items should be easy to remove, and your service manual should provide sufficient guidance. When removing the cylinder head cover, be extremely careful not to allow dirt to fall into the cylinder head.

4. If you’re working on an engine still in the bike, you’ll need to remove your seat and tank, along with any other components hindering your access to the cam cover.
Next, they will need to be positioned so that the clearances can be checked. Most service manuals specify setting the valve train so that the piston is at top dead center (TDC) on the compression stroke. Setting the valve train at this position ensures that the cam, or cams, are on their base circles, and that neither the intake nor exhaust valves are open. The base circle of the cam is the circular portion of the cam that does not influence valve lift.
As an aside and for future reference, while it is sensible to follow the service manual’s recommendations on setting the piston position and engine stroke when the engine is assembled, it is not necessary, especially when working on an engine that is being rebuilt. Checking valve clearance can also be accomplished with the cylinder head removed from the engine and positioning the cam lobes opposite the lifter buckets to ensure the clearance measurements are taken with the cam on its base circle.

5. Whether the head is still on the engine or you’re working on it separately, be sure the engine is either at TDC or the cam lobes are resting somewhere on their base circle and not applying pressure to the buckets like they would when opening valves. Your service manual outlines the required procedure to set the engine on its compression stroke at TDC. Most engines have mating alignment marks on the crankshaft and engine case, as well as the cam gear and cylinder head. It is imperative that you know and understand how to utilize these reference points because they are used to correctly set the cam timing after any valve clearance adjustments have been made.
Once you’ve positioned the cams correctly, valve clearance measurements can be made using lash (feeler) gauges. Lash gauge measurements can be tricky due to surrounding geometry and inexperience on the user’s part. To obtain the most accurate measurement, it is essential that the lash gauge is inserted between the cam and lifter bucket as close to parallel as possible. To facilitate parallel entry, bend the lash gauges as necessary so that their tips can easily slide between the cam and lifter bucket.

6. Whether the head is still on the engine or you’re working on it separately, be sure the engine is either at TDC or the cam lobes are resting somewhere on their base circle and not applying pressure to the buckets like they would when
opening valves.
Your service manual outlines the required procedure to set the engine on its compression stroke at TDC. Most engines have mating alignment marks on the crankshaft and engine case, as well as the cam gear and cylinder head. It is imperative that you know and understand how to utilize these reference points because they are used to correctly set the cam timing after any valve clearance adjustments have been made.
Once you’ve positioned the cams correctly, valve clearance measurements can be made using lash (feeler) gauges. Lash gauge measurements can be tricky due to surrounding geometry and inexperience on the user’s part. To obtain the most accurate measurement, it is essential that the lash gauge is inserted between the cam and lifter bucket as close to parallel as possible. To facilitate parallel entry, bend the lash gauges as necessary so that their tips can easily slide between the cam and lifter bucket.

7. Begin by using the gauge equal to the median-recommended valve clearance measurement in your manual. You may have to move up or down a couple sizes until you find the size that slides between the cam lobe and bucket with a slight drag. Record this measurement for each valve.
After each of the intake and exhaust valve clearances has been recorded, they should be compared to the service specifications outlined in your service manual. If the valve clearances fall within the manufacturer’s recommended range, no further work is required; however, if the clearances are outside of the specifications, determining what adjustments need to be made is the next step. To do this, unless the current valve shim thicknesses are known, the cylinder head will have to be disassembled so that the shims can be removed and measured.
Follow the necessary procedures outlined in your service manual to slacken the cam chain, and remove the cam cap, cams and lifter buckets. When removing the cam cap, be sure to follow any recommended removal/tensioning sequences. Once the cam chain is free, use a piece of wire to secure it to the cylinder head. If it happens to fall in the chain case, a pen magnet can be used to fish it out.

8. Be sure to slacken the cam chain before attempting removal. Remove the camshaft(s) and secure the cam chain so it doesn’t fall in the cases.

To remove the lifter buckets, a pen magnet or valve lapping tool are both excellent options. When extracting the lifter buckets from their bores, be very careful, and keep tabs on whether or not the valve shim sticks to the underside of the bucket. Oil underneath the lifter buckets makes sticking shims a common occurrence.

9. Use a pen magnet or lapping tool to remove the buckets. Be careful of shims that may stick on the underside of buckets. Through engine operation, the lifter buckets mate to their respective bores, so they should never be mixed around. To help keep track of things, draw out a simple cylinder head diagram on a piece of paper so that the lifter buckets and all the measurements can be tracked. Remove any remaining valve shims from the cylinder head. Once the valve shims have been removed, measure the shim thicknesses and the diameter of shims used.

10. Drawing a simple diagram can help you keep track of what buckets and shims came from where. Once everything is removed, confirm your shim measurements.

To determine what valve shim adjustments should be made, a simple formula is used:

New Shim Thickness = Recorded Clearance – Specified Clearance + Old Shim Thickness

Calculate the necessary new shim thicknesses for all the clearances that are out of spec. Valve shims are available from most OEMs, but helpful shim kits that come with an assortment of sizes are also available from the aftermarket. Before sourcing shims, you’ll need to determine the diameter of the shim you need, because there are a handful of different shim diameters used within the industry. Shown below are the standard shim diameters.

Size:   (mm) Manufacturer
7.48:   Japanese
9.48:   Japanese
8.90:   KTM
10.00:   KTM

11. Shim assortment kits are available from various aftermarket suppliers. Just be sure you know what shim diameter your machine takes before ordering. This kit was sourced from ProX Racing Parts.
When calculating what new shim thicknesses are required, it is best to target the specified clearance on the upper end of the prescribed clearance range, because valve clearances usually diminish over time. Valve shims are available in 0.025mm increments, so the shims that can be utilized will also influence the new clearances that can be achieved.
Once you have the correct shims in hand, the valve train can be reassembled. Use engine oil to lubricate the valve shims and carefully install them. The lifter buckets should also be lubed before installation. When inserting the lifter buckets into their respective bores, ensure that the buckets bottom on the shims and at no point come back up. If the bucket comes back up upon installation, occasionally the shim will stick to it and become displaced. The engine can quickly be severely damaged if the shim is not seated correctly between the valve stem and lifter bucket.

12. Using engine oil and assembly lube when reassembling your shims, buckets, and cams to help prevent premature wear and also helps your shims stay in place while re-inserting buckets.
Pay close attention to your service manual during installation of the cams and when setting cam timing. Double-check that the crankshaft is in its correct position. If you’re working on a twin-cam engine, it is best to install the camshaft that resides opposite of the chain tensioner first (typically the exhaust cam). Pull the chain taut from the crankshaft. Orient the cam gear correctly, and then wrap the chain around the gear. Once this is accomplished, the remaining cam can be oriented correctly and the chain wrapped around it. Double-check orientation of all components and that timing has been set correctly. Be sure to use engine oil to lube the cam bearing bores upon installation.

13. Make sure your timing marks on your crankshaft are lined up, then reinstall your cam(s). It’s important to make sure the timing marks on the crankshaft and cam(s) remain lined up simultaneously when reinstalling the cam chain.

When installing the cam cap, ensure the torque specs and sequences outlined in your service manual are followed. Deviations from either can cause the cam bearings to wear prematurely. Once the cams have been secured, use lash gauges to confirm the new valve clearances match the clearances that were calculated. Any deviations should be carefully scrutinized, because they may be indicative of calculation errors or shims that are not seated correctly. If there is a hint of a problem at this point, it is imperative that it is thoroughly understood and corrected before proceeding.

14. Be sure to follow the correct torque sequence and specifications when reinstalling cam caps.
Assuming everything checks out, the cam chain can be tensioned. Follow the procedure outlined in your service manual. Once the tension has been set, rotate the engine through at least four complete revolutions to help the automatic chain tensioners to set the correct initial tension and confirm that the engine has been timed correctly. Position the piston at TDC on the compression stroke and check that all timing features on the crank and cams remain in their specified positions.
Complete the job by carefully reinstalling the cylinder head cover, making sure to torque those bolts in a star sequence to recommended specs. Once the rest of the machine is buttoned up, it’s time to get back to riding!

 

 

 

 

 

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