Checking and adjusting valves on a motorcycle can be intimidating to some, however it is a fairly straightforward maintenance item that makes for a great DIY job, saving you from an expensive trip to the stealership’s shop. This article focuses on shim and bucket valve adjusters. Screw-and-locknut are another prominent type of adjuster. Desmodromic Valves and Hydraulic Valve Lifters are also found in motorcycles but are much less common. They all have their own benefits and downsides and you can read more about them in this informative Common Tread Post.
What is Valve Clearance and Why So Damn Important?
Valve Clearance, aka Valve Lash, is the gap between the cam lobe and the top of the valve assembly. As the cam lobe spins it operates the valves, allowing for proper combustion and exhaust. If the clearance is not big enough the bike will loose performance and potentially overheat and your valve could melt (yikes). So in other words, worst case scenario of unmaintained valve clearances is catastrophic failure. On the other hand, if the gap is too large the valve will be hammered during operation, resulting in a valve tick and eventually cause wear.
The Screw-and-locknut type adjuster is by far the easiest to maintain as an owner, but the bucket and shim system is not to be feared. A few years ago, a crotchety old motorcycle mechanic cringed when I told him I was doing an adjustment on my old F650 (Shim-Over-Bucket). He was of the school that only Screw-and-locknut should be attempted by a DIY’er. I just shrugged back and gave him the Big Lebowski “Yeah, well, you know, that’s just, like, your opinion, man.” He isn’t around the shop where I get my safeties done anymore, but I would love to tell him how easy it was the first time I tried it.
Valve check intervals vary widely depending on the bike you are riding, but generally you should be checking them every 15,000 miles or 24,000 km. You will find yourself checking them more than adjusting them, but piece of mind on a long motorcycle trip is worth it, plus you get to know your bike when doing your own maintenance.
Shim-Under-Bucket vs Shim-Over-Bucket
Unlike many modern engines that use hydraulic lifters, Shim & bucket style valve set ups allow the cams to directly operate the valves, eliminating rocker arms which reduces weight and cost. The cam lobe makes contact with the shim/bucket and in turn operates the valve. Once the cam loses contact, the valve spring returns the valve to its original position.
Shim-Over-Bucket: In this configuration the shim sits in a “bucket” and the cam lobe makes contact with the shim. The benefit to this is that the shim is more accessible and tends to be easier to change. Sometimes you can access the shim without having to completely remove the cams. “Valve float” is a concern with this design as there is the potential scenario that the shim shoots out of the bucket during a perfect storm.
Shim-Under-Bucket: In this configuration the shim sits under an “upside down bucket” so it is surrounded by the bucket walls and bottom. The downside to this configuration is that the shim is harder to access and cam removal is always necessary. Take precautions to ensure your timing chain does not move from where the links sit when you first open the engine.
What is a Valve Shim
Valve shims are small hardened-metal circles that range in thickness by 2 thousandths of an inch or .05mm. Shim-Over-Bucket shims are larger in diameter than the Shim-Under-Bucket shims. If the valve clearance is out of spec, a smaller or larger shim is installed to account for the difference.
You can buy valve shim kits so you always have them on hand, but just ordering single shims as needed is the most affordable option. They can be bought from any bike shop.
That Sounds Complicated; No Way I’m Doing That
Opening up a modern motorcycle engine can be intimidating to say-the-least. But doing a valve check and adjustment can be a fairly simple process with the correct tools, a general understanding of valve clearances and adjustment (you’re doing that now - congrats) and a good bike specific walkthrough (YouTube is littered with them). In fact I would be willing to bet that anyone that has enough interest in reading an article on Motorcycle Valves, likely has the mechanical capability needed to tackle the job. This is not like rebuilding a top end or splitting a crankcase, but actually closer to changing your own oil or tires. In true Zen and the Art of Motorcycle Maintenance fashion, you can and should maintain your own valve clearances.
Most of the tools, perhaps with the exception of the first one, you will likely already have in your tool box:
- Feeler gauges for checking clearances
- Micrometer to measure shim thickness (only if adjustment is needed)
- A Socket Set for removing parts, spark plugs and turning engine over
- Allen Keys for removing plastics
- Torque Wrench
- Beer (optional)
Preparing for the Valve Check
- Start by removing your bodywork such as the seat, plastics, faring, and gas tank.
- To gain access to the valve cover you may also have to remove things like rad fans, engine mounts and whatever else is in the way on your bike.
- Clean any crud off of your valve cover and the surrounding area so it does not fall into your engine.
- Remove the bolts holding your valve cover on and carefully remove the cover, making sure not to damage the rubber gasket, which is normally reusable.
Checking the Clearances
- Remove the spark plug and set the engine to Top Dead Center (TDC). Depending on your bike, there will be a timing mark on the flywheel or timing chain gears.
- Get the valve clearance tolerances (min – max) for the Exhaust Side and the Inlet Side from your motorcycle’s Maintenance Manual. It will look something like this: Exhaust: .15-.25mm/ .0059-.0098inch, Inlet: .10-.20mm/ .0039-.0079 inch
- Now check the clearances by trying to slide the appropriate sized feeler gage in the gap between the cam lobe and bucket / shim. Choose the feeler gage to match the size of the minimum and maximum clearance. This is known as the go / no go test. You want the minimum size to slide in and the maximum size not to fit.
- If all gaps pass the go / no go test, than you’re in spec and you can start reassembling.
If you have one or more valves out-of-spec, you will need to install a new shim.
- Use your feeler gage to measure the gap in the out-of-spec valve.
- Remove the shim from the out-of-spec valve. To do this you will need to move the cam shaft out of the way to gain access. In most cases the cam shaft needs to come all the way out. Tip - Tie wrap the timing chain to the cam gears to avoid losing proper timing.
- Lift the shim out, or remove the bucket, and figure out what the new shim size needed is: New Shim Thickness = (Measured Valve Clearance - Specified Valve Clearance) + Old Shim Thickness. If you’re lazy or terrible at math you can use this cheesy calculator.
- Install the new shim and reinstall the cam.
- Re-check the clearance to ensure you selected the correct shim size.
Valve check and adjustment is all part of owning and maintaining a motorcycle! Check out these Cheap Motorcycle Tools That Actually Work for more Motorcycle Maintenance talk.