Checking and adjusting valve clearance is part of common maintenance on a four-stroke motorcycle engine. Here, we go through the step-by-step process required to make sure the valves are in spec on your shim-under-bucket style valvetrain.
When it comes to four-stroke engines, adjusting the valves is just another tick on the maintenance schedule. It’s a manufacturer-specified service interval that should be adhered to, ensuring the long-term health and performance of the engine. In this write-up, we’ll look at what goes into adjusting the valves on a motorcycle with a shim-under-bucket valvetrain design.
NOTE: What we’ll be covering here isn’t simply a regurgitation of a factory service manual’s instructions. A service manual assumes the user has a certain degree of knowledge about the procedure, and thus, it will gloss over some details that can truly stump a first-timer. We’ll be giving some tips and tricks throughout the write-up that can give you a chance at success on your first shim-under-bucket valve adjustment.
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The motorcycle we’re demonstrating with is a 1994 Honda CBR 900RR with just over 41,000 miles on the clock. The bike was purchased without any service history, so there was no way of knowing the last time the valve clearances were checked. The bike ran well, but it’s better not to take any chances with the engine. There’s no better time than the present to dig in and set its service record straight.
Stripping It Down
With the CBR being a fully-faired bike, the first step in the procedure is simply gaining access to the engine. Everything from the side fairings to the fuel tank must be removed. Be sure to turn the fuel tank petcock valve to “OFF” before removing the main fuel hose. It’s also important to set the bodywork in an area where you won’t accidentally trip over it. We recommend organizing the parts in the order they were removed, giving you a clue of how to put it all back together.
With the fuel tank out of the way, you’ll next need to remove the airbox assembly. Pay close attention to the throttle bodies when you lift off the air filter – you don’t want anything falling inside them. Wadded up shop towels or rags stuffed into the openings can lessen the chance of something going inside.
The next step in the tear-down involves removing the ignition coil packs and unplugging the associated spark plug wires. This now grants you access to the engine’s valve cover.
From there, remove the valve cover bolts and gently pull the valve cover upward from the cylinder head. You’ll want to be careful pulling up on the valve cover because the gasket will have a glue-like sealant around the “half-moon” sections. If you’re delicate with the removal, you can often salvage the valve cover gasket and reuse it upon reassembly. A replacement gasket for this bike runs about $50, so we prefer to save it!
Space is at a premium when working on these CBRs, and with the valve cover free from the cylinder head, I soon found that there was not enough clearance to pull it from the bike. After stumbling around, we discovered that unbolting the radiator from its mounts allowed the radiator to swing downward and away, thus opening a door to extract the valve cover.
At this point, we could now see the engine’s valvetrain in all its glory. For an engine with over 40,000 miles on it, the inside looked brand new. This made us feel a little better about the bike’s mysterious service history.
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The next step is to remove the crankshaft ignition pickup cover located on the right side of the engine. The bolt that becomes visible with the crankshaft cover off allows you to turn the engine over in a precise sequence when measuring the valve clearances.
Lastly, to assist in turning the engine over, you’ll want to pull out the spark plugs.
Checking the Clearance
Now it’s time to start taking the valve clearance measurements. This procedure will vary depending on engine layout and bike manufacturer, but the process summarized here will be similar.
The first order of business is to turn the engine over by the now-exposed crankshaft bolt to bring the engine to top-dead-center (TDC). For this CBR, you know you’re at top-dead-center when the “T” notch on the crankshaft ignition pickup is aligned with the engine case indention and both camshaft sprockets have their alignment marks flush with the top of the cylinder head. Once in this position, we can take the intake valve clearance measurements for cylinders 1 and 3.
Now, pull out your feeler gauges and locate the gauge size that the manufacturer specifies as the correct clearance for the valve you’re measuring. For the CBR’s intake valves, the correct clearance is .16 millimeters (.006 inches). The manufacturer will list an acceptable valve clearance variance, and for the CBR’s intake valves, that range is plus or minus .03 millimeters (+/- .001 inches).
Access to the valve buckets is sometimes limited, but there’s usually just enough room to wrangle the feeler gauge into position. You want to be able to slip the feeler gauge between the camshaft lobe and the bucket.
Start with the manufacturer’s specification to measure the current valve clearance on the engine. If the feeler gauge slides in easily without any resistance, this indicates that the clearance is too large. In that case, you’ll select the next thicker feeler gauge and attempt the measurement again. Alternatively, if the feeler gauge doesn’t slip into the gap, then the clearance is too small. Follow the same procedure, but instead move in a thinner direction with the feeler gauges.
Continue this procedure until the feeler gauge has a slight amount of drag between the bucket and the camshaft lobe. This indicates that the feeler gauge matches the gap, which means you’ve just identified the valve’s clearance.
Once you’ve measured the first set of valves, continue rotating the engine until you’ve taken measurements of the remaining exhaust and intake valves. This is where it’s best to reference your bike’s factory service manual to see the relevant procedure. On the CBR (and most inline-4-cylinder engines), you’ll rotate the engine three more times in 180-degree intervals to capture all the valve clearances.
Now that you have all the valve clearance measurements, create a diagram that shows each cylinder’s valve buckets and their respective measured clearances. Having a diagram will make it visually clear which valves are out of specification and require adjustment. As you can see in our diagram, we’ve highlighted in pink the valves that need adjustment.
Adjusting the Valves
We’re finally at the point where we can start adjusting the valves. To do this, the camshafts will need to come out of the bike. The first step in this procedure is to bring the engine back to TDC, where the crankshaft and both camshafts are lined up with their respective markers.
Following this, we need to place marks on the timing chain roller pins that directly align with the markers on the camshaft sprockets. We used a yellow paint pen to apply a mark to the roller pins. This will guarantee that you can realign the correct chain links to the correct teeth on the sprockets upon reassembly.
Now we need to remove the tension from the timing chain so that the camshafts can be extracted. On the CBR, this is done by creating a custom tool out of 20-gauge sheet metal to act as a key for “unwinding” the spring in the timing chain tensioner.
With the chain tensioner backed all the way out, the timing chain is now slackened. From here, we can begin unbolting the camshaft holders. We recommend loosening the holder bolts only halfway on the first pass, then all the way on a second pass. This will help distribute the stress applied to the holder caused by the valve springs pressing up against the camshaft lobes. Be sure to record where each holder bolt came from upon removal, as they are often of varying lengths.
Before removing the camshafts, we recommend suspending the timing chain with a small dowel rod secured with a cable tie. Doing so will prevent the timing chain from falling inside the engine.
With both camshaft holders unbolted and removed, carefully extract the camshafts. Place the camshafts in a clean area with identification of which one is for the exhaust and intake.
At this point you will have access to the valve buckets. Review your clearance measurement diagram to identify which buckets are the ones that need adjustment. We always like to work from left to right, so we started with the exhaust valves for cylinder 1.
To adjust the valve, you need to first remove the bucket. Only adjust one valve at a time so that the parts removed always go back to their original spots (this is very important!). To remove the bucket, use a magnetic pick-up tool to slowly draw the bucket out of its recess in the cylinder head.
Next, use the magnetic pick-up tool to pull the shim out of the recess atop the valve spring retainer. Do not let the shim fall into the cylinder head – that could spell disaster, requiring a complete engine teardown to retrieve it.
Set the bucket and shim down in a clean work area where you can conduct a measurement of the old shim and then calculate the new replacement shim size.
The old valve shim may have a number printed on it – this is the measurement of the shim in millimeters. That number may have worn away, or the shim itself may have partially worn, so it’s always best to measure it with a caliper. In this example, we’re using a digital caliper to confirm the old shim’s size.
With the old shim size verified, it’s time to do a little math to determine the correct new shim size. The equation you’ll use to determine the new shim size is as follows:
So, for the first valve I adjusted (cylinder 1 exhaust valve), the equation looked like this:
Having calculated a new shim size of 1.952mm, the kit’s 1.95mm shim will fit the bill perfectly.
Now it’s time to get the new shim loaded into the engine. You simply plop the new shim back down atop the valve spring retainer by hand then slip the bucket back into place. That’s one valve done and only a few more to go! The procedure above will be the exact same for the rest of the valves requiring adjustment.
Putting it Back Together
With the all the valves adjusted, it’s time to reinstall the camshafts into the cylinder head. Before doing so, you’ll need to apply a liberal amount of assembly lube to the camshaft journals (on both the cylinder head and camshaft holders), the camshaft lobes, and the valve buckets. Doing so ensures that proper lubrication is present the first time the engine is turned over post-reassembly.
Sliding the camshafts into place while also aligning the camshaft sprocket markers with their corresponding chain links is a delicate act. It may take a few tries to get the sprocket teeth lined up with the correct chain link. However, getting this correct is critical. Have patience and eventually you’ll see it through.
With the sprockets aligned and the camshafts resting in the cylinder head, it’s time to reinstall the camshaft holders. Slide the camshaft holder bolts back into place, verifying that the correct bolt is going into the correct hole.
When bolting the camshaft holders into place, don’t simply run a single bolt all the way down in the first pass. Instead, tighten each one down about halfway on the first pass using the tightening order specified in the factory service manual. On this CBR, the bolt tightening order numbers are molded into the camshaft holders to help fool-proof the operation.
On the second pass, screw the holder bolts all the way in until they stop, but don’t tighten them yet. Now, use a 1/4-inch drive torque wrench to tighten the holder bolts to specification in the order stated in the factory service manual. For this CBR, the holder bolt torque is only 9 lb/ft.
At this point you can release the timing chain tensioner so that the timing chain becomes taut. To verify that the timing chain is aligned correctly to the camshaft sprockets, rotate the engine two complete revolutions (from the exposed crankshaft bolt) and bring it back to TDC. If the crankshaft and both camshafts align with their markers, then you’re ready to re-check the valve clearances.
Use the same method as before to re-check all the valve clearances to verify that they are all now within specification. Assuming your shim calculations were correct, everything should check out fine.
Now it’s time to reinstall the valve cover. Before doing this, you’ll first want to clean up any sealant residue from the cylinder head with a rag and brake parts cleaner. Be careful to not let the sealant residue fall inside the engine.
Next, clean up the sealant remnants in the half-moon corners of the rubber gasket. Following this, apply a small amount of sealant to the half-moon corners so that there won’t be any oil leaks. With the sealant applied, slip the valve cover back into place and bolt it down to the cylinder head.
Reinstall the spark plugs, plug wires and ignition coils. Secure the ignition pickup cover plate and tighten it to specification with the 3/8-inch drive torque wrench. If applicable, mount the radiator on its support brackets and bolt it into place. The engine should now be sealed up.
Synching It Up
At this point we’re ready to balance the carburetors (or throttle bodies for fuel injected bikes). A carburetor balance needs to be performed because by adjusting the valves, we have altered the amount of air being drawn into the engine. Even if the carburetors were balanced prior to the valve adjustment, they will more than likely be out of balance now.
Installing the balancer tool first involves removing the small vacuum plug screws in the cylinder head located below each carburetor. From there, you’ll screw in vacuum hose adapter nipples and snug them finger tight against their rubber O-rings, creating an air-tight seal. Next, connect the four silicone vacuum hoses up to the adapters and route them up to the balancer tool. With the balancer tool set up, it’s time to rig up an auxiliary fuel tank so that the engine can be started without having to mount the motorcycle’s fuel tank.
Now we can start the engine and begin balancing the carburetors. On a four-carburetor engine like this CBR, there is an adjustment screw located between each pair of carburetors (3 screws in total). Based on the reading from balancing tool, you’ll first adjust the screw for cylinders 1 and 2 until they read equal on the balancer tool. Then, move on to adjust cylinders 3 and 4. Finally, you’ll bring cylinders 2 and 3 into balance with their adjustment screw. You should end up with an equal reading across all cylinders.
Wrapping It Up
By this time the engine should be running very well with the newly adjusted valves and balanced carburetors. All that’s left to do is reassemble the rest of the motorcycle. As the wise service manual always states: “Install the removed parts in the reverse order of removal.”
Adjusting the valves on an engine with a shim-under-bucket valvetrain may seem intimidating, but with preparation and patience, even a beginner can accomplish it in a weekend. Even though it isn’t as straight forward as an oil change, it’s still a service that shouldn’t be ignored. Your engine’s health depends on it. As for this old CBR? Its service history is now back on track.
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