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How to Screw Up a Valve Clearance

I recently checked my valve clearances on the Spada and had the grand opportunity of having to do them twice. It's a screw and locknut setup, which is considerably easier than the old shim and bucket, but despite that here's some of the ways it can go wrong:

1. Not setting the piston to Top Dead Center

This is a no-brainer, but you need to rotate your cam lobes away from the valve before you can check your clearance. If you suddenly find that all your valves are extremely tight, check where your piston is at. There should be markings on the flywheel, but if in doubt, stick a long screwdriver down your spark plug hole being careful to to jam it up or get it stuck. Watch your valves. At some point as you turn the engine over by hand, both intake and exhaust will be open at the same time (or close to it). That's the exhaust leaving the cylinder and a new batch of fuel/air entering the cylinder. You should now know which set of valves are your intake (alternatively, have a look at what your headers are connected to - that's your exhaust port). Once the intake valves shut, compression starts and your screwdriver should be making its way up. Find the highest point it ever reaches and that's Top Dead Center. Simple.

2. Getting your Intake and Exhaust Mixed Up

For some reason, perhaps walking around the other side of the bike, people get their clearance screwed up on the intake and exhaust. On bikes where the clearance for intake and exhaust are the same, this doesn't present itself as a problem, but where the clearances differ, the clearance on the exhaust valves is almost always larger than on the intake. Make sure you note which is IN and which is EX.

3. Setting your Clearances Too Loose

The danger of setting your clearances too loose is that at high RPM, there's a large enough gap between the valve and the cam lobe that can cause the valve not to return to its seat in time before the piston returns on its way up. This is known as valve float and at best results in a poorly running engine (as a complete seal of the cylinder isn't achieved during compression) and worst in bent valves as piston meets valve meets go and get some new expensive parts.

When setting your clearances, check that the next feeler gauge won't fit. Don't rely on advice that gives you a figure and a "medium to light drag" on the gauge. If you're not experienced, you can feel a medium to light drag, but be far out of spec. If you clearance states 0.15mm, ensure that that feeler gauge fits, then try the next largest up and ensure that it doesn't fit, at least not without a lot of convincing. It's true that it's better to set your clearances looser than tighter, but don't settle for what "feels" right - make sure you check that you haven't left her wide open.

When you start your bike up after setting your clearances, she shouldn't tappity tap like crazy. A bit of valve noise should be expected, but not a complete ruckus.

4. Setting your Clearances Too Tight

Always double check your clearances after you've tighten up the locknut. Even then, I triple check the clearances once a set of valves are done. What you'll find is that when tightening the locknuts, you'll turn the adjusting screw slightly which may throw your clearances out. I usually keep a screwdriver in the adjusting screw to stop it from moving, but I haven't the steadiest of hands either. Check, check and then double check. Too tight and the valve will be held slightly open, resulting in valve burn and the opportunity to go and purchase more expensive parts.

If you're in doubt, it is better to have your clearances looser than tighter. What generally happens is that over time, the intake and exhaust ports get worn down. I'm talking tiny fractions of a millimetre here. So your valves are seating a little lower as time goes by. This takes up the clearance that you've set and if you've got no clearance, guess what happens? The valve never completely shuts and all that hot explosive fuel/air during the compression cycle tries to make its way out the tiny gap between the valve and its seat. Compare the hot, blasted, explosive environment of the valves compared to the cooler, lubricated camshaft and you'll see which side wears out faster. This is also why valves usually need to be loosened rather than tightened. If you're finding that your valves need tightening, you could have a lack of lubrication. Check your oil!

5. Not Paying Attention

This last bit is where I think most people fail, even your local mechanic (most probably due to time constraints than anything else). Pay attention to your bike! When you're putting her back together, are there any gaskets that look worn? Perhaps they need replacing or a touch of sealant now. When you took the plugs out, did they look normal? What could they have told you? Did you torque everything back up to spec? Could/Should those bolts you undid be greased (or are you happy to have them seize in the head for the next time you need to adjust your clearances?)

Paying attention to your bike is one of the key things most people don't do. Most people don't inspect their rides before they set off. Most people aren't proactive when it comes to maintenance. Our poor, poor bikes tend to grumble before dying, but unfortunately it's not until after something critical falls off or dies that we pay any attention to it. I would guess that there's not one bike on the road that doesn't have something that should be fixed, filled or checked. Even brand new bikes need to be inspected shortly after purchase. So with that in mind, every time you finish working on your bike, find something that you know you need to keep an eye on. If you don't find anything, you're not paying enough attention to her!