Have you ever received the bill for the valve-clearance adjustment on your overhead-camshaft motorcycle engine and mourned for the lovely dinner for two that money could have bought? There have to be a few thousandths of an inch clearance between each cam lobe and the tappet that opens the corresponding valve because valves and other parts expand as the engine warms up. If that clearance is too small, valve expansion will make it disappear, allowing the valve to leak. If the clearance is too large, you will hear it as a tapping at idle, becoming a clatter at higher revs.

Because, despite the excellence of modern oils, wear does exist; the valve clearance set at the factory may change as your odometer records thousands of miles of pleasant motoring. If the cam lobe and tappet wear, clearance increases. If the valve’s sealing face and the valve seat on which it closes recede slightly, clearance decreases. The hoped-for result is that the two are equal and your valve clearances (16 of them in a four-valve-per-cylinder inline-four) change remarkably little over thousands of miles.

But there it is in your owner’s documentation: At some specified mileage, the valve clearances are to be checked and, if necessary, reset. The mechanic removes the engine’s valve cover, under which are the two camshafts, one operating the intake cam, the other the exhaust. With the cam oriented so the lobe being measured points away from its tappet, the mechanic will slip thin feeler gauges between lobe and tappet to measure the cold clearance, then write it down. Once all clearances have been measured, those outside the specified limits must be reset.

That loose shim could also get caught between a cam lobe and the cam cover, breaking a great hole in it. And in your wallet.

Forty years ago, this was easy, for quite a few overhead-cam engines with inverted-bucket tappets had the circular clearance-adjusting shim on top of the bucket, sitting down in a shallow counterbore. Using a special tool, the mechanic depressed the tappet (requiring slightly compressing the springs on the valve under it), and with a pointed tool popped the shim out. Next, the mechanic got out the shop’s big box of all thicknesses of clearance-adjusting shims, which were like thin steel poker chips, each with its thickness engraved upon it. It was then easy to work out what thickness shim was required to restore factory clearance and to pop it into place and move on to the next valve and its clearance.

Trouble was, if it was so easy for the mechanic to pop that clearance shim out, it was also easy for the engine to pop the shims out as the rider overrevved on a missed shift or while basking in universal admiration as he or she raised billows of tire smoke during a redline burnout. As it says in the beloved Christmas poem, “There arose such a clatter!” And the clatter of operating with a spit-out shim wasn’t all; that loose shim could also get caught between a cam lobe and the cam cover, breaking a great hole in it. And in your wallet.

valve adjust
"The sheer convenience is worth the occasional inconvenience." In 1981, Peter Egan, then Cycle World's technical editor, wrote—and modeled for—"The shimmed valve adjust, a gap-it-yourself guide to the DOHC valve train."Cycle World archives

So, with regret, the industry adopted what was called shim-under-bucket clearance adjustment. In this system, you have to remove both camshafts, then remove and place in numbered locations on a bench each valve tappet and the shim so securely held beneath it. In the case of the pivoted finger followers now being widely used, valve clearance is adjusted by the use of various thicknesses of “lash caps,” tiny hardened steel cups that fit closely over each valve’s stem.

Once valve clearances have been correctly set, the cams must be reinstalled, taking care to align the timing marks on their drive sprockets according to the instructions in the service manual. Those instructions are not just lifestyle cues. Get the timing marks wrong and the valves open and close at the wrong times and power will be down. Even worse, there may also be expensive contact between valves and pistons.

By driving the cam through a gear whose center was on the hinge line, cam timing was not altered by swinging it aside for easy access to the valves.

Ducati owners especially dread this job because there are cams for both opening and closing the valves, with the use of a special “crunching tool” required for fully seating each valve retainer on its collets.

But 116 years ago, Scottish engineer Alexander Craig had a better idea. When he included overhead cam on a three-cylinder engine he was designing for the English Maudslay Motor Company, he mounted the cam on a hinged frame, secured by a couple of fasteners. By driving the cam through a gear whose center was on the hinge line, cam timing was not altered by swinging it aside for easy access to the valves.

With Craig’s system, you measure the clearances of all valves, then swing the cam aside, pull the shims or lash caps, select others that will make clearance correct, install them, and swing the cam back into place, fastening it with a few bolts. Check clearance again to be sure it’s right, then reinstall the cam cover and, in the much-used modern phrase, “You’re good to go.”

Craig’s invention would make valve-clearance adjustment quick and easy but extra cost and extra parts have been the death of many a good idea.