Most people have at least one inexplicable habit—something you do in private or late at night, thankful that no one’s watching. This probably marks me as a brand of especially boring weirdo, but I read old airplane service manuals. My favorite is a 1953 Air Force publication called Powerplant Maintenance for Reciprocating Engines. The front jacket holds a drawing of a nine-cylinder radial, and the book’s 452 pages contain everything from a treatise on the disposition of aviation fuels to prose and diagrams outlining the operation of a Curtiss electric propeller governor.

Dangerous stuff, if you’re wired a certain way. But its joys are mostly in the little things. The elegant, brutally efficient language of the government repair manual, applied to virtually universal topics such as cylinder heads. “It is important,” one page reads, “that valves open and close properly, and seal clean and tight against their seats, to secure maximum power from the burning air-fuel mixture, and to prevent burning and warping.”

The first time I saw that sentence, I read it three times in a row, marveling at its clear-cut density. Those 33 words sum up any head, on almost any nonrotary engine, with a kind of poetic clarity.

If you think of an engine like a body, its cylinder head is akin to a person’s head: an important gatekeeper, its job seemingly plain but more impressive with study. The head is one of the hardest-working parts of a reciprocating engine—the hottest during operation, containing both scorching exhaust gas and flame fronts moving at up to 150 feet per second. Given a supply of air and atomized gasoline, a cylinder head both controls combustion and helps time the ­delivery of the resultant torque. And yet the mechanism can be reduced to almost medieval simplicity—a four-stroke head on the simplest of one-cylinder motorcycles holds fewer moving parts than most people have fingers.

My 1975 BMW R90S is not the simplest motorcycle ever built, nor is it even the simplest twin. But it's not far off. When I bought the thing, several years and many thousands of miles ago, I was looking for a reliable, quickish classic on which to take trips. I had wanted a 90S for years, in love with the bike's Dell'Orto quack and role in BMW history.

Domed combustion chamber
Domed combustion chamber with two valves that ride on hard seats shrunk into the aluminum for perfect sealing. Carb clamps to smooth spigot; exhaust nut threads onto other side.Sam Smith

A pearl-orange 90S won the first AMA Superbike National at Daytona, Florida, in 1976. Mine is a version of that color, which the Bavarians naturally dubbed Daytona Orange. The previous owner lived in Phoenix, so I flew in, handed him a decent-size check, then ripped the bike home to my garage in Seattle, weaving over the mountain ranges from Los Angeles to Puget Sound. Two short years later, hard riding and age had caused the BMW’s heads to tire. The engine wasn’t as strong as it should have been, but more important, a blue haze of oil smoke followed it whenever the throttle was closed at high rpm. The bores and pistons checked out OK—BMW boxers are known for stout bottom ends, low of stress and large of bearing—but symptoms pointed to worn valve guides.

At which point I got a little excited, because that meant the heads had to come off.

Removing cylinders
It’s easy to remove the cylinders once heads are off. Slide jug outward to reveal piston pin, remove it, then piston and rings are undisturbed. Bores were good, so no work required.Sam Smith

BMW motorcycle design has changed little in the decades immediately following World War II. In 1970, the marque walked away from its antiquated Earles-fork bikes, redesigning its entire catalog. The result was the line known as Slash 5—telescopic forks, 12-volt electrics (replacing 6-volt), bright new colors (replacing black and white and not much else), and, chiefly, a new range of engines. BMW’s 1970 engines looked much like the ones sold in 1969—two horizontally opposed and air-cooled cylinders, an alloy crankcase, pushrods carried in external tubes, and peanut-shaped valve covers—but the new units were a complete rethink, aimed at greater output with no sacrifice in durability. As such, their combustion chambers and pistons bore a significant resemblance to the strong and potent straight-sixes used by contemporary BMW cars.

BMW ­flat-twin
In terms of presentational ­convenience, it is hard to top a BMW ­flat-twin for access to parts. Photography for this story was shot on a 1966 Leica M3 using Ilford HP4.Sam Smith

There is no small joy in pulling the heads from an airhead. Partly because the bikes are so straightforward. The 67 hp 90S is one generation of engine newer than a Slash 5, improved in several ways, and while modern standards can make its twin seem agricultural, the engine remains a good lens for the basics. The heads on a 90S were effective enough to win ’70s AMA Superbike races, revving as high as 10,000 rpm with unmodified stock rockers. Those heads can be understood on sight and serviced by anyone who can tune up a lawnmower. And because they’re hung out in the breeze, free of complex plumbing or cam drives, a quick mechanic can decapitate both jugs in less than 20 minutes.

More often than not, if you have everything apart, wholesale replacement is the greatest insurance.

So that’s what you do. You blow it all apart and look at what’s gone wrong. Cylinder-head maladies vary with bike design, but even the plainest of engines has a lot to get dorked. Valve seats, stems, seals, and guides can wear, changing when and how the cylinder breathes. If friction and subpar lubrication cause a valve’s stem-top retaining grooves to fritter away, the valve can drop loose and hit a piston. Pushrods can pit at their ends or bend, camshaft buckets can wear and cock in their bores, valve seats can pit or get beaten down from thousands of miles of pounding. An overly sharp edge on a valve face means it’s been overrevved and subjected to spring float, causing the valve to hammer against its seat. The list goes on. All of it can keep a head from sealing as it should, and cylinder heads live or die on proper sealing. The trick is that the whole thing doesn’t wear out at once—the life of each part is impacted by how you ride and service the bike. Even adjusting a valve a bit loose on a tuneup might result in that valve closing too quickly, which could lead to stem stretch and failure.

Cylinder and head
Round fins would encourage the bore to remain round as cylinders heat. Cylinder and head are held on by studs that thread into engine cases.Sam Smith

More often than not, if you have everything apart, wholesale replacement is the greatest insurance. I made a few quick measurements, then sent my heads off to a well-­regarded specialist in Northern ­California. They came back gorgeous—bead-blasted and jewellike—new seats and a stack of new parts replacing the worn. Just as much art as piece of machine. So I took a few pictures, which can be seen here.

Peanut-shaped gaskets seal valve covers to heads.Sam Smith

Motorcycles are lovable for a lot of reasons, but at-a-glance mechanical honesty is pretty high on the list. You look at the machine and you see what’s there, and you know what the parts are and why they work. And you know that if the bits are healthy, they will run for a long time, covering miles that make you happy.

Round gaskets
Round gaskets are for cylinder bases.Sam Smith

This knowledge tends to wash over you as you reassemble the machine, making you a strange kind of happy in advance, even though the age and use of the thing has caused you to expend money and effort. Or maybe you’re happy because the machine has given you reason to do those things. I don’t know, and thinking about it too much tends to drop sap on the purity and release of riding.

Either way, I now have a honking Bavarian Superbike that spits no smoke and rips into the hills better than ever. In a few weeks, the two of us are heading off on a trip to the desert, a week’s vacation, saddlebags full. Everything kosher under those alloy peanuts. And I’m once again wondering where I should point that front wheel.