THE CYLIN­DER HEAD

The art of me­chan­i­cal breath­ing

Cycle World - - Components - Story and Pho­tog­ra­phy by SAM SMITH

MMost 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.

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.

BMW mo­tor­cy­cle de­sign has changed lit­tle in the decades im­me­di­ately fol­low­ing World War II. In 1970, the mar­que walked away from its an­ti­quated Ear­les­fork 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.

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.

So that’s what you do. You blow

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

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.

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 well­re­garded 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.

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.

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.

ABOVE: 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. RIGHT: 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.

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.

ABOVE: 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. LEFT: Round gas­kets are for cylin­der bases; peanut-shaped ones seal valve cov­ers to heads. Pushrod tubes get rub­ber seals. This R90S has 63,000 miles. Not a bad valve-ser­vice in­ter­val.

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