® Camshaft facts


  • The basics
  • Installing a high performance cam
  • More on cams
  • Cam chains, bearings and related
  • The challenges to mechanics

The camshaft is one of the least understood parts of a motorcycle while at the same time a part steeped in lore and mystery and which folks want to pretend they know much about. Following are the thoughts of a career industry professional on the subject of camshafts and the parts related to them.


The basics
Camshaft. Few words stir the soul of the gearhead as much as this one. And few engine parts grace a parts catalog with more honor, more mystery, more significance. Yet, little real understanding of cams exists, and with very few exceptions (and those in even more unique contexts), aftermarket high performance camshafts yield less than hoped-for outcomes. Let's explore a little bit about camshafts.

Strokes versus phases
Focusing on the clues the engine itself gives us is instructive. There was a period when the four-stroke engine didn't even have a camshaft. Their valves opened and closed atmospherically, that is, at the prompting of these steel-pistoned, very slow-revving engine's positive and negative pressures. Interestingly, the valves' movements didn't coincide precisely with the top and bottom of the piston's stroke. Most of us would have assumed that they should. Then later, when lightweight aluminum pistons permitted higher rpm, camshafts were added to better control valve action, and engineers were surprised to discover that those first engines "knew" what they were doing. Their cammed successors worked best when their cams caused the intake valves to open before the piston's intake stroke actually began, and their exhaust valves to close some time after the exhaust stroke ended. The piston's stokes and the valve's openings and closings were not in lock-step.

Intake air inertia
The reason is inertia. The intake charge's momentum keeps it flowing into the cylinder despite the piston's upward movement after completing the intake stroke, and exhaust evacuation is more complete when it starts before bottom dead center. Engineers took the engines' cues and delayed the intake valve's closing to take advantage of this phenomenom, increasing cylinder filling and power. That's good. But it turned out to be kind of tricky. How long is long enough? If the valve is held open too long, the mixture's momentum dies and the gases actually back up in the port, sucking mixture out of the cylinder and causing a loss of power instead of a gain. On the other hand, if we close the valve too early, we've limited how full the cylinder can get. Either way, power is lost. The ideal thing, the perfect thing, is to close the intake valve at precisely the millisecond the mixture loses its momentum and stops, and not a nanosecond later when it begins to actually reverse direction. This is the whole point of making sure the cam, and in turn the intake valve, is in the correct position to make this happen. Whether manufacturer or modifier, this is what you strive for.

However, the manufacturer plays to a unique and very careful set of rules. The result is the stock cam times its valves conservatively, providing the cylinder very limited exposure to the atmosphere, unlike the performance camshaft's extended timing which exposes the cylinder longer. This is because extended timing harms intake air velocity and density. Therefore, though the engine's power can be increased through inertia tuning, that increase is focused more around a particular rpm range, and not spread as broadly over the rpm as before. In fact, the "cammier" the engine, though at full song it's more powerful, the less power it will have everywhere other than that ideal rpm. Moreover, that rpm will be higher than normal due to the resulting slowed intake air speed, which reduces combustion efficiency due to reduced fuel and air homogenation. Combustion, and thus torque, suffer. No free lunch, as they say. And manufacturers know better than to tune an engine this way.

Cam degreeing
This well-known drawback can be partly overcome by nudging cam timing to put the intake valve's closing at the point that captures that last little bit of the intake's inertia, just before it falls, as described previously, closing the door on it, so it can't back up. Trial and error finds this ideal point that maximizes cylinder filling and pressure and is the whole reason behind "degreeing" camshafts. Degreeing a cam is not simply mistiming it. The nudge is a mere fraction of one sprocket tooth's distance. It's very nuanced.

The ubiquitous high performance camshaft fallacy
A dealership once planned to give away high performance camshafts as part of a promotion. What a disaster that would gave been! It illustrates the way many think about hot-rod cams. Aftermarket cams aren't something you can just bolt up and go. Dozens of painstaking measurements have been used by engine builders for nearly a hundred years to ensure camshaft compatibility. More about this later. Moreover, the performance expectations of big cams are something of a myth in all but very few specialized instances. In fact, due to cam wear, cams remanufactured to stock specs, in which there is a growing popularity, often outperform bigger, "high performance" cams. Many times my customers have asked for bigger cams. I generally discourage them. When they insist they always find they have made a mistake. "I've lost my low end", one says. "Your engine is over-cammed," I reply. No one wants to believe it. They have to experience it for themselves. Something few seem to appreciate is how well the stock cam works in most Japanese multicylinder engines. Short of riding at full throttle, i.e. racing, aftermarket cams usually fail to satisfy. For the kind of riding most of us do, stock cams really are best. And the average person is completely unaware of the many pitfalls professional engine builders have learned the hard way to pay attention to in order to avoid engine damage. Wait for it; we'll get there.

The origin of valve clearance
The purpose of valve clearance is to compensate for heat expansion, right? Wrong. This hoary, tenacious assumption may be intuitive, but it is very wrong. Actually, as the valve in an overhead cam engine heats up, it expands more than the castings around it, resulting in the head of the valve increasing in size and the valve consequently being pulled into the combustion chamber, increasing valve clearance.

Why then, clearance? Valve clearance is wholly a product of cam lobe design (shape). It's tied to the specifics of a cam's intricate geometric design. A cam lobe is not just an ellipse machined onto a shaft. It's a complex shape with several distinct radii, each with its own job to do, and one of those dimensions is where valve clearance comes from. Cam anatomy, then. Let's start with the "base circle", also called the heel. The base circle or heel is that part of the cam lobe that looks mostly perfectly round, a constant radius. It's the datum line, home base, the point of departure from which everything else on the cam is measured. It is also the valve's fully-closed point, giving the valve its cooling time. Opposite the cam's base circle is the "nose", the point at which the valve is fully open. The nose affects valve open spring pressure, an important consideration in engine tuning. Either side of the nose are very steep areas called "flanks". They open and close the valve. On some cams the opening and closing flanks are symmetrical. But not on all of them. In addition, shim type engines, rocker arm engines, and pushrod engines each have their own distinct flank shapes, some flattish but many very rounded.

But the most interesting of the cam lobe's parts and one with many secrets is the "ramp". Detectable only with a dial indicator, the ramp is the tiny transition zone at which the radius of the base circle becomes the very broad surface of the flank. Though complex in design, the ramp's job is simple: to cushion the valve. For a handful of crankshaft degrees, the ramp eases the valve into its sudden climb up the lobe's opening flank. Then after the valve has opened and is skiing down the closing flank, it comes to an identical ramp on the closing side of the lobe that gently decelerates the valve so it doesn't crash onto its seat.

Paradoxically, there isn't actually room enough on the cam lobe for as much transition as the valve really needs. Here's where valve clearance comes in. Valve clearance supplements the cam's built-in cushion, augmenting the always-minimal ramp. Different motorcycles have different valve clearance specifications because they have different cams with different amounts of ramp. Modern engines run three to five times the valve clearance of engines of thirty years ago because their cam's larger lobes necessarily cut into the ramps, making them smaller. Less ramp means less opening and closing transition-- big cams need more valve clearance. Small ramps go with big valve clearance, large ramps with small clearance. Heat has nothing to do with it.

And neither does noise. Many folks seem to believe that should an engine develop a slight ticking noise this means the valves are out of adjustment. Actually, there is little direct connection between the two. I have known customers who wouldn't adjust their valve clearances until hearing this ticking, as if the whole reason for adjusting the valves is to ensure a quiet engine; and conversely, who believed no noise means all is well, no need to adjust anything. If the former is ignorant, the last is dangerous. It is not for nothing that factory valve clearance adjustment intervals are spaced just 3,000 miles apart on 70s and older Hondas. It's a very important part of maintenance, second only to ignition service.

The usefulness of valve clearance
Some obsess over getting valve clearances exact, sraining, striving. This is silliness. Valve clearances are not akin to carburetor jetting or piston-to-cylinder clearance, a specification indelible, sacrosanct, unchangable without marked consequence. Have you ever noticed that, much like spark plug gap, many engines' clearance specifications are given as ranges? Valve clearance is all about valve cushioning, remember, ramps, and has nothing to do with heat or noise or demandingly exact valve timing. Knowledgeable mechanics have learned to be relaxed about clearances, opting in most cases for slight looseness, and nearly every old timer has long since discovered that increased clearances helps the engine in numerous ways. It forestalls the valve's eventual burning due to insufficient clearance. It increases the valve's ability to shed carbon. It adds valve cooling time. It lengthens the engine's compression phase, boosting cylinder compression and by the same token shortens valve-open time for increased intake air velocity that improves low rpm performance. These are real world benefits long understood and valued by career techs. Don't minimize them.

Cam measurement
The official manual describes a camshaft measurement routine that is often misunderstood. It has you measure the entire cam lobe, heel (base circle) to nose. While having a purpose, the actual outcome of this measurement is not to gauge the potential of your cam or compare it to another cam. You can't do those things by this measurement; it's too gross, it spans too many of the cam lobe's important regions all at once, areas of the cam that need to be separated to get meaningful specifications. The goal of the manual's measurement is maintenance, not performance. It's not intended to tell you anything about the cam's specifications. It is simply a crude and easy way to track cam wear, and you'll discover an astonishing rate of wear doing this. Japanese standard model motorcycle cams wear very quickly, as much as 0.001" per thousand miles.

Some people feel that measuring heel to nose and then measuring again ninety degrees to that across the heel, then subtracting, is a way to derive the cam's valve lift specification. As with many things, this sounds good but ignores reality. Three problems make this idea unworkable. First, the cam's heel or base circle is not necessarily continuous for 180 degrees; in fact in most cams seldom is. Remember the ramps? So you're not really measuring the base circle with your micrometer at ninety to the nose. In fact you're not measuring anything of any consequence. Second, when the cam is from a rocker arm equipped engine, the cam's movement profile does not match the valve's; the engine sees different movement due to the rocker arm's ratio. Therefore valve movement, both duration and lift, on rocker arm engines is most accurately measured at the valve itself. Engine builders actually do so on all engines, whether equipped with rocker arms or with shims and buckets. It's that critical. Third, even on a given model engine, the cam's base circle diameter changes slightly from year to year, making the heel to nose measurement of absolutely no use in comparing cams. You can tell almost nothing significant from measuring the size of a cam lobe. The manual's reason for doing this is to track cam wear and nothing more.

Cam profile
It's actually a bit more work to properly measure a cam. The most important, most useful, and most widely used cam detailing method is what is called "profiling" the cam. The cam is left in the engine, a degree wheel is affixed to the crankshaft, and a dial indicator is placed onto the valve. As the crankshaft is rotated, the indicator's readings are marked on a piece of graph paper every five degrees, and the result is a right-angle, X and Y, duration versus lift depiction of both the intake and exhaust valve movement caused by that cam. This is a tremendously useful tool for getting a meaningful "snapshot" of a cam, and thus for comparing camshafts. Because it requires the same tools and setup, engine builders add yet another layer to this process that measures valve-to-piston clearance at various crankshaft positions, for a very complete picture of cam action. After reading this you might think cam profiling a very esoteric exercise. It's not. No one who properly modifies an engine neglects this.

Lobe center
Now let's combine ramps and measuring. We've learned we have acceleration ramps which gather up the clearances in the valve train before the valve is shot off its seat, and deceleration ramps which decompress those bits and cushion the shock of the valve's closing. And that they are what determine valve clearance. The problem is that these ramps accelerate the valve so slowly that interpreting exactly when it has opened or closed is difficult. Because of this, engine builders have developed a method of ignoring the ramps when profiling a cam. The ramp isn't there to move the valve anyway, its sole purpose is to graduallly take up clearance. To babysit the valve. Builders know to disregard the valve's movement until the cam has rotated past the ramp and the valve has lifted a certain amount, then to start recording it on the graph paper, thus ensuring that the valve is well clear of the ramp. That delayed amount is called the cam's "checking height." A checking height is merely a predetermined starting and stopping point in cam measurement.

This is great. It firms up the process of measuring the cam, makes it more accurate. But the problem is that ramps aren't all the same size. The ramps on a pushrod engine's cam for example are the largest, because that engine's spindly valve train demands very gentle opening and closing curves. The ramps on overhead cam engines by contrast are much smaller, because there are fewer valve-related parts flailing about needing cushioning. Each engine design has its own size ramp and correspondingly its own recommended checking height; some 0.020", others 0.040", and still others 0.050". Comparison of these different cams therefore becomes meaningless, as two different checking heights give two very different measurements. What's needed is a system of measuring cam timing which is independent of the checking height used. Is there such a thing? Yes! That's the lobe center method. That is why it was developed. Though it still requires the use of a checking height, the lobe center approach indexes the cam based on an average of its opening and closing points -- the cam's mathematical "center" -- and not one of those points, then counts the number of crankshaft degrees from there to top dead center (TDC). This means different checking heights can be used on a given cam and the lobe centers will still come out the ssme. No matter what the starting and stopping points are, the center of a thing is always the center. Make sense? Lobe center divorces checking height, once an insurmountable hurdle in ecumenticizing cams, from cam measurement, making legitimate cam comparisons possible.

You might argue that no one compares the cam from a Harley Big Twin with that from a Hayabusa and of course you're right. That isn't the kind of comparison we're talking about. Then why do we need the lobe center method to compare two cams for the Hayabusa? Their checking heights will be same, won't they? That's just it, they won't necessarily! Cam makers, each one having their own history and influences, continue to use different checking heights even on cams made for the exact same model motorcycle! Why then isn't the cam lobe's center measured directly? Because it isn't the lobe's physical center we're looking for, it's its mathematical center -- the average of its opening and closing points. In a few cams, the physical and mathematical centers are the same, but usually they are not. The cam lobes in overhead cam rocker arm engines for example are asymmetrical -- their opening and closing flanks are shaped differently to compensate for the change in geometry the system undergoes as the rocker arm slides over the lobe.

So that's lobe center. However, a strange thing has happened. In recent years lobe center has been appropriated by many for use in timing cams to the engine instead of merely comparing cams. These folks propose that a particular lobe center works best on a particular engine design, i.e. larger lobe centers on Kawasakis and smaller ones on Hondas. An intriguing idea yet actually pretty arbitrary. And even if it has merit, that's not what the the lobe center measurement was originally devised for and using it that way is a kind of rabbit trail and obfuscates the real goal of adjusting a cam by its intake valve timing. Admittedly there is nothing wrong with using lobe center for timing, if you want to go to that trouble, other than it unnecessarily adds to and ignores the most important valve timing event.

Firing order
A four-stroke engine's cylinder firing order is largely determined by its camshaft. A couple of examples. Generations ago, Yamaha 650 twin racers used to run cams that were cut in half and welded back together to make the engine fire its cylinders in a different arrangement for improved traction, and presently owners of these vintage bikes in search of enhanced engine smoothness are doing something similar, that is, rearranging the cam's lobes. Also, the exotic six-cylinder Honda CBX1000 had an error in its earliest official manual that when followed made the left side cams 180 degrees out, resulting in such a dramatic alteration from the bike's 1-5-3-6-2-4 firing order that all of this engine's famous silkiness was eliminated and the cam chain failed within a documented 25 miles. The point is that the camshaft is the arbitrator of the engine's firing order, a not-inconsequential thing and something not often appreciated.

Installing a high performance cam
The goal of fooling around with non-stock cams is the attempt to hold the engine's valve open longer and/or farther to increase cylinder filling. Despite the inherent problems of the negative effects on intake air speed and rpm, the use of an alternative cam can in fact be made to work. However, a camshaft that is foreign to your engine absolutely must be installed carefully and correctly, and part of this involves making several valve-related checks that demand to be made on the engine, which is what we'll focus on now.

Valve spring free-length
The first is valve spring free-length. This is the length of the valve spring uninstalled. This is usually the only specification given in the OEM service manual. Its purpose from Honda's point of view is merely to enable you to easily spot fatigued springs. However, when building a high performance engine we go way beyond this, using spring free-length as the basis for calculating other important spring specifications.

Valve spring installed height
Installed spring height is the height of the valve spring installed, that is, slightly compressed already. Even before the valve has moved. The purpose of knowing installed height is primarily to gauge valve seat pressure. Valve seat pressure is important in any engine, stock or modified, but the modified engine's increased cylinder pressure requires no-nonsense sealing. Also, at high rpm there is less time for valve cooling, so good seating becomes more crucial on that basis as well.

Spring full-open length
This is the spring's shortest working length, the hardest it can be expected to work. Among other things, knowing spring full open length helps the engine builder determine if there is sufficient spring pressure to control valve float. All performance camshafts increase how quickly the valve opens, adding to valve acceleration and making controlling valve "float" more of a concern. The spring's full-open pressure is your first line of defense against valve float, and it will often need to be more than stock.

Spring coilbind
This is when the spring's coils touch one another, metal to metal. The spring is compressed past the valve's full open point. This must never occur in operation, and there's something wrong with a spring that comes near to doing so. The purpose of checking for coilbind clearance is to validate your choice of valve springs; to point out when they are not the correct rate.

Valve free travel
Also called retainer-to-guide, this is the amount that the valve can travel from closed all the way to where the spring retainer hits the valve guide. One of the most important checks, many engine builders have failed to respect this one and have paid dearly.

Valve to piston clearance
Valve-to-piston clearance necessarily changes in an engine modified with a different piston or camshaft, or having remachined castings almost anywhere in the top end. To do this check, carburetor springs are put in place of the valve springs. A degree wheel on the crankshaft and a dial indicator on the valve tell us how much the valve moves at a point 30 degrees either side of TDC overlap, the danger zone, in 5 degree steps. This is unarguably the most important of all hot-rod engine building clearance checks. Don't believe what high performance parts retailers tell you. It's your engine, find out for yourself.

Valve follower travel
Whenever camming a shim and bucket engine, you must check that the bucket (follower) will in fact move the required distance. In some Asian cylinder heads, the bucket bottoms out in the casting less than 0.100" after the stock valve lift. Failure to check this will result in a considerable amount of damage, including a broken camshaft.

Valve protrusion
A commonly overlooked problem in shim and bucket type engines is that of assembling the cylinder head and then finding that the smallest of the factory's shims are needed to get the valve clearances to spec, leaving no room left for future valve adjustments. This is really not acceptable, yet can happen even on a head that has simply had a valve job done. The solution is to "tip" or shorten the valves 0.008"-0.010", bringing the valve protrusion back down to factory spec, and allowing the middle or larger size shims to be used for valve adjustment, a significant benefit to the customer.

Naturally, there are several other "gotchas" to look out for, such as clearance of a larger cam's lobe with engine castings, rocker arm interference, and a few others. Hopefully it is clear that only a very naive individual "throws" a cam at an engine.

More on cams
The fact is, there is a tendancy for folks to think they know more about cams than they really do. Did you know for example that Honda once made cams with angled lobes that made the valves rotate and thus clean themselves of carbon? A good idea but they had to stop because customers complained of knocking noises in the cylinder head. The cams were banging against their bearings from the lateral thrust the angles created. Just how much do you really know about cams?

Cam wear
With rare exception, Honda cams are made of surprisingly soft, easily-drilled cast metal and wear at a constant, fast rate. By 10,000 miles the cam is considerably worn, and valve timing thus appreciably shortened. I have perked up many an engine simply by ordering replacement cams made to stock specs (new cams from Honda are long gone), which sufficiently and happily restored good valve timing and full stock engine performance. For the few engines where that isn't feasible, I have found that advancing the cam a couple degrees can also help recover the factory valve timing lost to wear and brighten engine performance.

Certain models of Japanese bikes have much more than the normal amount of cam wear, actually severe damage, galling, tearing, of their cams. Many are unaware that Honda made/makes rocker arms whose required surface hardening consists merely of chrome plating. This unfortunate expedient has resulted, in many instances, in the chrome failing and the rocker arm then becoming a cheese grater and taking out the cam. The effect is mild on some engines, worse in others. One set of the really bad bikes to look out for are any with the four-valve Kawasaki ZX900 series engine, including the early 80s 900 Ninja, Concours 1000, and the 900 Eliminator. In addition, certain Honda models, namely all the early 1980s first-gen Honda V4s, 700cc and larger. These two sample engine types have the same two things in common: chromed tandem rocker arms whose four valve springs exert unusually high pressure on the one cam lobe, combined with (for the period) unusually high valve lift. This combination results in serious cam deterioration due to the follower's chrome eventually peeling due to the extreme pressure.

Valve springs and things
Honda valve springs are recognized among veteran mechanics as the best in the industry. Almost without exception, aftermarket springs do not offer any improvement. Whether resurrecting bikes that have sat for years, or racing vintage Hondas, the stock springs have proven their worth. Honda's valve guide seals were once equally famous; mechanics used to put Gold Wing seals on Kawasaki Z1s. And the cast iron guides found in Big Four engines are worlds better than the non-ferrous guides some racers use. They use them because they are more readily available, they ream easier, and theoretically they add cooling to the valves. They're a poor choice outside of racing however.

I'm always surprised when I find valve adjusting locknuts only gently snugged. This is risky. The velocities that the ends of the rocker arms reach are astronomical. That tiny locknut can and has rocketted right through a quarter-inch thick cast aluminum valve cover, leaving behind a gaping two-inch diameter hole. For one model prone to this Honda even issued a special extra-long tightening wrench. But overtightening isn't a good thing either. Though the adjusting screws are hardened, rocker arm screw threads go bad from overtightening of the locknuts, which then makes adjusting clearances an aggravation as the screw insists on finding its same place every time until the nut is tightened.

Cam bearings
Honda cams are fitted to their bearings very loosely, typically 0.005" or more. Yet noise due to this looseness is rare, and affects only one Honda model that I can think of. It's just the way they're made. Some tuners, including yours truly, have in special cases reduced this clearance through machining, for performance or noise reasons. The bearings are of course cast aluminum, which has never posed a problem because the Japanese aluminum alloys employed are extremely durable, and also due to copious oiling. I have seen steel cam journals wear more than their aluminum bearings. And, interestingly, Honda actually used an aluminum cam in one of their lawnmowers. That is a very trick application of technology. Don't think Honda isn't smarter than you.

It's common for folks to overtighten cam bearings. Part of the blame lies with oddly consistent torque spec errors in many OEM manuals. At least one Asian manufacturer's U.S. office has issued warnings intended to override their own official specs. Honda's listing of 12 ft-pounds for CBX1000 and early DOHC four cam bearing bolts is grossly in error, on at least two important fronts. First, a ft-lb torque wrench is never used on 6mm bolts because the setting on the tool is too low to be accurate for such a critical measurement. Second, those bolts should never be tightened more than 90 inch-pounds, which is little more than halfthat 12 ft-pound book spec. There is quite a lot of backstory to all of this, involving service bulletins, warrantied engine failures, and more, and includes Kawasaki and Yamaha as well as Honda. Suffice to say, the forums are completely ignorant of this and as with so many things make a complete mess of the issue, promoting as they do higher tensile cam bearing bolts than stock. This brings us to the issue of torque wrenches. If you are messing with Honda engines and don't own an inch-pound torque wrench, shame on you!

Cam design
Cams made for rocker arm engines have oddly assymetrical lobes. This is due to the contact point on the curved pad of the rocker arm changing as the cam is rotated. The cam's unequal lobe shape compensates for the rocker arm system's inevitable geometry change as the valve moves. Note for example the really peculiar shapes of the cams in Honda's 1970s DOHC CB450.

Cam chains and related
Something not many are aware of is that the cam chain is the highest wearing part in the bottom end of the engine. Yes, more than the clutch in fact. Cam chains
Hy-Vo type cam chains replaced roller cam chains on Honda's bikes in the late 70s. Hy-Vo, for "high velocity", is a trademarked name; the generic term is link-plate chain, gear chain or silent chain, the last rather ironic inasmuch as in motorcycles they can be noisier than roller chain when out of adjustment. And adjustment is problematic. The increased mass of the Hyvo chain over its roller counterpart, and its unfortunate application outside its original design parameters (wherein, in cars, they were centrifugally self-tensioned), makes for very creative approaches for pursuading it into tension in motorcycles. 5 It also wears surprisingly quickly, the chain's mass and tortuous encasement doubtless adding to this, resulting in a chain that needs replacement by 30,000 miles, and whose cam's timing is then markedly retarded (delayed), softening engine performance. Worse, many of the Hy-Vo cam chains originating in older Hondas are now discontinued as replacement parts. Worse still, Honda is being unconscionably deceptive about this and is putting inferior third-party (aftermarket) cam chains in wrappers with their names on them, just as they are also with ignition points and other vintage parts. I laud them for addressing vintage. I am scandalized that they are doing so in such a disengenuous, lowest-common-denominator fashion.

Where cam chain wear becomes evident is when the crankcase gets dirt in it. Dirt entry makes the engine oil abrasive. This came to be known many years ago when four-stroke dirt bike engines started appearing with odd and contradictory test results: low cylinder compression but good (low) cylinder leakdown. This odd but now-classic presentation is always cam-related. Either the cam lobes are heavily worn, or the cam is out of time due to severe cam chain wear, with the tensioner being fully deployed the important red flag. While dirt is liable to get into an engine through various means, a number of these engines had air filter issues. Either the filter was improperly serviced and maintained, or it was an aftermarket type (often K&N brand) that filtered very poorly. The resulting entry of contaminants ended up in the crankcase.

Cam chain tensioners are a problem in vintage powersports. One problem is that owners of 70s Hondas seem to not easily learn that their bike's cam chain tensioners are entirely manually adjusted. Despite spring assistance, properly deploying them is a manual, several step, specific procedure proposition, and even the official manuals often leave one in the dark. Another problem is these same folks seem to almost universally deduce that Honda cam chain tensioners' bolts are for tensioning when in reality they are merely lockbolts. Folks are breaking off these bolts inside the engines' crankcases by the score even as you read this. A third problem is the rack and pinion design of many of Honda's 70s cam chain tensioners. The gears are made of sheet metal, with teeth that are stamped not cut, and after almost 50 years now typically rust-frozen, thus requiring even more manual and unintuitive manipulation than even when new. If you know these issues, the parts work fine. But they are utterly demanding of this knowledge.

The challenges to mechanics
There seems to be very little respect for motorcycle mechanics among individuals who regard themselves as superior. But the depth of their practical knowledge when compared with that of the average powersports user is nothing to scoff at.

Cam timing
Many seem to stress over cam timing, assuming that Honda made the marks to line up perfectly. They didn't and they don't. Add to this the aforementioned cam chain wear issue and newcomers to the task can be challenged. Mechanics learn to judge correct timing by noting whether a sprocket tooth's roughly 15 degree difference is closer or farther relative to the mark. It becomes even trickier in many engines because as the cam bearings are tightened, the cam rotates slightly. Judging its final position takes a little skill. The shim-under-bucket engines common for many years now demand camshaft removal to adjust valve clearances. Thus cam timing is now a maintenance procedure and not simply something done only during a rebuild.

1970s Honda valves
One of the scandals of powersports is that Honda and Kawasaki from the mid 70s through the mid 80s employed soft, very fast-wearing valves in their engines. On those engines having correspondingly hard seats, the valves last less than 15,000 miles, and cylinder compression quickly becomes a fleeting hope. And since the OEM valve is plated (actually plasma-coated), refacing the valve as was commonly done on old cars is out of the question. Saving the odd NOS appearance on eBay or the expense of sourcing from CMS-- when they have them-- vintage Honda valves are all gone. Two Japanese companies have stepped up and are providing aftermarket replacements, and this keeps head work on these engines possible as time marches on. However, the quality of the replacement valves is not consistent. Out of four I recently ordered, half were out-of-round and had to be sent back. Of the two sources, the ones coming in green boxes appear to be better quality than the ones shipped in brown paper.

Valve seats
A cylinder head's valve seats are a mystery to many, with the usual hype and unfortunate misinformation obscuring the plain facts. No one for example needs five angles on their valve seats. In fact, without even trying, a good three-angle valve job becomes five-angle in practice. So it's just hype. Tooling for valve seat work takes a dizzying array of forms. There are too many choices for the initiate to make sense of it all. However, very good work can be done with tools that don't cost as much as your house, if you are careful, knowledgable and proficient. And even the valve seat machines that are as big as pickup trucks have their drawbacks. The real proof of whether a valve seat is sealing in partnership with its valve is the vacuum test, and this is the test pro engine rebuilders use. It's eye-opening.

The importance of cylinder compression
You can restore low cylinder compression by doing a proper valve job, and when cylinder boring is included you can expect to see in your 70s Honda four-cylinder the factory-specified 170 psi at sea level. I do this many times each year. Sometimes you can "cheat" and gain about 20 psi just by loosening valve clearances slightly. I tell all my customers: shoot for 150. Anything over 150 psi in a 40 to 50 year old Honda is a bonus. By the same token however, compression significantly under 150 is a compromise. The engine just will not be as sharp. It won't be "all there". Carburetion will not shine, acceleration will be saggy, ignition adjustment will not be very fruitful, and the idle will not be rock steady.

Where ignition comes into it
Ignition maintenance appears to be a lost art. Chalk it up to the current automotive ethic of pretending ignition doesn't exist. In the world of vintage motorcycles however, it matters. Whether points type or electronic, OEM or aftermarket, ignoring your Honda's ignition is, well, ignorant.

Summation
Food for thought? I hope you better appreciate cams, valves and cam chains. They represent much that is less known than it should be, and critical areas in Honda top end rebuilding. Informed tuning steps make a huge difference in vintage Hondas. Though the factory manuals are good, decades of experience is worth whatever you have to pay for it. There are many things to know, things not very intuitive and not easily discovered. And the veteran mechanic's finesse isn't just nice to have, it's imperative, such is the fiddliness of these old machines.


Last updated December 2021
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