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Cam Timing
H0W TO DO IT
BY KEVIN CAMERON
SEPTEMBER 1989 CYCLE


 Knowing the effects of cam timing is one thing, but how do you actually measure the timing of your bike's cams, and how
do you change it?

First, you have to know what your motorcycle's cam timing should be; specifications are usually given in a service manual
or in an aftermarket camshaft's instructions. To maximize confusion, there are three methods of cam timing:

(1) Timing at running clearance. Most timing diagrams in service manuals give opening and closing figures with valve lash
set at actual running clearance. Valve action at running clearance includes part of the clearance ramps - gradual slopes
ground onto the cam that take up valve lash gently, before the actual lift profile hits. Unfortunately, a valve lifts or closes
so slowly on these ramps that the exact angle when a valve first moves is hard to determine. But this method makes cam
duration appear longer; an advertising advantage when buyers think longer has to be better.

(2) Timing at checking clearance. To make it easier to time cams, aftermarket cam grinders generally use a more realistic
set of timing figures, taken at what is called "checking clearance." Checking clearance is usually either 0.040 or 0.050
inch. If valve lash were set to this distance, the tappet or follower would hit the actual cam lobe, not the clearance ramp
Lift at this higher point is rapid, making it easier to precisely determine at what crank angle the checking clearance is
reached.

(3) Timing by lobe-center angle. Cam-lobe-center angle is a simplified way of stating cam timing, for it specifies it with a
single number. This is the number of degrees of crank movement between piston top dead center and valve top dead
center
I recommend using the lobe-center method as the simplest and most understandable. Timing a cam by lobe-center angle
is easy to do because most cam profiles are symmetrical. Valve motion during lift is the mirror image of valve motion
during closing. Because this is so, any given lift on the opening curve is the same distance from the lobe center as is the
point of equal lift on the closing curve. To find the lobe center, we pick a lift, find the two crank positions that give this lift
at the valve, and the lobe center must be exactly halfway between.

Please note that the actual shape of the cam lobe will not be symmetrical in engines using pivoted cam followers (most
Suzukis and Hondas, for example). The action at the valve, however, is symmetrical, so the above paragraph still holds
true.

Let's take a specific example, and see how lobe-center timing can be calculated from either running clearance or
checking clearance timing specifications. In this imaginary engine, the intake begins to open at 37 degrees BTDC and
closes at 63 degrees ABDC. The open time, or duration, is 37 + 180 + 63 = 280 degrees. Because opening and closing
cam profiles are almost always mirror images of each other, the point of valve top dead center, or peak lift (that is, the
lobe center) will be located halfway between 37 BTDC and 63 ABDC. Half of the 280 degrees duration is 140 degrees,
and counting from the 37 degrees BTDC opening point, we get the lobe center location at 140 - 37 = 103 degrees ATDC.

WHAT YOU'LL NEED

First, you need general mechanic's tools for the operation of removing and replacing the cams. You'll need a large
diameter degree wheel and a means of tightly centering and affixing it to the crankshaft You can buy these readymade for
some engines, or can make something suitable on a lathe. There must be a pointer near the rim of the wheel to show its
position. It's traditional to use a bit of welding wire for this. You'll also need a way to turn the crank while keeping the
degree wheel in view. Don't try to turn the engine with the rear wheel; it's a headache and accuracy will be poor To
indicate valve movement, you'll need a dial indicator whose total movement is at least as great as your engine's valve lift,
and you'll need a clamp set to hold the gauge riqidly so its tip rests on the valve-spring retainer

FINDING PISTON TDC


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There are two ways of locating piston TDC-a dial gauge inserted through the spark plug hole, or the use of a positive
piston stop. Don't try to find TDC by locating the point at which the dial gauge stops moving; the crank can move several
degrees while the piston sits nearly still, making your TDC inaccurate. The right way to locate TDC is to define two crank
positions, one BTDC, one ATDC, at which the piston is equally far down (say, 8mm, for example). Because piston motion
is symmetrical, TDC will be halfway between these two positions.

A piston stop is just a spark plug shell with a short piece of bar stock projecting from it that stops the piston some distance
below top center. Before you screw it in, put a rod in the hole to show Piston position and rotate the engine to
approximate top center. and zero your Pointer on the degree wheel. This gives a rough TDC to work with. Remove the
rod, turn the engine ahead slightly (so YOU won’t screw the piston stop down hard on the piston crown) and screw in the
piston stop. Back the piston up against the stop and note the degree wheel position. Now slowly turn the engine forward
almost a full revolution until the piston comes up against the stop again, but on the opposite side of top center. Note this
position. Because your first TDC was just a rough one, you'll get two figures that are close but not necessarily equal-say
42 degrees ATDC and 38 degrees BTDC. Bend the pointer to split the difference, and check again in the two positions.
Now the numbers should be the same. When they are, your degree wheel is correctly fixed on the crank, and zeroed at
TDC. Make sure it's tightly secured.

If you use a dial gauge screwed into the spark plug hole, the principle is the same-find two positions of equal piston drop,
one BTDC, one ATDC, and TDC must be exactly halfway between them. Let the piston drop several millimeters to get it
well away from its low velocity region near TDC.

FINDING THE LOBE-CENTER ANGLE

The cam-lobe-center is the valve event's TDC-the angular position at which the valve is at maximum lift. As in the case of
finding piston TDC, it's very hard to locate this directly because with the valve fully open, the crank can move several
degrees with little movement of the valve. Instead, we choose some arbitrary lesser lift and find the two crank positions
that give that lift at the valve. The cam-lobe center will be exactly half way between those two positions-analogous to
finding piston TDC.

Here is how it's done: With the valve closed, use your clamps to put the tip of your dial gauge against the valve-spring
retainer making sure the gauge shaft is parallel with the valve stem. Make sure the gauge is indicating by pushing on its
case; the pointer should move, zero the gauge by rotating the gauge face. Now, picking some arbitrary lift, find the crank
positions, before and after maximum valve lift, at which the valve is that far open. The lobe center or position of maximum
lift will be halfway between them. You'll know if your work is reasonable because the range of useful lobe-center angles is
between 95 and 115 degrees. Intake-lobe center is ATDC, while exhaust-lobe center is measured BTDC.
Be careful not to bump or snag the degree wheel pointer, don't do anything that might make the degree wheel slip on the
crank, and don’t disturb the dial gauge. Any of these three deadly sins will ruin your measurement, and you'll have to run
through the procedure again. Bear in mind as you measure or adjust cam-lobe centers that it's possible to position the
cams such that the valves will either hit the pistons near TDC or will touch at the first trace of valve float. Either event will
bend the valves, destroying their seal, and render your engine useless.

Fortunately, there is a simple check for how close this is to happening. Put the piston just after top center on overlap (both
valves slightly open). Using any convenient lever, and being careful not to damage the surfaces of cam lobes or tappets,
pry the exhaust valves open and note how far they drop before contacting the piston. Street hikes usually have generous
"valve drops” of 0.060 inch or more. Tightly toleranced racing engines may be intentionally built with as little as 0.030- or
0.040-inch valve drops. Use your dial gauge for accuracy if it looks close, and try various crank positions to find the point
of closest valve-to-piston approach.

In closely assembled race motors with chain-driven cams, it's desirable to turn the engine forward only. Turning it
backward transfers cam-chain slack to the front of the engine, slightly changing the cam timing-perhaps enough to cause
valve-to-piston contact, In such engines you should find TDC with a dial gauge rather than with a piston stop. Use of the
stop requires turning the engine backward.

RE-TIMING CAMS

To change your cam positions, you'll need slotted cam-drive sprockets that you can adjust. You can buy these for some
engines or you may slot your own. If you buy pre-slotted sprockets, check that they have the same timing marks as your
originals. If you slot your own, make marks so you can reinstall the sprockets roughly in their original positions. Usually
slots allowing 3mm of movement either way are enough. Install pre-slotted ones at the midpoint of their adjustment.
Tighten one mounting bolt and leave the other loose for the angle-setting procedure (this is because only one of the bolts
can be reached at a time, and it saves time to use just the one). Use the camshaft remove-and-replace procedure in the
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factory manual to be sure you reinstall your cams correctly after changing the sprockets. Find the lobe-center angles of
the cams again after cam installation, so you'll know where they are.

To change lobe-center angle, rotate the engine until one valve on the cam you are working with is at maximum lift; friction
will tend to keep the cam from turning while you make changes. Loosen the tight sprocket bolt. Now you're ready to turn
the engine very slightly, counting off degrees on the wheel, to change the lobe-center angle. It all goes well, the cam will
remain motionless while you do it. If your intake cam was at 100 degrees and you want to increase it to 104. You back the
engine up four degrees and re-tighten a sprocket bolt. Turning the engine forward decreases intake lobe-center angle and
increases exhaust-lobe center. Turning the engine in reverse has the opposite effect. If it gets confusing,
stop, get yourself a little treat of some kind, and make a diagram.

MAKING IT STAY TOGETHER

It is critically important that you use the most careful procedure in securing cam sprocket bolts before running the engine
again. First, remove the bolt that is not holding the cam in position, and clean both it and its threaded hole of all oil, using
a spray solvent. This will allow effective use of a thread-locking compound such as Loctite. Cover the threads of the bolt
with the locking compound-don't just use a bitty little drop-and run the bolt into place. Tighten it carefully, so as not to
disturb the cam position, and torque the bolt to the maker's recommended torque. Turn the engine to bring the other
sprocket bolt up where you can get at it, and repeat the procedure. Failure to clean, glue, and properly torque
cam-sprocket bolts may result in slipped timing, with possible valve-to-piston contact and engine damage.
This procedure will definitely make your hands shake the first time you do it. The shaking will be diminished it you take
notes as you work-it's embarrassing to have to redo a step because you forgot a crucial number. Measuring twice is a
good way to be sure you're right. And it you find yourself barking at someone who has the thoughtless insensitivity to be
watching you, remember that mechanics and tuners do this all the time with no problems, With some mechanical
experience and proper equipment, so can you.




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