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Piston to valve clearance is a critical dimension,
as not enough clearance here can cause bent or even
broken valves. But few people take the time to
measure and adjust it. It's not difficult to do, and
I highly recommend checking this if the TDC (top
dead center) lift figures on your cams are on the
large side, say over .220" on either or both valves.
The TDC lift spec tells how much each valve is open
at overlap TDC, i.e. the TDC that occurs between the
exhaust and intake strokes. Both valves are slightly
open at this point. Since the piston is passing
through top dead center with both valves open, this
is where you may run into issues. The other TDC,
between the compression and power strokes, both
valves are closed.
A quick and dirty method for checking valve to
piston clearance is to attach a Trock fixture or
similar to the cylinder head, and with the engine
positioned at TDC, open each valve until contact
just occurs, measure how much lift it took, and
subtract the TDC lift figure. The result is your
clearance. A lot of people do it this way, but there
are some drawbacks. The biggest one in my mind is
that the method only checks for depth clearance.
Equally important and at least as likely to be an
issue when using large valves is what we call
"eyebrow clearance", the clearance between the
perimeter of the valve and the i.d. of the valve
pocket. The other disadvantage to the static method
is that it's not giving a dynamic dimension. Since
the valves and the pistons are both moving in the
real operation of the motor, the point where they're
closest together is not necessarily right at TDC.
So what I'm going to show here is how to "clay" a
motor. In my opinion, this is a better way to do it.
The first thing you need to do is position the motor
correctly:
In this picture, Jon has a ratchet and socket on the
final drive pulley nut, the motor is in gear, and
he's turning the motor over to position it. If your
motor is in the bike you can do much the same thing,
turning the rear wheel instead. Get the rear wheel
up off the ground by placing a small scissor jack
under the frame, or better yet, use a bike stand or
swingarm stand.
So where do you position it? We want to start this
process at "compression TDC", i.e. top dead center (TDC)
between the compression and power strokes. At
compression TDC, both valves are closed. Conversely,
as mentioned, at overlap TDC both valves are
slightly open. For putting the clay in, we want to
start at TDC with both valves closed, then we'll
roll it through overlap TDC to compress the clay.
To determine which TDC you're on, watch the lifters
as you turn the engine. When you see the intake
lifter going down, i.e. intake valve closing, the
next TDC is the compression TDC.
Okay, now that we're at TDC in between the
compression and power strokes, we're going to pack
some clay into the valve pockets.
The best thing I've found for this job is oven-bake
clay from my local hobby store. Peel off a little
bit, pack it into each valve pocket, and then ...
Take a sharp knife and trim away the excess. Too
much clay in the pockets will make it get between
the valve and seat when we turn the motor over.
You want it to look about like this. Just around the
perimeter of the pockets. Now turn the motor
slightly forward and ...
Put the head gasket into place. Something to watch
for here:
These Cometic gaskets are multiple layers of steel
that are riveted together. Notice how this cylinder
has a notch on each side to accommodate the rivets.
Most cylinders aren't like this. You need to make
sure that rivet doesn't get sandwiched between the
head and cylinder and keep the head from sitting
flat. It's usually only one side, the spark plug
side, that has an issue. We generally just grind off
the rivet and it's little tab. The alignment dowels
on the cylinder will hold it all in alignment
anyway.
Okay, now that the gasket is in place, carefully put
the head on, without scratching up it's deck, and
torque it into place:
Be sure to follow the order as specified for the
gaskets, or if the gaskets don't have an instruction
sheet, go by the service manual. For the Cometic
MLS/EST gaskets, torque in steps of 9, 14, 22, 35,
and finally 42 ft-lbs.
Most high performance springs will require you to
clearance your rocker box. This is really important,
because if that spring or retainer hits the rocker
box, it makes the valve land unevenly on the seat,
which can cause seat recession and loss of
compression. Make SURE you get this right. Remove
the rocker arms before you do this, clean it
thoroughly after you're done. Place it over the head
and line up the bolt holes and take a look. Go back
and grind on it some more until you're sure you've
got lots of clearance. When you've got it right,
clean it up one last time and then put the rocker
arms back in, with assembly lube on the bushings.
This is what you want to see, making sure the valve
is fully closed when you look at it (best to do this
check before you put the rockers back in so you know
the valve is closed). You want a visible gap all the
way around that spring, without causing damage to
the wall that captures the rubber gasket. Don't take
a chance with this at all. Get plenty of clearance.
It's important.
Put your pushrods in place (longer pushrod on the
exhaust), and then put the rocker box gasket and
rocker box itself onto the head.
Tighten down at least the four main bolts that hold
the rocker box down (shown above). No need for
pushrod covers at this point.
If your lifters are standard hydraulic units, and
they have oil in them, the valves will open as you
tighten down the rocker boxes. The pushrods will
have a lot of pressure on them and you won't be able
to turn them by finger. As the oil escapes from the
lifters, i.e. the lifters "bleed down", the valves
will close. When you can turn the pushrods by finger
you know the valve has closed and released the
pressure on the pushrod. Do not turn the motor over
until you reach this point! The result could be bent
valves.
If your motor has solid lifters in it, extend your
pushrods until all the slack is out.
If your motor has Hydrosolids or other travel
limited lifters, you need to extend the pushrods
until the valves are open and stay open, even after
ample bleed down time, and then shorten them until
the pushrod just goes loose, i.e. the valve
just closes.
This motor had new hydraulic lifters in it, but they
had no oil in them. As a result, with the standard
non-adjustable pushrods the plungers just pushed
down, without the valves opening, and the lobe was
going to have to push the lifter up about another
.100" before the plunger bottomed and the valve
started opening. So even though this motor has
standard hydraulic lifters, I still put adjustable
pushrods in it for the purpose of making this
measurement, so that I could bottom the plungers and
get the valves to start opening as soon as the cams
told them to. Watch for this if you're using new
lifters. If you ignore it, the valves won't open as
far during this test as they will in a running
motor, and your measurement won't be valid.
Now we're ready to turn the motor over. Watch the
valvetrain as you do this.
If you did everything right, you won't have to turn
the motor very far at all and the exhaust valve will
start opening. Notice how the pushrod end of the
exhaust rocker has moved up in this picture. Keep
turning, you'll see the exhaust start closing. Then
as the exhaust approaches closed ...
... the intake will start opening. It's hard to see
in this picture but both valves are slightly open,
the exhaust is almost closed and the intake has
started opening. This is the overlap period. The
piston is passing through TDC as you pass through
here and the valves are smashing the clay. Keep
right on going, don't pause or back up.
The next thing that happens is the intake valve
fully opens. Notice how the pushrod end of the
intake rocker is higher than the exhaust. Keep right
on turning, let the intake valve fully open, and
then close. As soon as the intake valve closes,
STOP.
Pull the head back off and hopefully you'll have
something that looks like this. Remove the gasket
and bring the piston up to TDC for clay
measurements.
I usually just stick a machinist's ruler into the
clay to do the depth measurement. Some people prefer
to remove the clay and measure with a caliper, but
it's very difficult to remove it without changing
it's size. You need to see at least .060" of
vertical clearance on the intake and .090" on the
exhaust. The piston chases the exhaust valve closed,
where the intake valve chases the piston down, so
valve float can cause contact with the exhaust
valve, but not the intake. So the exhaust clearance
needs more margin. The intake clearance conversely
is just to accommodate thermal expansion and piston
rock.
As you can see, this pocket has more than .100" of
vertical clearance, so it's fine. However, notice
the "eyebrow" clearance, i.e. the thickness of the
clay against the wall of the pocket. You want a good
.050" of eyebrow clearance. In the area near the
ruler, it's plenty, but at the edges (top and bottom
of the photo) it gets substantially thinner.
I carefully peeled the clay from the pocket and
measured the eyebrow clearance with a caliper. I had
plenty in the center of the pocket, but like I said,
I was concerned about how it lost clearance out away
from the center. The pocket diameter wasn't big
enough.
For minor adjustments like this, I generally don't
remove the piston and fixture it up into the mill
for a precise cut. Instead I just tape off or
otherwise cover any area where debris could get into
the engine, and using a die grinder I modify the
pocket. Here Jon is using a coarse grit Scotchbrite
pad. It works well because it leaves a smooth finish
and also rounds off sharp edges. A sandroll can also
be used to remove larger quantities of aluminum and
do it faster, but it's a good idea to finish off
with a Scotchbrite pad.
Once you're sure you have plenty of clearance, clean
everything up and do the final assembly. Here are a
few more tips ...
On 2003 and older Sportster motors, heads can be
installed first and then the pushrod tubes put into
place. On 2004 and newer models, you can't do it
this way, the pushrod tubes need to be there when
you put the head on. Here Jon is installing the
front cylinder pushrod tubes using NRHS billet
aluminum pushrod bases.
Here's an easy way to check pushrod length if you're
using standard hydraulic lifters. Finger tighten the
2 large rocker box hold down bolts on the spark plug
side, while the pushrods are supporting the rocker
arms and rocker box on the pushrod side. Now take
your caliper and measure how far the rocker box is
being held off the head. When you tighten down the
bolts, this is how far the lifter plunger will get
preloaded. The plunger has about .200" of travel and
ideally you'd like it centered, so you'd like this
measurement to be near .100". This one is nearly
perfect.
Pre-lube everything! I always put a dab of assembly
lube on each end of every pushrod, as well as the
valve tip, and of course in the rocker arm bushings
as well. And before I button it up, I pour oil
everywhere. It takes a bit for oil to get pumped up
to the rocker box and I don't want it running dry. Be
sure to pour some down the pushrod tubes, too.
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