Something we emphasize a lot is the value of getting
your squish clearance right. A proper squish
clearance makes the motor both more efficient and
less prone to detonation. When you're assembling
your high performance motor project, this little
detail is well worth the small amount of time and
effort it takes to get it right. You'll have more
power, better fuel mileage, and your motor is less
likely to "ping" on you.
In the ideal world, your squish clearance should be
set to about .030". Tighter than that risks piston
to head contact; looser starts reducing the chamber
turbulence effects that you're going for. Unfortunately, I don't think I've ever
pulled apart a factory assembled engine that had a
squish clearance of less than .050"! So you can see
what a huge opportunity this is to improve on your
So let's talk a little about what squish clearance
is and how to adjust it.
Put simply, a squish band is the area or areas where
the piston comes in close proximity to the chamber
as it passes through top dead center (TDC).
Fuel trapped in this space gets squeezed out at high
velocity and generates turbulence in the chamber.
Turbulence in the chamber promotes better
atomization and less separation of the fuel and air,
encouraging it to burn.
Shown above is an angled squish band, i.e.
the squish band is at an angle from the horizontal.
More common in stock HD motors is a flat or
zero degree squish band, which is the same
thing except that the fuel is squeezed out
horizontally. Here are some factory heads with flat
Evolution Big Twin heads have a zero degree
squish band on one side
of the chamber only.
Buell XB and 2004-up XL1200 heads have a pear
shaped chamber with
flat squish bands on each side. This is commonly
referred to as a "bathtub"
Twin Cam heads also have a zero degree squish
angle in a
bathtub chamber, although it's oval shaped.
So squish bands are common in HD/Buell motors. In
fact, the only modern HD models that don't have
squish bands designed in are the 86-03 Sportster
models, pre-Thunderstorm Buells, and all 86-present
In any event, the idea is the same whether the
squish band is angled or flat: the piston comes
close to the squish squish shelf in the head as it passes through top
dead center, squeezing fuel out and generating
turbulence, and that improves the efficiency of the
burn while helping resist detonation. And regardless
of which HD or Buell model you have, unless you've
optimized the squish clearance, it's probably got a
lot more clearance than it should have, just due to
the way the motors are mass produced. HD has to
design things such that in a worst case tolerance
stack-up situation, the clearance is still adequate
to prevent piston to head contact, and that means
the typical motor will have much more clearance than
So now that you know what the squish band is
all about, how do you go about measuring and setting
it? It's really not hard at all.
Since none of us are small enough to climb down
into the chamber and measure it directly, we have to
do it indirectly. The easiest and most
straightforward way to do this is with some small
strips of .065" diameter solder placed strategically
around the piston:
.065" diameter solder placed around the squish
area of a Twin Cam piston.
A small dab of grease works well for holding the
solder in place. Some
people use tape instead, just be sure to tape it
in an area outside the
squish area so the tape doesn't change the
Simply place some small pieces at
strategic places around the piston dome as shown
above, Turn the motor backwards slightly to pull the
piston down the bore a little, put your gasket in
place making sure the rivets (if so equipped) aren't
getting sandwiched in between the head and cylinder
(grind off any that do), torque the head into place, rotate the piston
through TDC, and remove the head.
With any luck, your individual pieces of solder will
look like this. Now simply measure with a caliper:
Squish clearance is then modified to the desired value
by adjusting the height of the cylinder, the
thickness of the head and base gaskets, using a piston
with a larger compression height, or some
Both head and base gaskets are available in
different thicknesses, select as needed
to achieve a .030 to .035 squish clearance. See
gaskets page for the various sizes.
As often as not, we don't use a base gasket! In
many cases the proper squish can only
be achieved if we leave it out. Three-Bond 1104
works great in it's place. Squeeze it out around the
base of the spigot and then spread it around the
base surface with your fingers. The excess can be
easily wiped off with a rag soaked with lacquer
thinner. In the future, when the cylinder is
removed, the old 1104 is also easily cleaned off
with lacquer thinner.
There are a couple of other things to think about when
you tighten the squish. Number one, since tightening
the squish lowers the height of the engine, you need
to be aware that your lifters are being preloaded
more and you’re reducing the valve-to-piston
clearance, as well as changing the way the intake
manifold fits. Generally, at the .020” to .030”
you’ll typically be dropping the top end when you
optimize squish, these things aren’t a concern.
Measure them if you’re not sure. If you’re lowering
beyond .050” or so (not likely unless you’re
specifically milling the heads for more compression), you
need to pay attention to these details.
Second, tightening the squish does cause an increase
in compression. Often it does so without any
introducing any pinging because the more effective
squish band has detonation reducing effects. But be
aware of it. At NRHS, we measure all chambers and
adjust the size to give the desired compression
ratio at .030" of squish clearance. So if you're
buying a matched set from us, you're good to go.
One more thing, if you're inside your motor doing a
hop-up project, consider angling your squish bands.
Zero degree squish bands are easy to mass produce
and they work reasonably well, but an angled squish
band offers a couple of advantages. First, the fuel
being squeezed out of the squish band is aimed more
directly at the flame front. Second, an angled
squish band reduces the shelf that the air must go
around as it enters or exits the chamber, thus
There's only one factory motor that came with
an angled squish band:
Buell Thunderstorm heads came from the factory
with a cast-in
15 degree angled squish shelf. A matching 15
degree domed piston
was used in these motors as well.
Buell Thunderstorm heads and matching pistons are
set up this way, with a squish angle of 15 degrees. However, since the Thunderstorm squish
band is cast-in, it's not very accurate. The squish
clearance will typically vary around it's perimeter
and it can even have fuel traps. We frequently
machine the Thunderstorm squish shelf accurately to
match the piston:
A Thunderstorm head that's had it's squish band
machined out to 30 degrees,
shown with a 30 degree domed piston
Unfortunately, the Thunderstorm head has quite a bit
of excess deck overhang around the perimeter of the
chamber, and as a result, we almost always have to
mill at least .040" off the head to remachine the
squish band at stock diameter. Often it takes .050",
it just depends on the specific heads.
"Lightning" heads, as used on 96-03 XL1200S models,
and pre-Thunderstorm Buells, also have a squish band
cast into them, 10 degrees in this case. But they
were paired with flat top pistons so they don't have
an effective squish band as delivered. The Buell
Blast actually comes with a 15 degree Thunderstorm
piston under it's 10 degree squish shelf, so it has
a mismatch from the factory, and it's in a bad
direction that creates a fuel trap.
We often machine out the flat squish bands on other
factory heads as well:
A 2004 XL1200 head, which started life with a
pear-shaped bathtub chamber
using zero degree squish bands, after machining
the chamber for 30 degree
Common squish band angles you'll see are 10, 15, 20,
and 30 degrees.
There's a downside to angling squish bands, though. An angled squish
band gives up squish band surface area for a given
dome height. The steeper the angle, the more squish
area you give up, unless you make the dome taller. For this reason, steeper angles
such as 30 degrees (which approximates the valve
angles) are usually only seen in conjunction with
higher domes and thus higher compression ratios.
Shallower angles, down to the zero degrees found on
many factory Harleys, are more common on street
motors. For most street applications, we recommend
an angle of 15 degrees, as this provides a good
balance of squish area and angle without an
excessively tall dome. For most race applications,
we use a 30 degree angle.
In summary, adjusting squish is really not a lot of trouble at
all, particularly if you’re in there anyway. It
takes very little extra effort to get this right.
And it pays back triple. You can make the engine run
better, reduce pinging, and improve your gas mileage
all at the same time.
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