The Cylinder Head part.2 (squish clearance)

If your machine is only for play, and that's the use to which many motocross bikes
are put, a wide squish clearance will not matter. You will not get peak power, but you
will possibly never know the difference. And you will probably never ride hard enough
to experience detonation.
However, if you want top power and no risk of detonation, the squish clearance
must be closed up. A squish band that isn't working is worse than no squish band at all
as it wastes part of your fuel/air charge. Wasted fuel charge spells less horsepower.

To give you an idea of how much horsepower you could be losing it would be good
to consider the example of a TZ250 Yamaha road racer. These engines have a bore
54mm in diameter and an offset squish chamber. The compression ratio uncorrected is
about 15:1, meaning that the trapped charge is compressed into a space 8.8cc in
volume. If the squish clearance is 1.7mm (lots of motors come from the factory like
that) 1.94cc of the trapped charge will not be burned until well past TDC, too late to
produce any power. 1.94cc represents 22% of the inlet charge lost. When the squish
clearance is reduced to 0.8mm the charge loss is reduced to 0.92cc or 10.5%. On paper
it would seem an easy way to pick up 11.5% more power, but losses reduce this increase
to about 5-6% on the dyno. Therefore maximum power goes up from 52 to 55 hp. Midrange
power can rise as much as 10%, so the bike is easier to ride and it doesn't
detonate.
Reducing the squish clearance is not easy, you can't just machine 1mm, or
whatever, off the head as the compression ratio would end up many numbers too high.
Also you must be sure not to reduce the clearance so much that the piston will bang
into the head at high rpm. The clearance required will vary from engine to engine, and
also on how careful you intend to be each time you replace a piston, rod or barrel.
Pistons usually vary in compression height by up to 0.2mm. Conrods are supposed
to be within a 0.2mm range but they can be up to 0.5mm out. Cylinder heights are
maintained within 0.4mm. In the worst case you could rebuild the motor with a new
piston, rod and barrel. The piston could be 0.2mm taller and the rod 0.2mm longer.
Together with a cylinder 0.4mm shorter than before, the new parts could reduce your
squish clearance by 0.2 + 0.2 + 0.4 = 0.8mm which would result in a blown motor if the
clearance was set at 0.8mm previously. Manufacturers realise this, so they purposely
set the clearance wide to make allowance for the worst possible parts size combination.
If you are willing to measure the squish clearance each time you do a rebuild, and
then compensate for inadequate clearance by fitting a thicker barrel base gasket or a
thicker head gasket, you can reduce the clearance down to the amount shown in

TABLE 2.1.
To find accurately what the squish clearance figure is, the barrel must be tensioned
down on a standard thickness base gasket. Clean all traces of carbon from the head and
piston. Place a strip of modelling clay 20mm wide by 3mm thick across the pistoncrown. Fit the head gasket and head and turn the crank to move the piston just past
TDC. Remove the head and then cut the clay down the middle with a sharp, wet knife.
Carefully pull one strip of clay off the piston and then measure the thickness of the clay
left on the piston. You have to be accurate, so use the end of your vernier calipers. As a
cross-check also measure the clay thickness on the other side of the piston. If the
thicknesses vary this would indicate that the head gasket surface has been machined on
a different plane to that of the combustion chamber. Also at this time measure and
record, for future reference, the compressed thickness of the base gasket and head

gasket. (FIGURE 2.3).

After the clay thickness is measured you can work out how far the head must be
machined to give the desired squish clearance. As mentioned previously, the combustion
chamber must also be machined deeper into the head to keep the compression ratio at
an acceptable level. If you wish to keep the compression ratio the same as standard, the
combustion chamber will have to be machined twice as deep as the amount skimmed
off to reduce the squish clearance, assuming a 50% squish band. Therefore if 0.9mm is
removed, the combustion chamber will have to be made 1.8mm deeper. A 50% squish
band is one having an area equal to half the cylinder bore area ie. an engine with a

54mm bore would have a squish band approximately 8mm wide (FIGURE 2.4).
To check that the machine shop recuts the combustion chamber to the originalcontour when it is deepened, you will have to make a template of the chamber shape
before you send the head off. The template can be made out of any light gauge metal or
even stiff cardboard. (FIGURE 2.5).

Most people like to see the compression ratio pushed up as high as possible. High
compression has always been equated with high horsepower. I agree that the
compression ratio should be made as high as practicable, but often the manufacturer
has already found the limit and built his engines accordingly. All you can do in this
instance is check that production tolerances have not lowered the ratio significantly
below that which the manufacturer intended.
Something you must always remember when dealing with two-stroke engines is
that increasing the compression ratio will not give a power gain equivalent to that
which you would pick up with a four-stroke engine.