The Cylinder Head part.4 (Compression Ratio)

As the Japanese motorcycling industry has been working with expansion chambers
as long as most, I am sure they are aware of the fact that the return wave actually forces As this bike is designed to run on 90-95 octane fuel the compression can safely be
bumped up to 13.7:1 for competition use on 100 octane racing fuel.
After all this confusion over all sorts of compression ratios you are probably
bewildered and wondering what compression ratio your motor will be safe on.
If you
take a look at TABLE 2.2, you will find the answer.

 These figures are what I consider
to be the maximum safe compression ratio for competition engines with good air cooling and an operative squish band. Water-cooled engines will generally run 0.5-1
ratio higher than indicated and motors grossly oversquare for their capacity i.e.,
Yamaha IT 175 usually require a number 0.5-1 lower than shown. Engines running a
pump fuel should not have the compression ratio pushed higher than the manufacturer
has specified. Most companies set their off-road engines up to run on 90-95 octane
petrol (gasoline) but some, like the Rotax engines used in the SWM and Can-Am bikes,
are really only happy on 100 octane. Road racing engines require 100 octane fuel on
standard compression.To keep all that tightly compressed fuel/air charge in the engine there must be a
perfect seal between the head and barrel. If you are really desperate, you could weld the
head to the barrel like they do in the twin turbo Le Mans Porsches, but the the more
usual method is to use an annealed copper or aluminium head gasket inserted between
the two.
McCulloch go-kart engines have used a thin 0.4mm aluminium head gasket for
years without too many problems, providing the engine remains stock, but I can't
really recommend aluminium for any other two-stroke as there always seems to be a
problem with leakage. McCulloch experienced a reverse problem in that they had used
aluminium gaskets for years then, on the MC-92 motor, they switched to copper. The
copper gasket always seemed to leak as you couldn't get the head tension high enough
to crush the copper gasket without the head distorting. On the new MC-93 they have
reverted back to the old aluminium gasket.
When a copper gasket is used, there is always a temptation to reuse the old one.
My advice is don't, unless the gasket has exactly the same inside and outside diameter
as the top lip of the barrel. Even then the gasket should be heated with a low heat gas
flame and allowed to cool and anneal.
Generally, I prefer to run air-cooled motors without a head gasket if the head is
recessed to give a spigot fit with the top of the barrel. In this instance I lap the head
onto the barrel, using valve grinding paste. When you are finished, be very careful to
get all traces of paste out of the cylinder and then clean the head and barrel so that the gasket sealant will take. Remember that removing the head gasket will raise the
compression ratio significantly.
Regardless of the type of gasket used, or even if you choose not to use a gasket, I
recommend the use of either Permatex No.3 or Hylomar SQ-32M gasket sealant. Both
sealants will provide a good seal at the elevated temperatures experienced in two-stroke
engines.

Two-stroke cylinder heads are easily distorted, so you have to be very careful not
to tension the studs more than recommended. Overtensioning will always cause the
head to warp. You must be careful to tighten the studs in at least three progressive
steps. If the head required 20ft/lb tension, you should take all the nuts down finger
tight and then to 10, 15 and finally 20ft/lb. After about 15 minutes go over the nuts
again and then, after the motor has cooled from a run for a minimum of one hour,
tension the nuts again.