The father of the high-power air-
cooled engine, S.D. Heron, noted in
a 1920s test that while a stainless
valve was relatively unaffected by
the new leaded fuels—which were
to be essential to ultimate Allied
predominance in World War II avi-
ation—a tungsten-steel valve of the
older technology, after three hours’
running “Looked as though it had
been dipped in liquid slag.”
Because more durable exhaust
valves were a continuing necessity
in the large aircraft piston engines
developed 1930–1957, a wide
range of materials and technolo-
gies evolved. These would include
internal cooling by partial sodium
filling and hard-facing of the sealing
surface with the everything-resisting,
Stellite, a cobalt=chromium alloy.
TOP LEFT: Burns Stainless head fabricator
John Bath lays artful weld bead. TOP RIGHT:
Admirers of good welds call it “a stack of
dimes.” LOWER LEFT: A fine-toothed band-
saw blade makes a cut. LOWER RIGHT: A hole
saw prepares for a tube joint.Fortunately for us, exhaust-valve
working conditions in most modern
motorcycle engines are benign by
comparison. Small diameter makes
it easy and quick for heat to flow
from the hot center of the valve
head to the valve seat that provides
most of its cooling. The adoption
of liquid-cooling in most modern
engines has put an end to valve-tem-
perature problems. Even in those
classic air-cooled designs still made
today, intensive local liquid-cooling
of exhaust valve seats has often
been adopted, by circulation of
either engine oil or water/anti-freeze
around or near the seat.
In exhaust systems, stainless
continues to compete with titanium.
For the lightness of titanium you pay
a premium price, but stainless pro-vides durability and enduring bright
appearance at lower cost.
The 300-series stainless alloys
are non-magnetic and cannot be
hardened, causing my dad some
frustration when he tried to sharpen
1950s knives made from them. I
first encountered high-strength 17-4
PH stainless in a motorcycle axle
that the distinguished constructor
Harry Hunt had made for one of his
projects. (Remember Hunt alumi-
num brake discs?)24 / CYCLE WORLD