Cardboard boxes filled with parts
and a bare frame were constant
reminders to Tom of his “sweet, old
motorbike.” He needed to find a
mechanic, and through a friend of a
friend, came up with an arrangement
that saw him deliver the components
to a backyard workshop. This didn’t
turn out well. “After a year and a half
of excuses, I showed up and think I got
most of the parts back — it was hard to
tell if anything was missing,” Tom says.
Triumph model development
Triumph’s TR6 was introduced in
1956 as a sporting machine aimed at
the West Coast desert racers. Before
delving too far into the TR6, however,
let’s look back into the development of Triumph’s 650cc twin.
Prior to 1950, when Triumph released its 650cc 6T model, the
manufacturer had only the 5T Speed Twin, developed by Edward
Turner and introduced in 1937, and the T100 of 1939 as the larg-
est bore machines in its range. Both were 500s. Immediately after
World War II, dealers and customers alike were requesting that
Triumph create a larger motorcycle, especially in North America
where wide-open spaces and long, lonely roads taxed a 500cc
machine. Turner didn’t immediately see the merits in boosting
the engine capacity of his parallel twin
engine, but he finally acquiesced.
On the inside cover of a circa-1950
Triumph brochure, copywriters wrote:
“1950 will be notable as the year of
the production of a new model to
be known as the ‘THUNDERBIRD,’ a
650 c.c. (40 cu. in.) high-performance
motorcycle suitable for solo or sidecar,
which we are confident will be as pop-
ular as the now classic ‘SPEED TWIN.’
The new highways being engineered
all over the world demand even greater
performance and reliability, and the
‘THUNDERBIRD’ has been evolved to
meet these modern requirements.”
Triumph’s parallel-twin 650cc engine
featured a three-piece crankshaft with
a central flywheel turning on roller bearings at each end inside a
cast aluminum crankcase. Alloy H-section connecting rods with
plain, or Babbitt-style, bearings had an 82mm stroke and cast-
iron barrels had a 71mm bore, providing an overall capacity of
649cc with a 7:1 compression ratio.
Geared camshafts, taking their cue from an idler gear turning
on a pinion gear affixed to the right-hand side of the crank, rode
high in the horizontally split crankcase, inlet cam to the rear and
exhaust cam to the front.