In 1963 Ford released the 289 High Performance which produced 271 BHP @ 6000 RPM.
Somewhat stronger connecting rods with 3/8" bolts, thicker main bearing caps,
solid lifter cam, screw-in rocker arm studs, machined valve spring seats,
forged steel exhaust valves, a dual point mechanical advance distributor,
and better flowing exhaust manifolds round out the package.
Those fitted to Shebly GT 350 cars rated at
306 HP also featured a high rise aluminum intake manifold and Tri-Y headers.
The 289 HP features a high nodularity cast iron crankshaft, Brinell hardness
tested to ensure quality. To prevent 4th-order harmonic vibrations from
destroying the crankshaft at higher RPM, the engine uses a different vibration
damper and an add-on counterweight. A portion of the 28.2 oz.in. imbalance
found in the normal small block damper is moved to the additional counterweight.
Moving the mass in towards the front main bearing reduces bending loads on the
crankshaft. The special damper also has a larger more massive inertial ring.
The add-on counterweight is 0.150" thick. As a result a special crank sprocket
is used, C3OZ-6306-A, itself being 0.150" thinner than the normal 289 sprocket.
The counterweight is both keyed to the crankshaft and indexed to the sprocket with
a 1/8" roll pin. The images below show the counterweight and sprocket with its
roll pin hole, as well as a comparison of the the normal and thin sprocket.