Honda CB450 Twin 1968-1975 Service Manual

Big Bike Reviews' own history nails exactly why this specific K-series generation matters more than the earlier, more famous "Black Bomber": introduced in 1965, the CB450 earned lasting recognition as Honda's "first big twin" and "the world's first mass-produced DOHC motorcycle" — but it took until February 1968's K1 update for Honda to genuinely refine the formula the market had been slower to embrace than expected.

Motorcycle Classics' own retrospective details exactly what changed and why: after the original K0's oddly humpbacked gas tank drew criticism for looking too unconventional next to British and American rivals, "the K0 was superseded by the K1, with a 5-speed gearbox, a more conventionally shaped tank, a larger oil pump to aid warm up and a longer wheelbase to improve handling." That 5-speed gearbox was a genuine mechanical upgrade over the original's 4-speed, replacing the earlier bike's transmission with a forward constant-mesh unit operated by a left-foot return-shift lever, mated to a wet multi-plate clutch and standard 530 drive chain.

Power comes from a 444cc air-cooled DOHC parallel-twin, a genuinely unusual engineering choice for Honda at the time — Bonhams' own auction description notes the CB450 "departed from Honda's usual practice in a number of ways, in particular its use of twin overhead camshafts, torsion-bar valve springs and constant-velocity carburettors," plus a tubular frame replacing Honda's traditional fabricated spine type. Output settled around 43-45 hp at 8,500-9,000 rpm with roughly 28 lb-ft of torque at 7,000 rpm, fed by dual Keihin CV carburetors, delivering a genuine 102-110 mph top speed depending on tune and test conditions. Big Bike Reviews' own spec breakdown notes the electric starter arrived alongside the K1 in 1968 (earlier models were kick-start only), while ignition stayed points-and-condenser across this entire K-series span before CDI arrived on later post-1974 models.

Genuine styling variants proliferated within this K-series run: the CL450 Scrambler, launched in 1968 alongside the refined CB450 Super Sport, gave the platform a high-pipe, off-road-inspired look after Honda found the original CB450's touring styling wasn't resonating with American buyers who wanted something sportier. By 1975, Curbside Classic's own retrospective notes Honda stretched the engine's stroke to create the CB500T rather than genuinely reengineering the aging platform — a stopgap measure widely regarded by period reviewers as making an already-dated bike's vibration problems worse rather than better, closing out the CB450 lineage before Honda's 1978 Hawk 400 picked up the middleweight-twin torch with an entirely fresh design.