If you want to get into classic road racing, the K4 Honda makes a fine companion – they’re plentiful, reliable and won’t break the bank. Mark Graham gets the lowdown on the whys and wherefores
The CB350K4 four-stroke twin remains one of the most successful classic racing grid-fillers at both entry-level and national standard. It is also one of the cheapest routes into classic road racing.
Honda sold more than 600,000 K4s from 1968-1972, more than half of that number in the USA. They are still in plentiful supply and you can pick up a complete 350K4 for as little as £1500.
As Stephen Walsh’s race engineer Graham Stinson says: “If you’ve a good pair of hands and you use the right bits, anyone can see 43bhp from a 349cc engine.” The stock K4 was 325cc and maybe 32bhp at the wheel. With a race camshaft (£400), stainless steel valves (£200), Kibblewhite valve springs and retainers (£300), and a pair of 66mm pistons (£350), you are in business.
The better developed engines are claimed to be nudging 48bhp on various dynamometers. “A decent K4 is 45bhp,” says Stinson. “Unless you’re Dominic Herbertson [2018 Junior Classic TT winner on a Davies Motorsports Honda 350K4], a few horsepower here or there is the same difference as being a 10-stone rider on a 43bhp bike or a 13-stone rider on a 45bhp bike.”
Few are better qualified to chat about the Honda CB350K4 as a race machine than multiple Classic Racing Motorcycle Club 350 Champion John Davidson. “There’s no better machine to start on than a K4 because even the standard chassis can cope with 40bhp,” he says. “With a Maxton fork conversion running 5 or 10 weight oil and some decent shocks, it’s fine.”
The stock frame is flawed in the swingarm pivot area. “You can either remove the rear footrest hangers and put gussets in to reinforce that area or buy an Asa Moyce/Drixton frame,” says Stinson. “But your first concern is braking. The road bike wheels are fine to start out with, but you’ll need a disc brake. Some people fit a Honda 400/4 front end, and with track-compound pads it’s OK. When you get quicker you’ll need a twin piston AP Racing CP2696 caliper, and an adapter plate to mount it.” You’ll also need a master cylinder of 5/8in bore.
The bottom-end internals are as robust as they come – and that’s key to why the K4 endures as a race machine. The pressed-up rol
