Honda's humble SS50 is iconic, but unremarkable. Built and sold in their thousands as the moped craze swept through the 1970s, the slick-looking four-stroke 'ped looked like a miniature motorcycle. Flat bars, upswept exhausts and a sexy slim tank, it was every boy racer's dream, until they prodded the kickstart.
For all its show, the SS50 had comparatively no go, and hot-headed 16-year-olds opted for the zippier two-stroke competition when they discovered the 3bhp Honda took 19 seconds to reach 30mph, 10 seconds more than Yamaha's 5bhp FS1-E, before topping out at just 40mph to the Fizzy's 50mph.
A modification to the carb helped things – slightly - but short of major surgery the little Honda just couldn't take the pace. Massively reliable, it just wasn't quick enough, and that really matters when you're 16.
Speed is important at 16
It mattered to specials genius and childhood SS50 owner Allen Millyard so much that 30-odd years later he decided to redress the balance by building his own special one-off. And, if you were lucky enough to attend the Devitt MCN Ally Pally Show in 2018, you will have seen it in action in the supersprint event.
Look carefully and you'll see his SS has an over-sized 250cc single from a motocross bike skilfully shoehorned in, making 32bhp at the rear wheel and actually weighing less than the original engine!
"I owned an SS50 as a youngster and loved it, but it wasn't until 1999 that I built my first SS50 V-twin. I'd been shoehorning big engines into different bikes for years, but it was time the little Honda got some attention. I did the complete build live at a motorcycle show over the course of a few days, and got it up and running on the final day. It ran perfectly."
A factory finish
"One of my overiding priorities is that it has to look as if it rolled out of the Honda factory like that, so I engineer it to suit - even down to the webs on the crankcases."
Yet the Millyard engineering madness doesn't stop there. For years Allen had nurtured idea of putting a single-cylinder four stroke motocross engine in an SS50, eventually opting for a Kawasaki KX250F motor.
"In an ideal world I would've used a Honda CRF250 motor, but you just can't find them for sale anywhere. The only way to get hold of one was to buy a complete bike and take the engine out, but that's a bit unrealistic and expensive.
Accidental Kawasaki power
"So a friend of mine who is a dealer mentioned that he had a kawasaki KXF250 engine in bits, it needed rebuilding but was a bargain so jumped at the chance. In some ways the Kawasaki engine is better, it's twin cam and looks are more pleasing to the eye.
"Because of all the extra power, the transplant isn't as straightforward as simply grafting the bigger engine into the frame," says Allen. "There are many modifications, mostly chassis related. The frame needed strengthening and all this was done internally so from the outside it looks completely standard. The forks have been beefed up by 2mm and there's heavy-duty Hagon spokes in the front wheel, a drilled standard disc and a KX85 twin-piston caliper.
"I had to build the swingarm from scratch, because there wasn't really a way of strengthening the original internally. It's constructed from cold steel rod 3.2mm thick, and needed to be longer than the standard one because the extra power caused the bike to be twitchy and unstable.
Coping with the extra power
"Even things like the gearing needed a rethink; for a start the drive chain needed to be uprated from a 420 to 520 to cope with the extra power. Then the final drive sprockets needed to be made, it's amazing how small the rear sprocket needed to be, it went from 48 teeth to a tiny 28!"
The detail of the build is true to how it would have been made if it was a factory-made production bike. The radiator and cooling system are beautifully finished, and the speedo is from a CB450 with a cleverly incorporated digital temperature gauge.
The real skill of building this bike is the seamless and natural way all of the modifications blend in with the SS50 silhouette. Which makes riding the SS250 a real shock, albeit an unforgettable and unique one.
Try not to flip it
As soon as the big-little single fires up it feels unnatural, and you have to remind yourself that you are sitting on a tiny SS50 chassis with its super-low seat and narrow bars. The instant response from the engine gets your attention and I remind myself that it would be foolish, not to mention embarrassing, if I were to rev it too hard and flip it over.
So it's with an air of caution that I put it into gear and let the clutch out with a sedentary amount of rpm… and it stalls! "You need to slip the clutch loads!" laughs Allen, as he reminded me of the SS250's insanely tall gearing, almost 60mph in first gear. With the feel for the gearbox and engine, I unleash the power and find the little machine is unbelievably fast.
It feels so unnatural to have a tiny chassis with so much power, in particular the way in which you have to ride it like a big bike, predicting how much throttle to give it flowing through fast bends, and optimising the gear changes, and of course shifting your body weight to counter the front end going light all the time! The riding experience from the 250 generates genuine raw excitement and speed. Simply amazing.
See the bike in action
Watch as Allen Millyard rides the bike as part of the sprint display at the Ally Pally, followed by 23-time TT winner John McGuinness and four-time BSB runner-up Chris Walker.
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