DUNSTALL’S BIG BORE
After a decade of transforming customer bikes into proddie racers, Paul Dunstall was so involved with Norton’s big twins that he’s credited with assisting the Commando’s development. ‘Dunstall machines were first past the post 17 times in 1968,’ boasted the firm’s 30-page catalogue, ‘with three lap records thrown in for good measure.’ So of course he’d develop a range of performance parts for the new Isolastic 750, and even build complete, brand new bikes by arrangement with the factory.
The ‘bigster’ was based around an 810 engine conversion kit with a 3mm bigger bore, which included barrels, pistons, rings, studs and assorted sundries. That little lot cost £65 (plus an entire 50p for UK delivery); £7 more and 10lb less than the standard items from Norton. A Commando owner could stop at that point or continue the transformation with Dunstall’s reworked top end: a reshaped combustion chamber, 1/8” larger intake valves set at a decreased angle, up-specced guides and springs, race cam, lightweight followers and a high-pressure oil feed, fuelled by two 32mm Concentrics.
Once you’ve gone to all that effort, you might as well go the whole hog with a special exhaust system, lightweight front mudguard, fibreglass tank and seat unit (‘of a quality seldom seen in the United States,’ according to Cycle magazine), rearsets, Quaife five-speed gearbox, twin 9” front discs and fork legs to suit, with heavier springs in the Girlings at the back and a big, fat front tyre to cope with the enhanced stopping performance. And a Domiracer half-fairing if you wanted to go racing…
You’d be treated to a mellow, low-pitched grumble from the siamesed exhaust system, ‘the most beautiful, rich, deep, smooth exhaust note this side of Milwaukee.’ Designed by Dr Gordon Blair of Queen’s University, Belfast (who also worked wonders with the Seeley G50 racer and many two-strokes of the 1970s), the exhaust system was credited with adding 5mph to the Commando’s top end and shaving precious seconds off the standing quarter mile. It was also significantly quieter than the standard pipes fitted to production bikes of the time.
All of this effort added up to one of the unsung superbikes of the Seventies. The Dunstall 810 Sprint (as it was officially known) was a fast but tractable flyer, a fire-breathing beast which could be managed by mere mortals. In 1972, Cycle magazine were surprised by its restraint – Dunstall could so easily have built a quicker bike. But that would have been hard to start, a handful to manage and a lot less likeable.
Instead the 810 pulled smoothly from tickover, and 70 appeared on the speedo just when the tacho needle quivered at 4k in fifth gear. Snick down a gear at that point and it delivered freight-train torque straight through to the ton. Dunstall claimed an output of 70bhp at 6800rpm, which would be good for a real-world 125mph. Dunstall also claimed that the 810 would cover a standing quarter mile in under 12 seconds, but Cycle couldn’t replicate that using standard fuel with an unfaired bike. The 810 was 0.3 seconds faster over the quarter mile than a standard 750 Commando, however.
The 810 was roundly applauded for its smooth running, being notably less buzzy than a Kawasaki 750 triple on full throttle. The Norton, however, displayed an unsettling lightness at the steering head. This could turn into a tremor in certain conditions, especially when equipped with the chunky 4.10 x 19 K81 tyre that accompanied the double-disc front end. Under hard acceleration through the gears, testers were disconcerted by a high speed, low-frequency oscillation as the front end wandered somewhat. But it was solidly planted on all corners, with predictable steering that forgave any imprecision on the part of the rider.
Around town, the 810 could be more than a handful. Both throttle and clutch were slow and heavy, requiring an iron grip and a certain determination which made clean downshifts a little tricky. Even the handgrips were hard on the palms. By contrast, the brake lever needed less pressure than a Honda 750 or Kawasaki’s triples. The Dunstall front stopper was considered to be superior to them all and to Norton’s own factory disc set-up. Dunstall claimed their twin disc setup could ‘reduce the stopping distance of your machine by up to 50%,’ and it did indeed halt a Norton 750 from 30mph in just 19 feet. It was ‘among the best ever tested,’ by Cycle, ‘smooth, accurate and progressive.’
British testers reckoned that the Dunstall 810 felt very similar to the Norvil production racer – yet without its fairing, an 810 Sprint could easily be mistaken for an ordinary roadster. Until you met it on an open road, of course…