Skip to main content


Bonneville land speed record attempt, part six

By Mario

It’s been a winter of enormous work and engineering development but Fergus Maynes’s record-chasing Bonnie is now shaping up nicely.

Last time round, progress on Ferg’s would-be Land Speed Record machine had hit a bit of a speed bump, in the shape of its pistons. With the need to create maximum reliable power, their design and performance are naturally crucial. So after talking with manufacturers in the USA, the UK and Dunedin, eventually a custom solution was sourced from JE Pistons in the USA 

Solving the piston riddle means that everything is now coming together quickly. Which it has to in order to cover a demanding schedule of dyno and road tests, including a planned outing at the New Zealand Land Speed Association’s December meeting. 

Engine build

With the beautiful billet crank away for machining, attention has turned to the top end. But building a one-off engine for a land speed record attempt has more challenges than a simple reassembly.

Precisely measuring how much the pushrods will have to be shortenedBecause the pistons didn’t realise quite the compression ratio desired, Graeme Cole the engine builder has dispensed with the base and head gaskets, expertly machining the facings. However, this means the pushrods are too long, so they need to be shortened to operate in the optimal ‘throw’ of the tappets. Not a biggie, but just an example of how one modification begets another.

Oversize, thin-stemmed Kibblewhite valves in situ, left

Another discovery along the way concerned the high-performance Kibblewhite ‘beehive’ valve springs. They were simply too strong. Designed for absolute maximum power in a 1/4 mile drag bike engine, they would have beaten the valve seats to death and put excessive strain on the rest of the valve train. Again, because the head has been modified to fit larger, thin-stem Kibblewhite valves, it’s not a case of just swapping for some softer springs. Something more ingenious was required so James Simpson Engineering in Albany is machining down the valve collars to reduce the pre-load on the progressive springs.

Other modifications have been taking place too: 

  • Spherical roller bearings have been sourced for the crank to reduce friction and deal with flex
  • An external oil filter is on order to do a much more effective job of filtering than the Triumph’s standard wire mesh. To meet the Southern California Timing Association’s Production Class rules this will be hidden away near the battery box and side panel
  • An exhaust ‘plug’ is being machined to give the option of closing off the Bonneville’s exhaust balance pipe
  • K&N air filter inserts have arrived, sourced from a USA supplier
  • An ultra-low output alternator will be made up and tested as an alternative to running total-loss ignition
  • Boyer-Bransden electronic ignition to replace the standard points and condenser set-up
  • A high-output coil package is now hidden away under the tank, with the HT leads emerging through the original, but empty, coils
  • A new custom made wiring loom will be made and fitted

K&N air filter inserts ready to fitK&N air filter inserts ready to fit

High output electronic ignition pack routed through original coil cansHigh output electronic ignition pack routed through original coil cans

First sponsor: Shorai Batteries

Congratulations and thanks to Ed Smith of Shorai batteries (, Ferg’s first official sponsor! Shorai NZ has supplied the team with state-of-the-art power in the shape of a tiny (113mm x 89mm x 58mm) but extremely powerful lithium battery. It’s an important part of the build because the small dimensions will make room for the hidden oil filter while the strong charge makes possible low-output or total-loss ignition. Although weight loss isn’t a target for Ferg’s bike (lead is added to increase traction) many racers will appreciate the 1.61kg reduction over stock.

Build and testing schedule

The T120R is about to enter the most exciting phase of the whole build when everything comes together for the first time and testing begins. First will be getting the carburation to a point where the bike starts and runs, followed by a simple few runs up to temperature and more tweaking of the carbs. Next, the engine will need to be run in. After checking, the bike will then be booked in for runs on the dyno, and this is where the serious work on cam timing and gearing will begin, giving the team their first indications of power gains and overall engine performance.  A spare set of exhaust pipes has been sourced and Lambda sensors installed in both pipes to allow a Dynojet module to measure and record the air/fuel mixture to fine tune the jetting.  Of course getting the set-up right at sea level is one thing; getting it right at an altitude of 4,400 ft is another. But the dyno runs will give a base setting and prepare the bike for its first competitive outing: in the NZ Land Speed Association’s December meeting at Goudie’s Road in the Kaingaroa forest.

The NZLSA have adopted the same classes and categories as the SCTA, which makes everything simpler. Apart from the chance to score a New Zealand Land Speed Record, the Goudie’s Road run will be a valuable opportunity to run the bike at speed over a race distance. Another meeting in February is a fallback.

SCTA events are run using a proprietary fuel, so Ferg has already sourced a ‘similar’from arom  a US supplier for set-up and testing. However, with the NZLSA meeting now featuring as part of the development, the five gallons sitting in Ferg’s shed is looking to be not nearly enough, so another order is on its way.

Next time

Our next instalment on the Bonnie should reveal something that looks like an actual motorbike! And, hopefully, a chance to hear the 650 twin burst back into full-throated life.