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Bonneville world record attempt - part two

By Mario

Follow Fergus Maynes as he sets about sorting out his Triumph Bonneville 'bike for his world speed record attempt.

Last month we began covering the plans of Fergus Maynes to take his 1970 Triumph Bonneville and break the world speed record for a production class pushrod-engined 650cc at Speed Week at Bonneville Salt Flats in 2018.
World record attempt: Taking a Bonneville to Bonneville

In part 2, the work starts in earnest with the engine being removed and sent for tuning...

It’s a long, slow road to being the fastest in the world. Expensive, too. Even so, there’s a dash of Kiwi can-do about Ferg’s assault on the world land speed record for 650cc pushrod production motorcycles.

Sitting in its nondescript garage in Takapuna, with a few chips in its odd orange paint, the Bonnie doesn’t exactly look like a world-beater. But nor should it. The Southern California Timing Association (SCTA) rules specify that production class motorcycles should appear completely standard, right down to the footrest rubbers.

Ferg's Triumph Bonneville before it was dismantled.

Okay, let's get to work.

Okay, let's get to work!

The engine’s internal dimensions, too, must be as it left the factory: not even a rebore is allowed, necessitating brand new standard-size cylinder liners. A refurbished set sits proudly on Ferg’s shelves ready to accompany the rest of the engine to the tuners. 

Strip it, strip it good

But first comes stripping the motor and gearbox (this being the ‘unit’ version of Triumph’s venerable twin) out of the frame. 

Triumph Bonneville with its motor and carburettor removed.

Here's Ferg's Triumph Bonneville looking pretty bare!

Ferg's dismantled Triumph Bonneville on some shelves

If you want me, I'll be just putting my 'bike in the cupboard...

Ferg had already purchased and fitted a spare set of exhaust downpipes, into one of which he’d welded an oxygen sensor. The purpose being to understand, and precisely set, the fuel-air mix that would be crucial at over 4,000 ft altitude. But having established that it fitted and worked, it was one of the first things taken off the Bonnie on a mild, clear Thursday night in Auckland, altitude 35ft. 

Taking photos of the rest of the dismantling wasn’t just to accompany this article. Having a record of where every nut, bolt, washer, bush, spring, carburettor needle, spade connector and snipped wire fits is damn useful when it all has to go back together again. 

Close up of a Triumph Bonneville dismantled.

Labeling's important - you don't want to connect the horn to the brake.

First problem

Tearing down an old ‘bike is never going to go without a hitch. And so it proved when it came to getting the tank off.  

It should be said that all three of us – Ferg, myself and Ferg's mate Martin – have previous convictions for Triumph twin ownership. (For me, it was a 1970 T100R Daytona: kind of the 500cc younger brother to the 650 Bonneville.) Even so, this was an American-spec ‘bike with subtle differences to the machines Ferg, Martin and I knew. One was the tank mounting, which somehow also incorporated the US-market side reflectors.  

It looked like a stud projected down from the tank, through a bush on the selector housing, with a nut and washer on the end. Simples. Except the right hand bolt just rotated, or so we thought.  

Close up of the Triumph Bonneville stud and not assembly on the 'bike frame.

Here's a close up of the stud that was causing us problems.

Attacking the left-hand one revealed that it was a stud, not some squish-bush or other weird arrangement, so the decision was taken to apply a bit of force to lift the tank off the frame. It came away, and so did what was left of the Araldite holding the tank mounting stud to the tank. Oh dear. 

Still, the odd bodge or problem was only to be expected. If that’s the only one encountered it will be a miracle. In the meantime, there’s a small welding job on the tank to do, so every last whiff of vapour will need to be out of it. 

Here's Ferg's Triumph Bonneville in an early stage of being dismantled.

Shopping list

Because SCTA production rules leave quite a bit open in terms of engine tuning, there’s nothing stopping you spending an absolute fortune chasing horsepower. Apart from your bank balance, that is. Ferg is trying to keep costs down, spending what he thinks will be ‘enough’ to take the record without going overboard.

Ferg jokes by holding the handlebars of just the front part of his 'bike.

On your marks, get set – hold on – where's the engine!

Even so, the shopping list for parts is a long and expensive one. On the night of the engine removal, Ferg showed off a grab-bag of exotic valve train components he’d just received from America. Tasty parts included larger valves with thinner stems for lightness, but they were still steel, not titanium. These were matched to specially-shaped, high-strength valve springs, guides, special collets, retaining collars etc. All-up, including the excise man’s cut, there wasn’t much change out of $1,000 for these alone.

Of course there’s a whole load of other things to splash out for in the engine/gearbox rebuild. The work amounts to a complete nut-and-bolt rebuild, including porting plus specially made crank, pistons and rods, but it must all conform to original 1970 model T120R specs. 

So, now the engine is out and its removal documented, the rebuild will begin. Next time, we’ll give you a little more about Ferg’s previous adventures on the salt.

Triumph Bonneville engine sitting on an old tyre.

It's a nesting Lesser Spotted Triumph Bonneville engine