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Bonny to Bonneville: May update

By Mario

May Update

It was all looking so promising at the NZ Classic Motorcycle Festival.

It was all looking so promising at the NZ Classic Motorcycle Festival.

Piston broke: engine dramas hit Bonneville record attempt

After making an appearance at the Classic Motorcycle Festival, Fergus Mayne's highly modified Bonneville was started up for the first time. only to result in a major engine seizure.

Ferg’s hope of taking a 1970 Triumph Bonneville to the salt flats in search of a land speed record currently lies in tatters. Or, rather, in a series of boxes strewn around his garage, while a hurried search was undertaken for replacement parts and expert help with reassembly.

Long story short, when the bike was fired up for the first time after its engine mods it didn’t run very well. After some fiddling, it became borderline rideable but then it seized up on its first real run on the road.

Disassembly revealed a litany of problems. The most obvious was a significant piston-to-bore sei-zure, but there was evidence of other potential problems too. It was time to call in expert advice–from someone unconnected to the build.

Valve lips visibly protrude below cylinder head. Disaster awaits.

Valve lips visibly protrude below cylinder head. Disaster awaits.

Phone home

As a British bike, there’s an enormous fund of knowledge about tuning Triumph twins to be found in the UK. In Ferg’s case, using a personal contact from one of the team, overtures were made to Hans Dixon based in Hampshire.

Hans has built engines that have won several British Championships, the most recent of which were with a 650cc Triton. Slipping a Bonneville engine into a Norton featherbed frame is of course a time-honoured route in classic racing, stemming from café racers of old. In Hans’s hands the en-gine was configured to a 270º crank throw and fitted with a full Nourish-Weslake top end. Piloted by Simon Sloan, the bike won the UK’s BCMRC pre-1974 Unlimited class twice. Funnily enough, it now resides in NZ.

After taking the head off Ferg’s engine, Hans was asked for his diagnosis via measurements, pho-tos and video calls.

The bad news kept coming

Skim has cut into valve seats, chamber material already being picked up.

Skim has cut into valve seats, chamber material already being picked up.

Initial fears of detonation damage were soon set aside, but there were some obvious and serious problems. The chief of which was insufficient piston-to-bore clearance, compounded by insufficient piston ring gaps. The seizure was inevitable. Overheating from the seizure and the distortion it caused made it hard to determine whether the gudgeon pins also lacked sufficient clearance. Either way, the whole engine had to come apart to ensure every last particle of metal from the seizure could be cleaned out, and any bottom-end damage assessed.

Attention then turned to the cylinder head and there were problems here too. Over-enthusiastic skimming of the face had started to cut into the valve seats. It meant the edges of the oversize Kib-blewhite inlet valves were protruding below the head. Pictures of the head before and after running showed material was starting to come away where the combustion chamber joins the rest of the head. It was, frankly, a disaster. Especially when new–or even good used–cylinder heads for 1969-1970 Bonnevilles are rare as hen’s teeth.

Stripping the motor down revealed two missing machine screws meant to hold the crankcases to-gether by the barrel, and somehow the 21-tooth front sprocket hadn’t been installed, a 20-tooth mysteriously taking its place.

Things looked briefly better once everything had been taken to an engine reconditioners for ultra-sonic cleaning. The Carillo rods looked okay. The bearings ditto.

Amongst the other parts, the Kibblewhite pushrods and valves were undamaged, as were the rock-ers.

With the engine all apart it was the perfect opportunity to reassess, with Hans’s expert guidance, how the engine was to be rebuilt.

From the US, high-compression MAP pistons were ordered and arrived in double-quick time. A new barrel was sourced locally and honed to the correct clearance for the pistons.

But other parts proved less easy to lay hands on. Out-of-stock cams and a secondhand replace-ment cylinder head that was too ropey to bother with meant a disappointing reality dawned. With the bike due on a boat on the 17th June there was insufficient time to get the bike rebuilt, running, run-in, tuned and packaged up.

The quest was over for 2019.

After less than a hundred kilometers expensive pistons are scrap.

After less than a hundred kilometers expensive pistons are scrap.

Future plans

After all the disappointments, let-downs and frustrations Ferg is understandably taking a break to clear his thoughts. For the time being, the Bonneville is parked at the back of the garage with a blanket over it. Ferg may attend this year’s event, helping out another team. And a crack at the record in 2020 remains a possibility, though by no means certain.

We’ll keep you posted on any developments and we will be going to Bonneville to stay in touch with some other Kiwi competitors and their progress. So watch this space.