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Mark Walker’s GN Thunderbug – Man and Machine

Mark Walker’s GN Thunderbug – Man and Machine Classic and Performance Car

What has two cylinders, 4.2-litres of displacement and a GN chassis that’s almost 100 years old? Mark Walker’s savage Thunderbug...

It’s got a GN chassis that’s nearing its centenary and a Riley V-twin hot-rodded with a couple of pots liberated from an aircraft engine: the 4.2 badges are no joke.
Four drive-chains whirl an inch under your behind, and there are no front brakes. This is the favourite creation of Mark Walker, who built the Monarch (8.3-litre Curtiss OX-5 V8 in a 1913 chassis), and set multiple records at Prescott hillclimb in the Parker GN (6.2-litre Cirrus Hermes), having previously owned a 3.2 Ford-powered GN. Last summer he spectacularly blew up his 25-litre 200hp Darracq: do we see a theme emerging here?
‘They were all fabulous, but none of them are really like a GN, and I wanted a proper little cyclecar. Its looks are based on “BHD” – Basil Davenport’s “spare” car. I made the ash-framed aluminium body, which is why it’s heavy and not very strong…
‘I wanted a bigger engine, so I went to put Gipsy Moth barrels on the GN bottom end – but they were wider than the crankcase. The VSCC requires you to concentrate on cars that might have been possible in period – a good thing – so I needed a big V-twin. Duncan Pittaway [restorer of the ‘Beast of Turin’ Fiat S76, current owner of the Monarch] said his dad had a Riley V-twin, which has a big crankcase with internal flywheels. They came in three sizes; this was the biggest. But then the Gipsy barrels looked too small. Luckily someone had the remains of a nine-cylinder radial Salmson engine, and I didn’t mind nicking a couple of pots off it because it had only five left.
‘So I made the engine. I had the remains of a Liberty [L-12, a 27-litre V12 aero engine] so I stole the rods out of that, and made the crank – it’s quite simple, with the pins cold-shrunk into place. All you need is 50 quid for two gallons of liquid nitrogen from BOC…’ The pushrods are prodded from a machined-down GN timing case in the middle, which also drives the oil pump. ‘It’s roller-bearing and only holds half a cupful of oil.’ 
It first turned a wheel in the late ’90s. ‘It’s a brilliant road car, with fantastic steering. It weighs 495kg, and I quickly had to make it four-speed as three wasn’t enough – it’s got quite a narrow rev band. I used to get valve bounce at 1500rpm so I’ve put in some stronger springs, but I tend to stop looking at the revcounter when it gets to 1800. In top, it’s geared for 65mph per 1000rpm… and I’ve calculated that it fires once every five feet – so rather than “once every lamp-post” it’s about once every fencepost. I drove it to Rétromobile and Montlhéry this year, and we’ve been to Phoenix Park in Ireland.’ He’s even done the ‘Pom’ in it – which prompted a dash home from Silverstone to make and fit plywood mudguards to pass scrutineering.
‘It’s my favourite. You can’t win anything in it, except on handicap, but that doesn’t really matter.’

And the secret of those 4.2 Litre badges on the sides? Not Jag, so Jeep? ‘Nope. Holden.’
Words and photography: Paul Hardiman
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