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Period Roadtests

Kool Kat "Katana" (From Hot Jap Bikes.
Authors name not provided)

Sean O'Brien gets into some serious lateral thinking and does it his way.

A fateful trip down from Dubbo to Oran Park for the ’88 Aussie Championships set Sean OO'Brien on the course of serious modifications to his trusty Katana.

”Yeah, I saw this YB4 Bimota with Ohlins upside-down forks,” he explained. “So I thought I might have a go at making some myself.”

Sean had owned the bike for about three years at that point and it was already reflecting its owner’s skill and imagination in motorcycle design. He’d paid $1000 initially. “It was rooted,” Sean told us succinctly. “But at least it was going and registered.”

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The 1982 750 Katana scored a heart transplant to an 1100, Sean salvaging the big-inch motor from a wreck. The motor scored a quartet of forged pistons but little else internally, although Sean cleaned up the ports and valves while it was in bits. Carburettors are 36 mm flatslides of an early Slingshot GSX-R1100: “They bolted straight on, even the cables fitted.”

Sean made up the ram tubes himself. Exhaust headers are Tingate, while the muffler Sean made up from a photo copier roller! “I’ve done the whole bike as cheap as I could,” Sean explained, “Using whatever was laying around…”

And then there’s the alternator. In an effort to narrow the engine for cornering clearance and lighten the bottom end of the motor for better rev responses, Sean removed the flywheel and alternator. The ignition cover was lifted from a GSX1100EFF (with a Yoshimura emblem milled into it) which is about an in inch narrower than stock.

An alternator from a Daihatsu Charade was then mounted on the swingarm, driven from the final chain! All it took was a bit of experimentation with alternator sprocket sizes to make it work, which it does from about 30 km/h on. Ingenious, simplicity carried to an ultimate extreme as a famous Italian motor engineer might have put it.

The swingarm itself has been braced, as has the 750 Kat frame: ‘mega’ bracing up the front also acts as cool air ducts to the carbies, which are protected by an owner-built heat shield. Dave Little, himself a owner of a very radical home-built Trident, lent a hand here, as did Sean’s mate Greg. The extra weight of the bracing is compensated by Sean removing a heap of unnecessary brackets.

Other mates to lend a hand include Jeff and the CBX-riding Stan. Brothers Max and Patrick got involved as well, as did Sean’s parents: Dad silver-soldered all the oil cooler fittings while mum made up a seat cover. Thanks go too, to Sean’s tech teacher Rick who let him loose in the college workshops.

Patrick was responsible for the first yellow paint job, which only lasted a week before Sean threw it down the road (“The bikes been down more times than Linda Lovelace,” he said). Bathurst-based Jeff did the repaint in acrylics, figuring Sean would fall off the bike again before the paint did…

Later, after another bloke in nearby Wellington sprayed his bike in similar colours, Sean commissioned Bridgewater Signs in Dubbo to add some graphics.

Not that you’d mistake Sean’s bike for any other, not with those home-grown upside-downers gracing the front of the bike.

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The forks once suspended an RM125 Suzuki motorcrosser, and are thicker and stiffer than the standard Kat forks, Sean says. Dave Little made a set of triple clamps from aluminium to take the inverted forks (which were conventionally mounted on the RM). Sean shortened the sliders to fit, and the pressed on the Katana bottoms to take the axle.

Instead of stuffing about with internal spring and damper adjustments to account for the greater mass of the Kat, Sean once again put his lateral thinking cap on for another exercise in creative simplicity. A massive fork brace was constructed which would also act as a lower shock mount for a single centrally-mounted Fournales shock absorber. Easy!

Sean admitted that it was a bit of a headache constructing this and matching the whole shebang up to a pair of GSXR750 discs and calipers. But it was worth it all the same: “The forks are stiffer than a 13 year old watching a porno movie,” he reckons.

The front brakes run off a CBR1000 master cylinder and braided lines, while at rear a GSX-R caliper grips a Honda disc. Rear shocks are a pair of Ohlins piggy-backs.

Front wheel is a mix of 16-inch Akront rim and Ducati hub, while the rear 18-inch Dymag is having trouble coping with the good ol’ country roads in Sean’s neck of the woods and so he’s currently modifying a few bits so than he can fit a pair of GSX-R1100 17-inch Marvic-copy wheels (together with a GSX-R1100 swingarm). Tyre choice alternates between Dunlop radials and Michelin slicks, dependent on mood.

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Sean’s likely to continue to modify his budget testbed, with the focus turning to few more horses (and bigger balls) once he’s sorted out the wheels and swingarm. And then there’s the GSX-R750 dry clutch that he’s modifying to fit, which he intends to operate hydraulically. Maybe we’ll return to the Central West in a year or two to see what he’s been up to then.

In the meantime, check out Fryz’s superb photos and wonder at the creativity and skill of this 27-year-old abattoir worker. That the bike took the award for ‘Best Modified’ at the recent Streetbike Sportcycles Katana Show is no surprise.

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If you need work done on your classic machine, from basic service to full rebuilds, contact me, I can help.

Period Roadtests

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