It could be said that Yamaha started the new era of 4-stroke motocrossers with their YZ400F a few years ago.  Nobody could ever have imagined the incredible effect this would have on dirt racing around the world.

Other manufacturers have been playing catch up ever since the YZ400F, that model evolved in to the YZ426F, now for 2003 Yamaha look to have raised the bar once again.  Meet the YZ450F.

A new longer stroke 449cc engine retains the trademark 5-valve cylinder head but now boasts an improved automatic decompression system.  A new piston and crankshaft contribute to a claimed 20% reduction in rotating inertia for quicker, more aggressive, response.  The 426 was not a machine for the feint at heart, the 450 sounds as though it will be an even more aggressive beast.

Thankfully the clutch has now been strengthened to cope with the extra grunt from the bigger engine.  Also new on the YZ is a four speed gearbox, one ratio less than the previous model.  The WR retains the customary five-speed gearbox but gains from the same clutch improvements as the four-speed YZ.

The frame has also come in for attention and combines with a new tapered swingarm to help bring the YZ down to a claimed 100 kilogram dry weight, five kilograms less than the 2002 model.  It is unclear what Yamaha will claim for the Australian road legal spec' WR450F with electric start.

Suspension has been slightly refined with the 46mm upside-down Kayaba forks offering improved levels of damping.  Kayaba also supply the revised fully-adjustable rear shock absorber.

Braking is handled by a 250mm disc up front and a 245mm disc at the rear.  Those discs mount to Excel rims, 21" up front and 19" at the rear.

For the WR model the addition of electric start will be an absolute bonus and will ensure that Yamaha's WR models will retain their positions near the top of the sales charts.

Specifications

• Engine  - 449cc, liquid-cooled, 5-valve, DOHC, 4-stroke

• Bore x Stroke - 95 x 63.4mm

• Compression Ratio - 12.5:1

• Induction - 39mm Keihin FCR flat-slide carb', throttle position sensor

• Ignition - Digital CDI

• Gearbox - 4-speed (WR 5-speed)

• Front Suspension - 46mm Kayaba inverted telescopic fork, adjustable compression and rebound damping, 295mm travel

• Rear Suspension - Fully adjustable Kayaba single shock, 310mm travel

• Front Brake - 250mm floating disc

• Rear Brake - 245mm disc

• Length - 2137mm

• Width - 815mm

• Height - 1282mm

• Seat Height - 980mm

• Wheelbase - 1462mm

• Ground Clearance - 365mm

• Dry Weight - 100 kilograms for the YZ but no weight has been hinted at for the electric start WR

• Fuel Capacity - 8 litres (WR will get a bigger 12 litre fuel tank)

• Availability - Best guess is for a November release for the YZ, while we may have to wait until  around March for the WR

• RRP - ?