CZECHPOINT

Jawa Four Hundred (1966)

By Pavel Husak
(Translation by CzechExperts)

In my capacity as a chief engineer of Jawa off-road motorcycles I was trying to design for the 1966 season a two-stroke single-cylinder motorcycle with the torque characteristics and maximum power output substantially higher than all other machines – except for motorcycles with full size four-stroke 500 ccm engines that were, however, handicapped by their weight. Another condition for achieving good riding capabilities was a new crankshaft with the best possible inertia.

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The 1966 Works Jawa 400 Moto-cross

I was trying to base my efforts on the construction of the last season's moto-cross and ISDT Jawa 360 motorcycles but here an overall higher cylinder capacity would have to be achieved only by using the bigger cylinder bore value. Since we had to substantially improve the torque characteristics in particular I decided to increase the piston stroke. However, I had to design and manufacture a new engine case with a new crankshaft, cylinder and piston – actually the entire new engine.

The engine case again consists of one basic casting with covers. All castings (except for the cylinder head and the cylinder) are made of the magnesium to minimize the weight. The crankshaft is encased in the cage-type needle bearing on the left side, and in the double-row angular-contact ball bearing on the right side. The crankshaft diameter is 140 mm. It is a kind of medium value compared to heavy crankshafts of old types of Jawa moto-cross and trial motorcycles, and very light crankshafts of new Jawa brand motorcycles.

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The unique double expansion chambers of the 66 works 400

The traditional bearing of the connecting rod in the needle cage at the bottom and on needles without cage on the top was not changed. The piston is made from the aluminum casting and it is sealed using two chrome-plated steel piston rings. The aluminum cylinder has a pressed-in steel liner. It has only one exhaust port but there is an insert with a partition in the exhaust pipe, (double expansion chambers). The Jawa magneto ignition was left without a change, as well as the carburetor Jikov 2932 SZ.

The gearing was designed with a view of maximum reliability, and this is why the clutch was installed on the gearbox. The primary gearing is provided by gear wheels on the right side that are connected with a dry type multiple-plate clutch of the most contemporary design. The Ferrodo friction plates have a reduced height involute gearing engaging with the internal gearing of the clutch drum, which prevents the gearing damage. The declutching system is based on the well-proven pinion and rack system with a special declutching bearing.

The basic concept of the last year's five-speed gearbox remained without a change but the tooth system was enforced by using a larger module and fewer teeth.

The basic frame was again a dual cradle, (Banana), frame, since Jawa moto-cross and trial motorcycles were supposed to promote the stock types of Jawa motorcycles with a similar concept of frame. The advantage of this frame type for the off-road motorcycle was a break resistance; the disadvantage was in a slightly higher weight.

The front fork with the front wheel was identical to the fork used on CZ motorcycles; the spring mounting of the rear swing fork used Girling spring units. The rear wheel with a very effective cushion drive always worked very well on this motorcycle.

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Vlastimil Valek on the 1966 Works"Banana Frame"400 Jawa
(Picture courtesy JiriVrublosky)

 

 

 

 

 

Valek chases Paul Friedrichs, Valek finished 4th in the 500cc world championship in 66 on the Banana Frame 400. (Picture from Czechoslovak Motor Review)

Giorgio Invernizzi Photo

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