John Blackburn came across an article in a magazine that was originally written by Jim Lindsay and takes a look at the workings of the DOHC 895 and 894 Jawa Models.
Jim went along to the Jawa headquarters in King's Lynn to watch Czech factory mechanic Rudi pull apart one of the Jawa 894 engines. We should point out at this stage, that the 894 was designated for Speedway, whilst the 895 was designed for the Longtrack circuits.
Simplicity and strength are the two most obvious features of the engine's design. The crankcase split vertically and the halves comprise very hefty, thick walled castings. The bosses in which the main bearings are housed are very substantial, as are the bearings themselves. The strength is necessary to cope with the forces generated by the large, out of balance mass of the hefty crank with its full circle flywheels and the conrod which carries a dustbin sized piston. The heavy flywheels are needed to maximise the torque output of the engine in the lower end of the rev range.
The crank runs on cylindrical element roller bearings contained in a rather unusual cage. To handle the additional stress, two sets of rollers are used on the drive side of the engine as opposed to the single set employed on the timing side.
The crank is a pressed up affair with the big end running on a needle roller bearing. A big surprise was the amount of vertical play in the con rod, necessary said the factory's chief designer, for the engine to be able to rev freely. No actual figures were quoted for this free play, it was preffered to leave it to the feel and preferences of individual mechanics.
The piston is conventional except for the large number of grooves therein from just below the piston ring locating grooves to the bottom of the piston skirt. Their purpose is to retain as much oil as possible, thereby ensuring the best possible film of lubricant between the piston and cylinder wall.
The cylinder barrel is also of conventional design, being an aluminium alloy casting with pressed in steel liner. A tunnel for the cam chain is incorporated into the left hand side and the two eccentric wheels through which the cam chain adjustment is achieved are mounted inside the tunnel just over halfway up. Access to the adjusters is via a pair of threaded alloy plugs.
On the drive end of the crankshaft is a taper on which is fitted a spline collar to carry the drive sprocket, transmitting the power to a rudimentary clutch, whence it finally goes to the rear wheel. Inside the timing case, a three stage sprocket picks up the drive from a pinion located on a pinion on the end of the crankshaft and distributes it, via chains, to the camshafts and the magneto.
The Head follows the normal edicts of design for power, having a shallow combustion chamber with four valves set at a shallow angle to each other. The twin overhead camshafts run plain in the head and, of course, this layout allows the spark plug to be centrally sited for the most efficient combustion.
There is a raised lip on the barrel which fits into a register on the underside of the head. The gasket consists of a thin copper ring. The barrel fits over four two-piece studs which protrude beyond the top of the barrel, the ends of which are threaded. There are tapped holes in the underside of the cylinder head, and the head and barrel are tightened down by inserting a spanner between the cooling fins of the barrel and screwing the studs up into the head.
Camshaft to valve contact is via
buckets and shims, the shims being contained inside the buckets rather
than resting on top of them. Tensioning of the cam chain is achieved by
means of a pair of eccentric adjusters mounted inside the chain tunnel
in the ctlinder barrel. When tensioning the chain, an equal amount of slack
must be taken up on each adjuster. Unless it is done in this manner, variance
of up to eleven degrees can occur in the valve timing.
Being a bit puzzled by this set up, the factory engine designer was asked why they did not use a more foolproof system. He replied diplomatically, that it was foolproof if the mechanic knew what he was doing and that the present system was extremely simple in the mechanical sense and therefore, very reliable. The magneto is held in the timing side crankcase half by a crude but effective metal band and is driven by chain from the same composite sprocket that also drives the camshafts. The nut which holds the drive sprocket onto the magneto shaft is equipped with a slot into which fits a spigot on the end of the oil pump's main shaft. The oil pump is mounted on the outside of the timing chest cover and is fed from a small tank mounted on the frame of the bike. There is no return feed as the lubrication system is total loss.
And that is how the whole lump goes together. The safe rev limit is 10,000rpm, high for a big single. Claimed maximum power is 65bhp. It's a very basic power plant, tough, quick and reliable.
|Engine||Single Cylinder||Single Cylinder|
|Cooling||By Air||By Air|
|No of Valves||4||4|
|Comp Ratio||13.5 to 1||13.5 to 1|
|Plug||NGK B95EN||NGK B95EN|
|Engine Oil||Castrol R40||Castrol R40|
|Gearbox Oil||Castrol R40||Castrol R40|
|Rr Suspension||None||Pivoted Fork|
|Fuel Tank||2.5 Ltrs||2.5 Ltrs|
Harry Lahti from Vasa, Finland happened upon the site and sent in a couple of pictures of his machine. Harry writes:
I'd like to attach a photo of my Jawa 894.0 short stroke, year probably 1977. This is my first speedwaybike ever. I got it as a birthday present off my wife almost four years ago, on my 50th birthday! At that time it was in a rather poor shape, so I stripped it and rebuilt it. At first my intention was only to check that the engine was sound, but by the time my project developed to a complete rebuild.
The engine proved to be very difficult, a long stroke crank, short stoke head and cylinder. There was a 6 mm aluminium spacer at the cylinder foot to
compensate for the longer stroke. As I was (am) not so experienced in speedway bikes, it took some time to find all the parts needed to put it back in
original shape. There are still some items which are not as when new, such as the blue bellows on the forks, blue handle grips, not original handlebar,
Mikuni carburettor and K&N filter, newer clutch and blue clutch and carb wires. You may see something more that I don't know about.
The first start attempt was made in front of my garage on the 26th of September, and it was a success. At that time the nearest speedway track which is 90 km away, was already closed for the season, so now I'm waiting that the ice would become thick enough that I could make a private speedway track on the ice at our summer place.
I have a set of ordinary speedway tyres equipped with a lot of sharp screws scewn in from inside. They are said to give a good grip. Let's see.
In addition, we now have a set of "Before and After" restoration pictures of a Jawa 895 Longtracker, kindly sent to us from the ever expanding collection of Rick Newlee.
However, this page is becoming too large for quick loading, so please follow the link to view the Newlee 895 Longtacker
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