Friday 2 September 2011

Fitting New Pistons to Ural Motorcycles

Fitting New Pistons to a Ural.

When fitting new pistons it is important to ensure that they fit the cylinders properly.
Before you start to measure anything carefully inspect the piston and gently remove any burrs from the bottom edge of the piston skirt with some 1500 grade wet and dry paper.

The gap between the cylinder and piston side should be checked.
Fit the piston to the cylinder with no rings. A 0.07 or 0.1mm feeler should just slide between the piston skirt and the cylinder at right angles to the gudgeon pin.
If this is not correct the cylinder should be bored or honed to the correct diameter.

The end gap of the piston rings must also be checked.
Fit the ring in the cylinder and push it down about 30 mm using a piston to ensure it is straight and at right angles to the cylinder. The gap should then be measured. The correct gap is between 0.25mm and 0.5mm. If it is too small the ring must be removed and  the ends carefully filed. Then check the gap again. This applies to all one piece rings but not the European three part rings.

The gudgeon pin fit must also be checked
The gudgeon pin should be oiled and fitted to the small end bush. The pin should rotate easily but not wobble or have play in it when pushed or pulled.
If the gudgeon pin is too tight and it is hard to turn in the small end bush the bush should be carefully honed or reamed to give a sliding fit.

The piston must be hot and the gudgeon pin should be placed in a freezer before the piston is fitted. The piston will expand slightly and the gudgeon pin will then slide easily into place. At no time should the gudgeon pin be tapped or hit as this will stress the crank and damage the big end roller bearings.

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