VHM 12º Kolben KTM SX 125 21-22 HVA TC 125 14-22 Gas Gas MC 125 21-23

HOW TO MEASSURE ANY DIRT BIKE´S PISTONS

The pistons of any internal combustion engine, two or four-stroke, are conical and oval, to control its expansion in its operation. To get to know your maximum measurement you have to measure at 10-12 mm. of the skirt, in the opposite part to the bolt.

When we see a mark on the head of the piston, for example, 80 mm. or we have a cylinder of 80 mm. this measure is corresponds to the cylinder, when meassuing the piston we will see that actually measurement is 79.95 mm., the tolerance between piston and cylinder would be 5 cents (0.05 mm.).

In the aluminum cylinders with hard chrome interior treatment, already in disuse, or NIKASIL the measure of 80 mm. It is usually fixed to 80 mm.

The pistons work with tolerances between 4 to 6 hundredths (0.04-0.06) for two-stroke engines and 3 to 4 hundredths for four-stroke engines (0.03-0.04). To fix a deformed cylinder, the interior of the cylinder shall be polished first and the measurement is adjusted with pistons that are over-measured for this purpose, marked with letters A-B-C-D (selection); the lowest measure being A, increasing 1 hundredth per letter; that is, selection B 0.01 mm. greater than A, C 0.02 mm. greater than A, etc.

EXAMPLES

New Cylinder Ø 80.00 mm. mounts a piston of Ø 79,97 A Factory Tolerance 0.03 mm.

Polished Cylinder Ø 80,01 mm. mounts a piston Ø 79,98 B Factory Tolerance 0,03 mm.

Polished Cylinder Ø 80,02 mm. mounts a piston Ø 79,99 C Factory Tolerance 0,03 mm.

Polished Cylinder Ø 80,03 mm. mounts a piston Ø 80,00 D Factory Tolerance 0,03 mm.

The important thing is to maintain the tolerance, which the manufacturer recommends, between cylinder and piston.