Spare parts cylinder/separating cylinder
- at SIP Scootershop
It doesn't always have to be a new cylinder ... Once a cylinder has covered a few thousand kilometres, the piston rings or even the piston should definitely be replaced to continue to ensure reasonable compression. We have the most important spare and wear parts in stock for all common tuning and original cylinders: pistons, piston rings, piston pins, piston pin circlips and gasket sets.
Commonly used are nose pistons, domed pistons and flat pistons. Whether nose piston or flat piston, the essential components do not differ. A small pin in the ring groove (ring lock/anti-rotation device) ensures that the piston ring remains in position and prevents the ring ends from hooking into the channel openings and breaking.
If cylinders or pistons are worn or the piston clearance has become too large, so-called oversize pistons are required. Example: Cast iron cylinder PIAGGIO Vespa PX 200, cylinder dimension = 66.5 mm, i.e. the inside diameter of the cylinder bore is given in mm. If the cylinder bore is worn, it must be reground. The engine repairer takes the measurement from the oversize piston, adds the piston clearance (manufacturer's specification) and thus obtains the new grinding dimension.
To measure a piston properly, the largest piston dimension is determined, lengthwise to the direction of travel, below the piston pin. The measurement is carried out at 20° C with a tested outside micrometer. Calipers do not provide reliable values.
Caution: PIAGGIO often gives high piston clearances in manuals, which are then measured below the piston rings, which is generally unusual. The cylinder grinder of your choice can help you best with his experience.
Aluminium cylinders also wear or need to be honed, which is why you will often find a letter on the pistons of coated aluminium cylinders. The letters, e.g. "A"-"D", stand for size specifications, whereby "A" is the standard size and "B"-"D" a correspondingly larger oversize. A step sequence of 1/100 mm is usual. When reordering pistons, use the next higher letter.
The main function of piston rings is to seal the combustion chamber from the crankcase and to ensure the flow of heat from the piston to the cylinder. In addition, they are also of great importance for regulating the lubricating oil balance. Accordingly, they are heavily stressed, subject to wear and should be replaced regularly. Loss of compression, reduced performance and poor starting behaviour are a clear indication of worn piston rings. The specified bore/Ø always indicates the dimension of the cylinder.
Piston rings are usually made of cast iron and in their highest-quality form (with nodular graphite) are already extremely break-resistant. Only the significantly more expensive steel piston rings, which are rarely used in the original Vespa range, are of even higher quality. The running surfaces of the rings are now chrome-plated to minimise wear and for better running behaviour. A distinction is made between rectangular and trapezoidal or L-rings and their mixed forms. The classic rectangular ring has established itself as the standard. In contrast to trapezoidal or L-rings, there is normally no prescribed installation direction for these.
SIP installation advice:
Fit piston rings carefully, do not overstretch, otherwise there is a risk of breakage.
Observe piston ring butt clearance, type and design.
Trapezoidal rings have only one installation direction!
Piston pin & piston pin clips
The piston transmits power to the connecting rod via the piston pin. This is mounted in a bore in the piston in the inwardly thickened part of the skirt. This bore often has grooves at the end for circlips/piston pin clips to limit lateral movement of the piston pin. These should be replaced each time the piston is disassembled. The clips are available as C, G, double G wire rings in 1.0 and 1.2 mm thickness or as circlips. Wire rings cannot be interchanged with circlips, nor can different thicknesses.