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Measuring pistons and re-boring clearances PDF Print E-mail
Written by David Garner   
Saturday, 24 July 2004
Two-stroke engine pistons often run hotter than similar capacity four-strokes because the working strokes are doubled. Exposure of the exhaust ports and contact with hot eases leads to further increases. The heat expansion is therefore greater than with the four-stroke-engine. Greater assembly tolerances are required for pistons. gudgeon pins and piston-rings. to compensate for the greater heat expansion. Transfer ports and inlet ports contribute significantly to the cooling as the fuel mixture is drawn under the piston. However, the piston loses support as it passes the open ports and thus further stresses are placed on it. Over time a carbon layer builds on the piston-crown. Carbon particles may form that cause pre-ignition. Glowing carbon layers lead to over-heating of the piston resulting in high wear and possible piston-seizure. When removing this carbon, take care not to scratch the piston crown. A scratched piston crown would encourage renewed local carbon formation. Take care not to crack the piston skirt area as it may lose compression and cause the transfer port tinting to be changed. Remove all the carbon from the ring grooves (a broken piston ring is ideal for this). I acknowledge the valuable assistance of Heiko Zimmermann in supplying technical data for these notes. When selecting a new piston, choose one that is just larger than all the wear marks that exist from the previous piston. If you select the largest available piston, it will reduce the number of times that the cylinder can he re-bored. Too large a piston will alter the port timing and thus adversely affect performance.

MOC supplies pistons in a large range of over-size diameters. If you have an original bore that has very light wear, it may be possible to use an original size 65mm diameter piston. (In this case the bore will need to be lightly honed to remove glaze and uneven wear). Partsmart can usually supply these front stock. Other engines may have been re-bored using the original Sachs/Mahle pistons. These increase in size by 0.5mm and replacements can be supplied to special order.
Last Updated ( Monday, 06 October 2008 )
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