65.4mm new piston
65.4mm new piston
65.4mm new piston

65.4mm new piston

Regular price
£137.00
Sale price
£135.00
Tax included. Shipping calculated at checkout.

Technical notes for measuring pistons and re-boring clearances.
Two-stroke engine pistons often run hotter than similar capacity four-strokes because of the working strokes are doubled. Exposure of the exhaust ports and contact with hot gases 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 (American: Wrist pin) 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 of the carbon, take care not to scratch the piston crown. A scratched piston-crown would encourage the renewed local carbon formation. Take care not to crack the piston skirt area as it may lose compression and cause the transfer port timing to be changed. Remove all the carbon from the ring grooves (a broken piston ring is ideal for this).  
When you select 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 be re-bored. Too large a piston will alter the port timing and thus adversely affect performance.
Concerned about the state of your cylinder? Contact Schmittparts – we can supply cylinders either ready bored to suit or good used example to rebore at your local machine shop.
Pistons have been available 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). Other engines may have been re-bored using the original Sachs/Mahle pistons. These increment in size by 0.5mm and replacements are occasionally available.
Original Mahle pistons are able to run with what most two stroke specialists consider to be optimal clearance.  However, ASSO and GPM have a different coefficient of expansion and will suffer multiple seizures if standard clearance is used.  Clearance for new ASSO or GPM piston 0.07 to 0.08mm (bore to piston) e.g. bore diameter 65.5 = piston diameter 65.42 to 65.43. UK stock North West England 65.4mm 65.6mm  and 66.0mm £135.00 each