Pistons are made from aluminum alloys as they are light. The content and mix of the alloy is determined by the application and engine system. A 2 stroke piston is made from different material than a 4 stroke piston as the forces and heat distribution in the engine differs.
Cast pistons and silicon:
Cast pistons are able to contain higher silicon content compared to forged pistons. The casting proces allow this higher silicon content. Next to this 2 stroke engines have lower cylinder pressures compared to 4 stroke engines. Cast pistons are ideal for 2 stroke engines. ProX Racing Parts pistons are gravity cast from an aluminum alloy with approximately 18% silicon content.
Why is silicon important:
- It decreases thermal expansion. Heat changes the geometry of the piston. A cold piston has a different shape than a piston that has reached operating temperature. Low thermal expansion characteristics allow pistons to run at tighter tolerances, which increases performance. The better the piston fits the boring of the cylinder, the better it performs. Better performance equals more power!
- The property of silicon in pistons has a great effect on the running properties of the piston in a cast iron or nicasil cylinder boring. It gives better wear characteristics and increases the lifetime of the piston by decreasing wear on the piston skirt.
Most of the ProX pistons come with a coating. Coatings on the piston skirts improves the wear in process when you just started the engine. A new piston needs to get the chance to adjust to the cylinder like the piston rings. We have developed a new skirt coating that not only assists the wear in process, but also helps reducing friction after the wear in. The black coating is made from a delicate mix of materials and will increase the performance.
Anodizing helps resist heat of the aluminium alloy of the piston. Although aluminum is light and the silicium content is important to give the best features for running in a cylinder, it is not the best material to cope with heat. The combustion proces creates a lot of heat, so anodizing the piston areas where heat plays an important role is key for specific applications. Not all applications need pistons with anodized areas as the performance or cooling of the engine does not make this necessary. So a non-anodized piston is still a very good piston!
The following parts of the piston are mainly anodized:
- the piston crown
- the ring grooves
- the piston pin holes
A break in procedure is necessary for proper ring seal. Refer to the service manual for proper break in procedures, air filter service, and oil premix ratios.
When a part runs in an engine, oil will take care of the lubrication. To be sure the oil reaches the right parts of the piston, we have added specific lubrication holes in most of our pistons. The engine system determines the way the engine/piston is lubricated. 2 Stroke engines lubricate the piston by the oil in the fuel (mix). 4 Stroke pistons are lubricated by the oil in the engine crankcases. The oil is forced pumped to specific areas in the engine.
2 Stroke pistons have lubrication holes in the pin bosses, to lubricate the piston pin and in the exhaust skirt to increase lubrication if the exhaust port of the cylinder is bridged.
4 Stroke pistons have lubrication holes in the pin bosses, to lubricate the piston pin as well and you will find many lubrication holes in the groove of the oil ring package. These holes will bring and return oil to this specific area of the piston.