Because Japan has been and continues to be the premier motorcycle manufacturing country, the Japanese Standards Organization (JASO) has led development of four-stroke (4T) motorcycle oil standards.
The three major requirements of JASO’s 4T motorcycle oil specification, introduced in 1999, are physical properties, engine performance, and clutch friction performance. JASO MA is the category for high clutch friction oils and JASO MB is for low friction oils. In 2006, JASO revised the 4T specification by raising acceptable oil quality levels, segregating JASO MA into JASO MA1 and JASO MA2, and introducing phosphorus limits.
Recently, the JASO standard has been updated again. Yoshinnobu Yashiro from Yamaha Motor Co. Ltd. who is Chairman of the JASO Motorcycle Oil Working Group describes the key changes that needed to be made: “We needed to update the JASO 4T specification to introduce new friction plates and reference oils for the clutch friction test because the existing materials were no longer available. However, we wanted to keep a level of consistency with the existing 2006 specification in terms of testing severity, test oil performance discrimination and classification ability.”
Gear pitting is an important parameter for motorcycle oils, particularly as oil viscosities decrease in order to offer fuel economy performance. As Yashiro-san explains, “Gear pitting is one of the biggest differences between motorcycles and passenger cars. Motorcycle engine oils demand anti-gear pitting performance as they lubricate the engine and the gears. As the viscosity of passenger car motor oils falls we naturally want to set up a criterion for gear pitting performance for motorcycle applications to ensure the oil is fit for purpose.” Though Yashiro-san had hoped a gear pitting test would be included in the most recent update, he said a gear pitting test was not included because “we noted that oils are not the sole cause of gear pitting – engines and gears can also influence this phenomenon. That makes it quite difficult to develop a test which can evaluate the gear pitting performance of the oil alone while establishing any correlation with field or engine test performance.”
“Recently, Yamaha has been putting more focus on High Temperature High Shear viscosity (HTHS) and we are using this when evaluating oil performance for engine durability, including gear pitting. MCO HTS viscosity is not being lowered as much as that of passenger car engine oils, where HTHS needs to be 2.6 or higher” Yashiro-san said. “We knew that returning to a 2.9 HTHS oil would improve the engine durability, but we decided to continue our races without encountering any critical problems.”
