An automatic transmission includes a parallel-axis gear pair including a first gear having a shaft portion and a gear portion, and a second gear which meshes with the first gear; a case that has a supporting wall which is adjacent to the gear portion of the first gear and rotatably supports the shaft portion of the first gear; and a frictional engagement apparatus that includes friction members, a piston that presses on, so as to apply, the friction members, and a hydraulic chamber that holds hydraulic fluid which moves the piston in the direction of the friction members. As a result, both the shaft to shaft dimension as well as the axial dimension of the automatic transmission can be reduced.
A piston for an automatic transmission including a pair of mutually meshing gears such as counter drive and driven gears mounted on respective two parallel shafts, the piston including a bottom portion, and a cylindrical portion to be disposed radially outwardly of one of the mutually meshing gears, wherein the cylindrical portion has not only a cutout formed in a first circumferential part thereof for permitting mutual meshing engagement of the mutually meshing gears such that a point of the mutual meshing engagement is located within a circumference of the cylindrical portion, but also a through-hole formed in a second circumferential part thereof so that a speed sensor extends through the through-hole, for detecting the rotating speed of the above-indicated one gear disposed radially inwardly of the cylindrical portion. This arrangement of the cylindrical portion permits size reduction of the cutout, and assures sufficiently high degrees of strength and rigidity of the piston.
