Between YM1 and the R-series, Yamaha made the leap to horizontally split crankcase, speeding assembly and cutting costs by allowing crank, both gear shafts, and kick start shaft to be set into the upper case. Because it was inefficient to produce vertically-split 250s and horizontally split 350s sharing no common parts, Yamaha engineers soon replaced those designs with a same-stroke modular system. Both would henceforth be horizontally-split, the 250 having 54 X 54-mm bore and stroke and the 350 64 X 54. Because crankcases, gearboxes, cylinder stud pattern, &c were shared, both could be assembled on a single line. Low production cost is the key to low market price.