I can't help with the 21" front but for a stock 18" drum rear, the spokes are about 150mm long. An old Yamaha shop manual lists them as 151.5mm for the outer, 152mm for the inner. But, it all depends how you measure them. There doesn't seem to be a standard way. Some places go from the bottom side of the bent top section to the end of the threads on the other (straight) end. Some go from the center of the spoke diameter on that top bent part, others from the center of the head. So, this can give you 3 different values, differing by up to a couple MM's. When I measure rears from the bottom side of the bent top, I get about 148mm. Yamaha looks to be measuring from the center of the head.