Contrary to popular belief, you cannot tell a 4-speed Muncie is an M22 by looking at the input shaft splines. All M22s use a 26-spline shaft made from a stronger alloy than the other 4-speeds. However, GM made both 10- and 26-spline versions of the M20 and M21. There are two ways you can tell the difference between the M21 and the Rock Crusher. The M22 has 27 teeth on the countershaft gear, while the M20 and M21 have 29 teeth. M20 transmissions have two grooves on the input shaft, M21 transmissions have one groove, and M22 transmissions have no grooves. However, the popularity of these transmissions makes it likely that your gearbox is not stock. Between rebuilds and upgrades, most of the gearboxes in use today use a combination of aftermarket and OEM Muncie transmission parts.