Disconnect a battery cable, drop the flywheel cover, and observe how the starter gear engages the flywheel when a helper pushes the start pedal. The most common cause of grinding is worn pivot points in the linkage that allows the armature to begin spinning before the starter gear engages the flywheel. It's also possible you've gotten a 12 volt starter instead of the 6V you need. Both starters have the same number of gear teeth, but the diameter and pitch is different due to a different number of flywheel teeth. A 12V starter has a smaller diameter shaft, and the pinion gear turns slightly as it engages the flywheel. A 6V starter has a bigger shaft, and the pinion gear moves straight to engage the flywheel.
Jerry
