What you're hearing is not likely gears grinding, though it always sounds like that. The starter drives the crankshaft through what is basically a sprag clutch, which freewheels one direction but grips the other. the rollers inside the clutch, which is attached to the back of the alternator rotor, are slipping instead of gripping when you're using the starter motor. It's usually a simple enough fix but you have to remove the front sprocket cover and the alternator cover, and then the rotor to do the repair. Parts aren't real expensive unless the outer clutch housing is cracked and needs replacement. Go here to get the part numbers to look for, only buy them there if you can't find them elsewhere because the shipping to the US from them is really high. You'll need #3 and #4 (3 each), possibly #13 (3 of those depending on condition) and worst case scenario, #2 as well
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