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Putting your rear cover back on your engine? Double check that your gear shift linkages are correctly aligned, or else you might find yourself unable to change gears properly. Here's how it should look.

If you can't change gear once the rear cover goes back on, or you get one gear change followed by a clunk and no further changes (make sure you are waggling the output shaft whilst attempting to shift) then these teeth may be misaligned.
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