I was hesitant to reply to this, because I've never done maintanance on an M.How about it, M experts? Does M have clutch inspection ports? I know it is a frame-built tractor and Model C is unicoque, so maybe not... But Model C has TWO inspection ports in the bell housing (one of which, the one on the bottom, ALWAYS has mice nesting on it). If M has such ports you might start by locating them and opening them up and looking for something obvious. In addition, one of the ports on Model C reveals a zerk fitting that is used to directly lubricate the throw-out bearing. Since most throw-out bearings are lifetime lubricated, many owners miss this fitting when doing routine lubrication. If your M has such a fitting, I would probably start by hitting it with some grease, then trying to get the clutch to work back and forth on the shaft. If you're lucky, it ought to come completely free. These old tractors are very forgiving. Then, if it's still stuck, you can start spraying brake fluid on stuff through the ports without removing the starter...
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