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These are from a ZF10M gearbox that came in recently. This box was showing a reluctance to go into gear. The ZF10M (or the HBW100) is a mechanical gearbox with two plate clutches. This is two of the clutch plates and the forward gear wheel: The plate on the left is from the reverse stack, the one on the right from the forward stack. Comparing the drive splines we get this: The bottom one is from the reverse stack and is pretty much as it was made. The top one is a different matter. The teeth in the spline are almost completely worn away This is the forward gear: The clutch plates work on the splines at the top, which look like this: The plates have worn grooves into the teeth. This makes going into and out of gear tricky. The box will either not engage, or will be reluctant to come out of gear These are the parts of the driveplate: The mushroom shaped bit on the big plate has been driving the gearbox through the pegs on the small plate. What should have been happening is that the drive is cushioned by parts of the white, plastic part - which are now missing So, with this metal to metal contact, any pulses in the driveline from the engine are passed straight into the gearbox. This vibration causes the gearbox input shaft to rotate with a varying speed through each revolution. The inertia in the rest of the drive (gears, propshaft, propeller) means that it wants to run at a constant speed. Net result is that the splines on the gear hammer against the splines in the plates - and wear them away rapidly Not nice Richard More on ZF and Hurth gearboxes here: http://www.canalworld.net/forums/index.php?showtopic=60619&p=1153456