Gear Box Failure

I finally got the time to strip the gear box down today.  Thankfully, the damage is limited to the main shaft’s third gear wheel, with no damage to the corresponding gear on the lay shaft or any of the other gears.  All the shafts, synchros, bearings and seals are also intact.  The 2nd/3rd gear bush is broken, though, where the third gear sleeve meets the thicker thrust piece between the gears.  On some versions, this is a two piece bush, but this was a single piece.  The break is very clean, and oddly the break shows no sign of movement – the two parts fit together precisely with no gaps or movement, with the crack all but invisible.  It makes me wonder why it cracked in the first place, if there has been no movement between the two parts.  A stroke of luck was finding a brand new 3rd gear wheel in its Genuine Parts box in my box of transmission spare parts.  I knew I had a spare input pinion and lay shaft input gear, plus one other main shaft gear, but I had thought that to be for 2nd.  I still have several unused circlips for the main shaft from the last rebuild – they come in bags of five, and a selection of shims to go with it to match the new 3rd gear.  So, I just need to buy the new big bronze phosphor bush to replace the snapped one and a gasket kit, and then I can put it all back together.  In fact, I was pretty surprised to find no discernible wear at all on the bearings or bearing surfaces of the shafts – I had expected after nearly 40,000 miles, with three quarters of that behind the Tdi, to find some wear on at least a couple of bearings, but there isn’t a trace.  That just leaves the concern over what caused the gear to break two of its teeth.  They were either side of an oil way cross drilling, which must have weakened them, but with no signs of overheating, oil starvation or foreign object damage, it still begs the question of whether the Tdi and 3.54 diff combination is too much for the gear box.

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  1. Hi Nick,

    Good to hear you didn’t find any other major damage apart from the bronze bush and the missing teeth haven’t damaged the layshaft gearwheel. Maybe the gearwheel was weakened with stress cracking around the oil drilling and had finally decided it had had enough.

    At least your motor won’t be off the road for long waiting for parts.


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