Dealing with the nut itself is relatively easy, it�s the just a bit of a faff removing everything to get to it. It is worth your while (as a supporter) downloading the free Yamaha Service Manual before you begin.
http://www.xt660.com/forumdisplay.php?f=58
Otherwise, before you start you will need to make sure you have got the following at hand:
- 2.9L of engine oil (and filter if you want to change that)
- 1.2L of coolant
- Water pump �O�-ring and gasket
- Right hand crankcase cover gasket
- Oil transfer pipe banjo crush washers
- Crankcase cover oil seal
- A new tab washer for the final drive nut
- A 36mm socket and torque wrench
First remove the water pump. Drain the engine oil (from tank and sump) and coolant using the coolant drain screw on water pump body (lower left hand bolt). Reinsert and tighten the drain screw when finished. Remove the hoses from the water pump body. You can remove the water pump as a whole unit by just undoing the three bolts with the recessed hex (Allen) heads (leave the other two in place). You can now take out the water pump.
Next take off the rear brake pedal. Then detach the oil transfer pipe from the crankcase cover.
Finally undo all the cover bolts working diagonally (undo opposing bolts in turn. Pull off the cover watching you don�t lose the two dowels.
To remove the crank nut normally you would have to lock the primary gears in place with a rag, wedge or I use an old leather belt, but if it�s loose, you won�t have to bother. Remove the tab washer and tighten the nut. Although it says tighten to 80Nm in the manual, we do know that those who have taken their bike to the dealer to have this problem rectified have had it tightened to 105Nm. Refit the tab washer and bend up the tabs.
Refitting the right hand crankcase cover is the reverse of taking it off except for the fitting of a new gasket and oil pipe crush washers (banjo bolt is 20Nm). You can also at this point fit a new oil seal in the back of the cover (under a retaining plate with two bolts), but this isn�t always necessary. Don�t forget to replace the water pump �O�-ring and any water pump gaskets you�ve disturbed. The torque for the cover bolts is only 10Nm, which isn�t that tight, so be careful!
Refit the hoses, brake pedal etc. Change the oil filter if you want, then fill the engine oil and coolant.
