Cross threaded final drive shaft! ...and the fix

This actually occurred during my pre riding season spring maintenance back in May 2011, but although it was well documented on the VSRI forum, I thought it was time to add to my Vstrom blog. 
So here is the extracts from the original VSRI topic www.vstrom.info

I recently needed to change the chain kit on my DL650. What should have been a routine task turned into a big problem.  The retaining nut for the front sprocket was proving difficult to remove. It took a substantial amount of force using a breaker bar to release the nut. The resulting damage to the threads on the output shaft was not good.  The thought of how the damage occurred was either when the bike shop changed the last sprocket, they crossed threaded the nut with an airgun, or that whilst I was undoing it, the nut tangs broke or maybe there was too much gunk on the threads?  We'll never know.

The nut was finally removed revealing the horror of the stripped threads

The stripped nut

There was much discussiuon on the foirum as to what to do.  I did phone my local Suzuki dealer who said it would cost a mimimum of £1500 to strip the engine all the way down to get to the shaft which sits at the very bottom.  Therefore, I decided it was going to be a good home fix.  

The idea was to use a split die, view this video for an explanation YouTube.  However, a standard die off the shelf wouldn't work as I wanted to cut the thread in reverse, by starting it on the good thread that was left neaerest the engine, and turn it anti-clockwise out, cutting a new thread as it went.  So I spoke to a local tool maker  who we use at work.  He knows his stuff, and is a keen biker too.  Recently he restored a Bantam and a 1952 Bonnie T100.  

My drawing shows the amount of 'good' thread remaining on the shaft

The plan is to buy the standard M20x1.5p Die Nut, and he will use a wire-eroder to slice it in half and utilise a grub screw or something to hold it back together.  Below is the M20 split die that he made for me.  You can clearly see that my toolmaker has sliced it in half using the wire erroder.  The shims are to replace the width of the cut.
You can also see that I got him to machine a recess at one end.  This is so that it will fit over the splined shaft and make sure that all the cutting edges of the die start on the good threads.
The die has been reversed too, as we are cutting counter-clockwise.

The reversed thread split die

I bought a 38mm socket to turn the die.  My toolmaker machined some grub screws to ensure that the two half's of the die are pressed together when inside the socket.

38mm Socket with machined grub screw points to hold the die together

It was impossible to show in a picture, but this is the socket with the die inside.  And I am just nipping up the grub screws.  It was in fact extremely tricky to get the die & shims in the socket.
Its one of those things that seems simple when you go over it in your head - a different beast altogether in reality!  Perseverance and patience was the key here.

The assembled socket containing the split die

It was extremely nerve-racking when cutting the thread.  I would cut a bit, stop and dismantle the whole socket and check and make sure I wasn't damaging the good thread.  
I did this so many times, and to be honest I was cacking it!  I knew that if I fooked this up, it was strip-down engine time.
I used lots of lube, and finally the die was through.  This is the finished thread.  Now it might not look that great, but I am very happy with the result.

The new thread

The very badly damaged threads were never going to return, there just wasn't enough metal left to cut on to.  But it did ensure the good threads were cleaned up and that the thread would
start nicely and a nut would go on without cross-threading.  Here I had purchased an M20 std hex full nut to try out first before using the locking nut which comes with locking compund already.

Testing the new thread with a standard nut

And so the finished thing.  I have torqued the nut to 113Nm and put a marker as suggested by Greywolf/Preload. After a few rides and miles, I'll check it a few times.
But with the amount of good thread (ended up with about 1 thread protruding the nut), the lock nut, locking compund, good torque and a washer, I'd be surprised if this sucker moves!

Finished.  The new sprocket fitted

Thanks to everyone who offered suggestions.  They were all very warmly welcomed.