Stripped Engine Shaft!

During a run-of-the-mill tech day earlier this year, it was time to do the chain and sprocket change.  Unfortunately it proved to be nothing like a run-of-the-mill days.   


We had terrible trouble trying to remove the front sprocket retaining nut.  At one point, my strapping fellow biking buddies Steve and Dino were standing on the breaker bar to try and turn the nut.  Eventually after much sweat and an almost toppled bike, the nut finally came off, and with it left behind a complete nightmare.  The nut and output shaft threads were completely
chewed to pieces!!!  


We reckon that the shop who replaced it last time, managed to cross thread the nut and forced it on with an air gun.  A call to my local Suzuki dealer confirmed in the 'likely' event that the thread is not salvageable, this would mean a complete engine re-build to replace the shaft that runs right through the gearbox.  This would cost over £1000 for labour alone, more like £1500 with parts.  So not being happy with that news, I set about a repair.  


Here's what happened..

With the nut finally off and a clean of the shaft clearly reveals the nightmare stripped shaft

You can see that there is good thread on the inner part of the shaft. Hmm.. 

Here is my rough drawing and measurements of what I have to play with.
I have caluated that if I can get a new nut on, there is
sufficient thread remaining to hold it

So the idea was to use a Hex Die and split in in two halfs

This is the M20 split die that I bought.  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.

This is the 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.

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 infact 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!

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 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.

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!