Hi,
Refurbed the fuel tank cap on my GPZ Unitrack this weekend.
Here's a bit of info that might help someone else.
I ordered a replacement lock but couldn't work out why it looked so different to the original.
Anyway, it turns out the piece highlighted in the picture is actually a cap with an O'ring around it. It gets so gummed up it needs a good pull with a pair of pliers and out it pops revealing the lock rotor underneath.
If I hadn't seen a picture on the net showing all the parts laid out I would never have sussed out the removable cap as it looks like its a part of the lock rotor and I didn't want to start levering away and risk breaking the casing.
To remove the lock rotor, there's a spring loaded brass plate that you have to slide into the lock body using a thin screwdriver or blade and then push the rotor out towards the key hole. You can see the brass plate at the top of the new lock rotor in the picture. The tumbler pins are underneath it.
Fitting the new lock rotor is a reversal of the removal process.
I believe that style of lock fitting was used on quite a few different models too.