I have come back to add to my review. Despite advising that this piston does appear to be 1mm narrower than stock I have spent soem time checking over and have got this working.
I have had to make a few modification by using a dremmel to remove the 1st tooth to allow the sector ger to engage correctly. and dremmel a little bit off the last tooth to ensure the sector gear clears the last tooth of the sector gear to release the piston to fire.
I also made sure that this gearbox was shimmed correctly to minimise piston side movement, and now it is working.
It seems to be a strong piston and light weight but may require modification to get it to work correctly. So as it does not work out the box I have given this a 3 star.