Nice post. I did mine a few years ago, it has a 37,5mm extension with an o ring as the seal. Mine is 177 and still made power like no other gun I've had before (including the HW80).
Mine has a short stiff spring in it now, the piston was lightened bearings added and all sorts. Took a lot of stripping and re-assembling to get it where it is now. Finding the right spring is quite a lottery in my experience. Here's a pic.

Regards

Max

http://i953.photobucket.com/albums/a...diana52003.jpg