I can't seem to edit the above post.

I re read through your posts and saw that you fitted a Tin Bum kit. So the spring and guides will be fine. So concentrate on sizing the piston seal.

There was a post on here where someone resized them a different way. They degreased, then fitted new seal. Then put piston ( with new seal ) inside comp tube. They then dropped the comp tube and piston inside a pot of boiling water and boiled the lot for about 10 mins. Then remove from water and allow to cool. Dry completely with paper and hair dryer. Apparently the seal is sized. I've not done this myself.

Other advice is available.