Take no notice of the shops & stick a meteor spring in, airsporter spring is slightly too large outer diameter & will bind on cocking. Grease it with moly & make a pop bottle lube sleeve for in the piston, that'll quiten it down a lot. You'll be surprised how bucked the old twin springs are whe you pull it apart I''ll bet. I'd wipe the piston washer with alcohol ,dry, then soak in hempseed oil wrapped in cling film overnight, just pushed into the end of the cylinder to stop it swelling too much. That makes re-fitting much easier next day. I think there's a knack to getting the piston past the trigger too but can't remember it. No doubt someone will tell you on here I've only done one tornado for a freind & had to glue the barrel in on that as it was loose & rotating slightly so check the barrel joint too while you have it apart.Don't expect top power from it either . The most they'll make is 8-9ft/lbs tops.