1) With springs and tops fitted, compress the fork leg by pressing it against a wall or bench, then turn the screw. This usually does it well enough.
2) Use an impact air or battery wrench! (mainly for removing of course!) not only does it work but the mere mention of such tools is really useful for winding up the grumpy old know it all who lurks on here waiting to take a pop at anyone he feels inferior to him/her. This basically means anyone if he hasn't had his medication. Hours of fun guaranteed
