I need to change my rear derailleur because it is broken.
presently I got 7 speed Shimano LX derailleur and Grip Shift SRT-400i shifter (does I don't like). I would like to have a rapidfire shifter but I can't find a 3x7 LX shifter.
Is it possible to put a 8 speed cassette on my bike (presently I got 7-speed, 11 - 28 teeth cassette) so I can put LX 3x8 shifter and LX derailleur?
You wont be able to use an 8 (or 9 or 10) speed cassette on a 7speed freehub as the 7speed freehub it is narrower. You should be able to use 8speed shifters and mech but you will have an unused click on the rear shifter. Just make sure to adjust the limit screws on the rear mech so it doesn't shift off the big cog.