in theory you can do this with the head still on but in practice it is a real fanny on. the cotters are usually tight to remove even with a valve spring compressor and the head on a banch let alone with the head still on. somehow you have to loosen them and the only way i've found to do this is to pour oil down the plug hole and then turn the crank till the oil comes out, put the plug back in and wack the retainer with a socket (about 27mm i think) on an extensing. this loosens the cotters and you can then compress the spring. if you dont do this, when you try to compress the spring the valce just goes down with the springs. its really a complete fuck on and its better to be off ya head!