valve springs on or off head?

just one question replace valve springs on head or off. just got new cam and springs just wanted to know which way is best.

if the heads are on block and you dont plan on taking them off . shoot some air in the cylinder, i like to use a leakdown gauge and pop the old off and replace with the new.

thanks for the advice i've neverdone them on the motor so i might just take them off.

If you don't trust the "air" method or don't have a compressor, you can use nylon rope. Pull the plugs, put the end of the rope in the plug hole and then turn the motor so that the rope wedges against the bottom of the valves. Change the springs, turn the piston down and pull the rope out. Don't use the yellow rope, it sheds to many fibres. Use the white flexible rope.

thanks i like the rope idea