AS stated the slot length can be varied by lossening the torx lock screws. To make it much easier Summit Jegs sells a Mallory (the MP distributor is made by Mallory) kit with spacers that make setting the amount of timing a snap. The kit also comes with a 1/2 dozen differnt sets of sprngs for adjusting the rate. The instuctions include graphs for the different spring combos so you will know what you ae getting. The kit number is 29014.