Both of these parts are standard equipment on the MSD Circle Track HEI distributor but will work on most GM HEI's. MSDs mechanical advance lockout plate replaces the mechanical advance weights and simply slides over the pins on the shaft and the advance assembly to eliminate any mechanical timing advance. The vacuum advance block off kit bolts in place of the vacuum advance canister on a GM HEI distributor to neatly and effectively lock timing solid. MSD offers two kits to make locking out mechanical and vacuum timing advance in HEI distributors an easy task for Circle track racers. No more welding a tac on the mechanical advance weights or trying to find a way to keep the vacuum advance from moving once the canister is removed.