Vacuum Advance Lockout For GM Hei Quick Easy Components To Lock Out Timing On GM Hei Distributors MSD IgnitionBoth 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.