i have been replacing the headgasket on a 1986-1989 four winns 160 freedom with a 3.0l mercruiser. i was told i had to adjust the valves before running the engine. only problem is i have no idea how to adjust the valves. i have been reading up on it and some people say zero lash and others to do it on compression stroke. i am an automobile technician so i know some stuff abou tthe engine but i have no idea what zero lash is or how to tell if i am on compression stroke. is tdc when pistion is at the top of head?