Neither rope nor air pressure is absolutely needed as long as you are certain the piston is at TDC on the compression stroke.


I just finished a valve guide/stem seal job for my friend in my driveway on a 2008 X5 E70 4.8L N62TU. While doing bank 2 (I started on the driver side), I worried constantly about dropping a valve into the cylinder. At the same time I kept asking myself "How can a valve drop into the cylinder on a high-compression engine at TDC? Since the combustion chamber volume must be small to create the high compression, the clearance to the top of the piston at TDC must be tiny."

Calling and talking with AGA about that very question - they said I was correct; with no air pressure at TDC, the valves would drop onto the piston top, moreover, are serviceable by replacing the valve keepers using a flat screwdriver with a pat of grease to hold and position the keepers. The down side to the rest-on-the-piston method is the valve stem drops too low for the AGA Keeper Tool to be used. One of their mechanics told me he never uses the compressed air method, preferring the speed of not having to deal with attaching the air supply via the spark plug hole.