Before you begin the valve lash (actually it's called the pre-loading) process, there are a couple of things worth mentioning:
Before you begin, it is vital that any lifter pre-load that you may previously applied is completely released. Since both your cam and lifters are new, it doesn't sound as if this will apply to you. Work around the engine and back off all the rocker adjusting nuts. Allow some time for the plunger springs to drive the plungers to the very top of their strokes. The socket ( the cup that the pushrod rides in) of a fully decompressed hydraulic valve lifter will be hard up against the bottom of the circlip. See the images below.
Also, be aware that this process is a one shot deal. By that I mean; once the spring is compressed - whatever you use, 1/2 or 3/4 turns past zero lash - you cannot go back around the rocker nuts and .... let's say - "check your work". That is, unless you start all over again by releasing any applied preload.
However, before you close everything up and start the engine, there are a couple of things you can check to verify that you have properly preloaded the lifters:
With the lifter off-stroke (i.e on the heel of the cam), check the gap between the socket and the bottom of the circlip. When it was unloaded, the gap should have been zero. Now, with the proper pre-load applied, the gap should be virtually identical to the cross-sectional diameter of the circlip. I don't know whether this is by design - or just a lucky co-incidence - but it is a valid check of proper pre-load/valve lash on a GM hydraulic lifter. See the second image below.
A second quick check is to apply pressure to the pushrod side of the rocker (again with the lifter off-stroke). The socket and plunger should move downward easily (the plunger springs are tiny and weak). The distance you can depress the plunger should be the same as the gap between the socket and circlip. IOW the proper preload will position the plunger at exactly the mid-point of it's total possible stroke.
Unloaded lifter:
You must be registered for see images attach
h
Preloaded lifter:
You must be registered for see images attach
Point is, when you are all done preloading you can easily check your work by rotating the engine. As each piston reaches TDC on compression, look down into the top of the lifter and observe the circlip/socket gap. If it is about the same as the thickness of the circlip you are good to go.
