I am replacing a head on a 4.2 and have removed the intake and exhaust manifolds. In the process I have notice an accumulation of carbon build-up in the old head. I am assuming this will carry through to the pistons, etc. What is your opinion or expereince of removing built up carbon deposits on the pistons?

I always leave the pistons alone! I'm aftraid of getting particles on top of the rings. There shouldn't be much on the pistons to worry about anyway. If the valves have a buildup on the manifold side I take em out and scrape them real good.
After rereading your post I see you do NOT have the head off yet.. Not to worry the combustion chamber does not look like the backs of the valves.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

My valves were dirty as hell!! It takes some patience and a lot of scrapin. Let them soak in warm degreaser for about 24 hours and then go to town with a wire brush, and I also used a small screwdriver for the really stuck gunk.

For the first few hours of soaking make sure you keep adding warm water to the degreaser... the longer it stays warm the better it will work. The degreaser I used had to be mixed 1 part degreaser to 6 parts water. Start out just barely covering the heads of the valves using mostly degreaser so as to not make the solution too weak after adding warm water a few times. When I say warm I mean bring some water almost to a boil, then add it to the degreaser.

I also did this same thing to the entire head after tearing it down. I only used a wire brush and compressed air to clean the head.