i had my heads cleaned and machined. The tech referred to the cleaning as "cooking the heads overnight". How hot do they "cook" the heads? could this damage the valve stem seals that they did not replace?
The short answer is Yes. Most machinists immerse cylinder heads to clean them, The chemical used is safe for metals, but not nesessarily the material on the seals. It is always a good idea to replace the valve stem seals anytime the heads are removed to save you the cost of having to remove them again in a very short time, to address a buring oil or smoking conditions.
You will need a valve spring compressor to remove the springs locks and keepers. I would do them 1 at a time unless you mark each valve to its original guide. The seal can be removed with a needle nose plier (gently). I have a 1/2'' about 5" long wood dowel I have drilled a hole the size of the seal with a second drilled step for the seal lip to install. this can be done with a light weight rubber mallet to drive the seal over the guide. Also wouldn't be a bad idea to get some lapping compound and relap the valves in their seats, before you re-assemble.