what is the usual method of doing this?

I read several threads and some people were saying that they were using special valve spring compressor either purchased or made their own.

what kind of compressor is it? the usual ones need the heads off the engine..

 

You should use this tool. It is the easiest to use. Other styles are much more expensive and require you to manualy position the retainers one at a time which can become tedious and almost impossible to do by yourself.

http://buy1.snapon.com/catalog/item....re&dir=catalog

Be sure to use compressed air (an adaptor from a compression test kit works best) to hold the valves up in the head or they will fall down into the cylinder.

 

+1...that is the one we use.