It partly depends on how much access your gadget gives you once in place. It is possible to extract the collets one at a time from a partly-compressed valve if you push the holding cap to one side and fish the collet out from the enlarged gap with fine nosed pliers. But for easy all-round clearance you need at least 1/2" I'd say.
There was a lengthy recent thread on making compression tools. I contributed my bit, but someone else came up with photos of a much cleverer (but more complicated) tool.