I saw this trick somewhere, probably on the internet.
Pick a suitably sized socket, 17mm was perfect for me. Then, insert a strong magnet into it (or two)
Put the socket (with magnet inside) against the retainer…
…and hit it with something hammerlike, preferrably soft mallet. The hit will compress the spring, loosening the valve keepers which will stick to the magnets which would otherwise disappear after short flight inside the garage.
4age valve spring free length should be 41.09mm. If shorter, throw them into bin. Mine were longer for some reason…
Next, I’ll have to find a similar way to get everything back together 🙂