Now this is curious. All the radial engines I've heard about use a single cam ring to drive the valves. This engine looks like it is using an individual camshaft for each cylinder, or perhaps for each pair of cylinders. Huh, imagine that. It requires a lot more gears, but they are all small, not like the great big cam ring, and all the camshafts would be identical, so maybe that would be a viable alternative. This was news to me, so I sent a note to Simon at the Fleet Air Arm Museum, Yeovilton, Somerset UK, and he tells me that the Bristol Centaurus was a sleeve-valve engine, which means it doesn't have a conventional valve train (camshafts, lifters, pushrods, rocker arms, poppet valves and springs). Sleeve-valve engines are insane. As I recall the Germans used sleeve-valve engines in their WWII torpedos. Not a bad application I suppose, you want maximum power, minimum weight, and it only has to last a couple of thousand yards. To use one in something you expect to last more than a couple of minutes, well, I am surprised they managed to make this one last as long as it did.
Bristol City Museum has (had?) a display model of the Centaurus's inner workings. You can see that on the back side of the timing gears instead of camshafts there are crank stubs. These stubs engage a socket in the base of the sleeve. The eccentric motion of the crank produces an oscillating motion in the sleeve that opens and closes the ports in the cylinder walls.
Update: More pictures here.
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