Why not blood? You just need a pair of valves to control blood inflow and outflow from this “piston.” That’s how the penis works, isn’t it?
Well I supose if you stripped the muscles from the bone on the top of the forearm, and then put the claws up against the bones there, and then reattached the muscles on top…but then how would he be able to rotate his wrists? The ulna and radius can’t be locked together like that and still torque. But how do we know that Wolverine can twist his wrists? Maybe over the years he’s learned to deal with things like doorknobs and car keys in his own special way…
I suppose his wrists would have to be ball-and-socket joints like hips and shoulders.
Wolverine breaks his wrist bones every time he moves his hands, buut he apparently doesn’t even notice it anymore.
An easier way to do these for us mere mortals would be to install them such that the blades extend from the top of the back of the wrist.