These are two designs, the first of the two [mine] puts out 1 cobble about every 2 seconds. It uses a 15-clock redstone masheen and 2 pistons to push the cobble along to the mining point.

The second one puts about cobble every 1-4 seconds. It uses a 5-clock which is 3x faster than the 15-clock, thus moves faster. However, the cobble is only created every 1/2 - 2 seconds, and the piston often comes too soon and messes up the water mixing with the lava.