Minecraft wiki entry for Piston, associated text: This rapidfire clock is fairly simple to build. It requires 2 non-sticky pistons, 2 repeaters, and 6 redstone, and 5 solid blocks. The repeaters must be set to at least 2 ticks, and they must match. Place the mobile block last to start the clock, and take output from either end or any wire. Note that this clock is not switchable, and if it does stop for any reason, it's difficult to restart by a redstone signal. The player can restart it by breaking and replacing the mobile block, or by changing the repeater delays (both of them; the clock restarts when they match and are set to at least 2 ticks).