I recently bought a used 400EX with no tools. Could not find a Craftsman or Snap-On socket that would fit down on plug for removal. Finally found a metric, long socket set at Wal-Mart that had a 18mm socket with thin walls. Fits perfectly. The whole set of Taiwain sockets was about $10, cheaper than a stock wrench and included more trail sockets for the old tool box.

Also, use a piece of old rubber gas hose that can be fitted over the plug to ease screwing the plug in and out while it is loose.