It depends on what plug you have. The very common NGK CM-6 plug has a 14mm hex and I prefer to use a deep socket on a 3/8" drive ratchet. Other common sizes are 5/8" and 3/4". If you want a low cost, dedicated wrench, just about any good saw shop should have one to fit your plug. These usually fit (2) different sized plugs and have a short handle welded in place.