A head shim or gasket is the same thing. If your leaking around where the head seals to the block then chances are the head is loose or tightened down improperly (MAYBE). Pull the head it will be a good experience for you but most of the time the leak is coming from the top of the exhaust gasket where the header attaches to the block. It pushes fuel around the head like the head gasket or shim is leaking. You might very carefully check it out [:-] Torque the screws on the head evenly. Make them just a little snug and torque them a little at a time in a cris cross pattern, never tighten one down all the way before any of the others............good luck and HAPPY RC'en