a shorter reach plug or a thicker head gasket (you may need 2 gaskets stacked).
but, thicker head gaskets will lower the compression so you may be defeating the purpose of the high compression head by adding a thicker head gasket(s).
you will have to do like oddzball said...remove the head and screw the plug into it and see how much of the plug is in the combustion chamber. if the threads are in the chamber, you need a shorter reach plug. BUT, you will still have to check the clearance between the top of the piston and the electrode pn the plug. A little secret is to put a thin layer of clay on top of the pistion. then, put the head back on and torque it to specs. install the spark plug and rotate the engine by hand (make sure the plug wire is off). after one rotation, remove the head and look at the clay. if there is an indentation in the clay from the spark plug, you can do some measuring to figure out how much clearance you have between the electrode and the piston. it all depends on how much clay you put on the piston tho because if you put a really thin layer on it, and there is plenty of clearance, the electrode will not touch the clay to make a mark on it. you don't want to put so much clay on the piston that it will make a mess if it gets squished. just put the clay in the center of the piston, right below the spark plug. there is no need to cover the entire top of the piston with the clay because that will just make more of a mess.
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