Hi all I'm working on my 1952 Martin and I'm a bit stuck. (See http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=17;t=038440 for some pictures). The original plugs were champion j8c with a 3/8"(9.5mm) reach. The previous owner put in a couple of h10 with a 7/16th" (11.1) reach. On the top cylinder the first couple of threads have gone and I'm sure there is not enough thread to go back to the original plugs without ripping the rest of the thread. The "oversize" plugs get a few more threads to bite on and do torque up however.... after spinning first by hand and then firing it up the electrode just leaves an imprint in the carbon on the top of the piston. 
Oh and by the way the head hasn't been off in 50 years. <br />So the options are<br />Helicoil it in situ and go back to original plugs<br />Put a stainless washer under the plugs to lift them by 040" or so.<br />Rip the thing apart, broken bolts, sheared studs, tears and drama...
Oh and by the way the head hasn't been off in 50 years. <br />So the options are<br />Helicoil it in situ and go back to original plugs<br />Put a stainless washer under the plugs to lift them by 040" or so.<br />Rip the thing apart, broken bolts, sheared studs, tears and drama...