bad design

or maybe poor machining with only three threads available ..... it can blow one of three ways ....

1... over torqueing , only three threads so its easy to pull the threads

2.... spark plug under torqued , just as bad as over torque .... plug rocks back and forth on compression stroke and the "only" three threads won't take it forever

3 ... early designed heads had the spark plug base protruding into the chamber , after many miles will build up carbon on the tip .... no problem until you remove the plug and the carbon wipes out the only three threads available ... newer heads don't have the plugs protruding into the chamber and have a chrome type coating that the carbon doesn't stick to , .... so new heads are better , but still don't have much threads