I have been in this hobby for 30 plus years and I did not know that glow plugs have different threads . I am not so sure about that . I am going to have to check that out before I believe it . I have both four stroke and two stroke plugs and I am going to check this out . Okay what I see is the Four stroke plug threads are the same but the threads only go part way down the plug . The last 1/16 of an inch has no threads . the four stroke plugs have no idle bar . So if you were putting a two stroke plug in to a four stroke engine it might only go so far before becomeing tight but not sealed . then it might try to cut threads lower down in the head where there is not supposto be threads and when you back it out it may ruin the upper threads. You could probably get away with putting a four stroke plug in two stroke moter but not the other way around .. I am glad you pointed that out . I learned something new today and from a newby ..Thankyou .