I have never built a H&M engine so I was wondering what the popular spark plug sizes are. Having purchased Dale Detrich's stock of plugs and parts last year it's time for me to get going on the endevor. I have 1/4-32 and 8-40 plugs that dale designed as well as my own 8-40 design from the Demon project. I also plan to make my own 1/4-32 and 10-40 plugs for sale. For the hit and miss crowd I was tossing around the idea of making copies of the X3 plugs in 1/4-32. I have done some reading and it appears that that was a popular plug in the full scale H&M engines way back when. With there being so many different engines and sizes i would like some help on where to start. Would the X3 be a good choice or is there a different plug that would be more appropriate for a H&M engine? Of the models out there would 1/4-32 be a good place to start or is 10-40 more popular a thread size on the H&M's Any information or suggestions for future designs are very much welcome.