the holes just go down to combustion chamber, but there are at least a couple of different kinds of plugs and they won't interchange. i think the problem you are talking about is the threads are same but outside will be too big to fit in the recessed seat where the plug seats and not allow plug to thread in. i think when this happened i went to part store and they looked and found right size, i had to regap for hei. i have changed and swapped several of these heads 250's and 292's and haven't had any problems with interchangability. i think the small 194's had a smaller combustion chamber but all rest are same i'm told. i have a bunch of heads and can get you plug #'s if you want. good luck sounds like you got a good deal. hr
