From my experience, Picco plug definitely doesn't fit Nova. Not sure of other engines' fittings. O'Donnell for sure fits Nova engines. Lots of guys use them in Nova engines w/o leakage so I'd say there's no issue there. I switched from Nova to O'Donnell plug during the heats and I didn't really notice any big change but I'm sure there will be some change to some more knowledgeable people. The fitting and thread and taper all that are important and I honestly don't know the measurement and all but I'm just saying from my experience. Once I messed up my Ison with Picco plug having leakage pretty bad when I tried to break it in. I guess I need a new button LOL. But it's not even worth it since the engine was so cheap.