This is a model of a NGK ZKR7A-10 spark plug from an old Voxhaul (Opel) Astra engine. I know this is not true to the real thing inside but externally is it as accurate as I could get it with a pair of veneers. If you want to print this you would have to make some changes which is why I have attached the .f3d file however I will be working on making one that's easily printable soon and I'll update it here when I do.

https://youtu.be/MXhnITSSP0c