It's a 10mm double hex (12 points). At least I'm 90% sure it's a 10mm double hex, might be 12mm. You can get expensive specialty tools or go to your equivalent of Pep Boys or Autozone and get a ~$10 set including 6mm, 8mm, 10mm and 12mm double hex bits that fit in a 1/2" socket and be ready for the head bolts of many modern cars.

Sixto
95 S420
87 300SDL