grounding j-box on new range
I want to update the range outlet from an older style 3 prong (10-50R) surface mount box to an inwall 4 prong (14-50R) for my new range. The new range is tighter to the wall so the inwall connection makes better use of space. The existing cable is non-metal sheathed 3 conductor 8 awg with a 10 awg ground. I have put in a double gang metal box for the outlet and run the cable into it with proper clamping.
What is the required grounding procedure for the metal box? I have 14 awg grounding pigtails and I have the special green #10 screws for 20 amp, 120V service but this seems inadequate compared to the 8 awg wires that are carrying current in. What is the required wire gauge for grounding the box? How do I attach that cable to the box? Can I use the pre-threaded hole with a #10 screw?
Thanks much! And apologies for double posting, I put this in the appliances forum before I realized this is the better place for this question.