I want to replace a standard G2 ballpoint refill with a rollerball refill.
Will your Black Schmidt P812 Rollerball Refill Fine Part no. P8126F-500 fit?
The dimensions appear right on length at 97.5mm versus I believe 98.0mm for the G2, but the diameter at 6.8mm is slightly larger than the G2 at 5.8mm.
No, the P812 will not fit a pen that accept the G2 standard ballpoint refill.
The correct refill for your needs is the Schmidt P9000 Easyflow refill - this is a rollerball refill in an identical G2 package: