I've done a fair amount of searching on this forum regaurding improving stock rods and never came across any pictures of what makes the F2 rod bolts different. Here's a pic of a stock K4 bolt next to a F2 bolt just in case people would like to see the difference. The F2 bolt also seems to be a .001" interference fit in the rod.
Part numbers are: Bolt 13213-KV0-000
Nut 13214-410-003
Cost was $90 CDN