Just received my S2000 master clutch cylinder part no 46920-S2A-003. Most of you would have known that it will bolt right up easily, except that the push rod too long, resulting in the clutch pedal being much higher than normal. There shouldn't be any improvement in performance over the original master clutch cylinder, except that it makes the engine bay neater by eliminating the clutch oil reservoir from the strut towers.
The solution for the longer push rod is either to get a spacer to mount it further away from the firewall or...
Use the M8 x 1.25 die to create more thread along the push rod, so that more adjustment can be made.
As the diameter of the rod is slightly smaller then the original thread area, you can see the difference in thread between the original thread and the part which was added on. But the nut will still screw on just fine.
The reason for adding threads instead of the using the spacer is that having the cylinder seating directly against the firewall will allow the cylinder to seat more firmly.