The Nissan SR20 GTiR Head takes a plain 6.0mm id cap shim on one side, and a Rocker Guide cap shim on the other side of the rocker arm.
The Nissan SR20 GTR Head Takes a 10.0mm dia flat shim on one side, and a flat bottom Rocker Guide shim on the other side of the rocker arm
The Nissan SR20 VE Head Takes 10.0mm dia Flat shims on both sides of the rocker arm.
This tool is for measuring SR20 shims, cap shims and guide shims directly on the shim, under the rocker arm fork.