Per strict Keirin regulations, "The distance from the center of the frame to the center of the chainring should be 42mm +/- 2 mm."

To use, place the V-groove of the gauge on the seat tube and check the placement of the chainring to the center of the slot cut out of the right side. If the installed chainring is positioned within the slot, congratulations! Your chainline meets Keirin inspection requirements. If your chainring is outside the slot, you'll never be a Keirin racer. Sorry!

Laser-cut from 1.5mm stainless steel and polised to smooth out the cut edges,  improving safety and workability. When measuring, care should still be taken to prevent scratches to your seat tube when it comes in contact with the cross section of the V groove.

Adding a bit of playfulness, Runwell laser engraved a mimic IC chip on the credit-card sized gauge.

Please note that this gauge was made for inspecting chainline on NJS frames with a 68mm bottom bracket shell. If your frame has a wider shell, the measurement may not be 100% accurate.