We recommend following the manufacturer's recommended instructions for installing your Cannondale Si Stages Power meter to your Hollowgram Si or SiSL2 crankset. Please see the attached manual from Cannondale which covers the instructions for installing the SiSL2 and Si left crank (same procedure for both). 

For pre-2020 cranksets, see the attached document. For instructions for 2020 and newer cranks, the left crank features a self-extracting bolt.