A blue shaft is inserted into the gudgeon pin hole. Ensure no gap there. Move the piston untill contact with both edges of the yellow arm to get value E1 shown by the indicator.
Turn the piston 180 deg. and do the same for value E2.
Error in perpendicularity is P = (E1-E2)/2A
A is center distance between the indicator and pivoting axle of the yellow arm.