SPA Section V Belts 13 x 10mm.
SPA Section Belts are measured on the PITCH (Lp) length in mm - The Pitch (Lp) measurement is taken from in between the inside (Li) and the outside (La)
Use the following to convert your measurements:
Outside (La) to Pitch (Lp) - 18mm
Inside (Li) to Pitch (Lp) + 45mm