The 39mm Vgas I do believe. Didn't ask to be honest. My numbers are also better than the PCW Flatslides which would be 41mm I believe, based on his website which recommends 41mm for 1425cc and larger. Would have to ask John Gainey at PCW to get an answer. John is recommending Sean's jet kit to be installed in the 1428cc engines like mine to his customers.