3.8mm on average to be more precise. The maver internals size 5 fit, some kits will be tight and some loose. Use a wrap of ptfe plumbers tape if sloopy.
If using light elastics with the mavers <8 then you might consider sleeving them with a drennan internal yellow or white to let you use small stonfo's.
Alternatively find someone with a lathe to turn you some to fit out of PTFE bar, the Malman one may be customised if specified I believe. Mick Wilkinson used to do it too, but you do need to leave them with the top kits to get an individual fit.