hi does anyone know the air gap for the rvf upside down forks? and also how do you check the air gap? thanks in advance.

You remove the top bolt, spacers, etc then fully compress the fork with the spring in place. The air gap is now the measurement between the top of the fork assy and the oil level. Air gap = oil level. Can't remember the exact measurement, someone with a Haynes manual to hand will be along soon enough.

Get yourself a Haynes and this will all be explained - its quite easy.