Welcome to APUG.

The Gaoersi like other 617's with interchangeable lenses uses different lens cones for each focal length. So firts you need to make sure you have the correct cone. I have a first series Gaoersi and a cone set up for a 90mm Grandagon so can measure that for you. When I bought my camera they asked what lenses I was going to use and then set my cone specifically for them.

I will go get the cone, finding a ruler may be more difficult

Ian