This is the combination I shall be working with: Fujimi Bridgestone Potenza tires and Hobby Design Rays G25 rims (part of!)
First thing to do is to decide on the amount of outer lip you want to show on the front part. So insert the rim inside the tire all the way until the required lip is reached. In my case, I decided to opt for the lip above, around 0.5mm outside of the tire wall
Then, turn your attention to the back side where the extra material is! Use a permanent marker and draw a line with the marker tip touching the tire.This will be your cutting line
Here's a comparison between a marked rim and one that is not to see the extra material that we need to cut out
To perform this procedure, I always use a rotary tool with a metal cutting disc. You can also use a regular cutting disc, but I find the metal one better and more efficient too while cutting. Instead of the rotary tool, you may also utilize a knife, like the photo-etched one pictured above. This will however take more time to do the necessary cut and you need to be extra careful with the amount of pressure applied on the knife as this will easily break a resin rim, and I'm sure you don't want this to happen!!
Now, here's the rim with the extra part cut out. When using a rotary tool always make sure to wear safety glasses. You never know where the extra resin flash pieces tend to fly to during cutting! For resin rims, I usually set my tool at around 8,000 rpm
Once the cut is done, I simply smooth out the surface using a 320-grit sand paper, removing all flash pieces left by the disc
Finally, here are the finished rims which now fit perfectly inside the tires and with the front lip as planned earlier
Overall, this is an easy and straight forward procedure. When I showed pictures of these rims being modified for the BRZ, a friend asked me how its done exactly. So having earlier photo-graphed the whole procedure, I thought of composing it into a tutorial! As always, thanks for watching and enjoy modelling!