Surfing Waves

Hi, folks. I’m a brazilian longboarder and last summer I wanted to replace my 3 FCS fins for a single bigger FCS one I asked for a friend to make to me. Being afraid my regular and short screws wouldn’t hold my bigger fin in place I still didn’t use my longboard with that configuration.
Anybody could help me ?
Best wishes from Brazil.

"Single fin doesn't fit" - Do you mean the fin's tabs do not fit into plugs or that the Single fin does not fit the style / design of a thruster set up ?

1) If it does not fit, and your friend made the single fin out of fiberglass, then just sand them down. slowly a bit at a time, until they fit.
2) What FCS fins system do you have ? FCSI ( twin tab with grub screws ) or FCSII ( tool-less installation but you can add grub screws for more security or if you want to run older FCS twin tab fins )

"afraid my regular and short screws wouldn’t hold my bigger fin in place" - Longer screws won't hold any better. The "box" is still plastic. The problem is if the larger fin will rip the plugs out of the fiberglass bottom.

In my opinion, installing a "single large" fin in to the center tabs of a thruster set up won't give you the desired results of having a Single fin Log/longboard set up for noseriding ( if that is what you are hoping for )

Lay down your board next to a true single fin longboard and see where your rear center tab matches up to the center box on the single fin board.

Hi, Waikikichan.
Thank you for your time, man.
My system is a FCS I actually. My single fin has double tabs and maybe drilling holes on them and screwing screws all the way through them could hold it in place.
I think that the best solution would be buying a real single fin longboard as you told. Once we are in winter downsouth, prices are not prohibitive.
I’m looking forward to ride the waves again in Florianopolis beaches (we have 42 in the island).
Thank you, man.

Pretty good number of beaches Sydney Australia just beats you with 48 listed surf beaches, not counting gnarly reef breaks!
Hope you get your fin sorted for your surfing!