Drilling a carbon fiber seatbase.

Hey everyone. Im just wondering how you drill the holes in a carbon fiber seat base and how you make the holes square to fit coachbolts (i think they’re called that)?

PS. has anyone done backwards shifty’s cause i was doing some off a 2 set today?


You drill the holes with a regular wood drill. The carbon fiber is very easy to drill, you can even drill it very easily with a manual hand drill (one that you crank by hand).

Drill the hold slightly undersized for the square part of the carriage bolt. Then use a file to file the hole square to fit the carriage bolt. It is best if you can find a square file. It is much easier to file a square hole with a square file. However, I had a very difficult time finding a suitable square file. Square files of that size are not very common, at least here in the US. I had to make do with a triangular file. It works, but is very difficult to get square 90 degree edges when making the square holes for the carriage bolts. Eventually I did find a square file and used it for my later seat. Much easier with the square file.

It is very easy to file the carbon fiber. Do the drilling and filing outside and wear some sort of a mask because the carbon fiber dust is not good to be inhaling. Gloves are good too because little carbon fiber slivers in your skin would not be good either.

With everything involved in drilling the seat base you want to measure twice at least. Measure twice, cut once.

I covered the underside of the CF base with masking tape. Then carefully marked a straight line in down the center of the seat base. I used that line to help keep the holes in alignment.

Use the seatpost as a jig for locating the holes. Position the seatpost and mark the first hole. Drill the hole. File it round. Double-check while filing the hole to make sure you don’t drift as you’re filing. Position the seatpost back on the base and put the carriage bolt in place. Now mark the second hole. Drill and file it square. Double-check while filing the hole to make sure you don’t drift. Repeat for the other two holes. Then do the same procedure for the handle and the rear bumper. If you’re careful everything will come out square and in alignment.

I used 1/4 inch carriage bolts cause that’s what we have in the US. If you have metric you’ll want to find a suitable metric size.

I placed a fender washer between the CF base and the metal part of the seatpost. The seatpost brackets have a sharp pointy edge that will create a stress point on the CF base where it could start to crack. You want to spread that stress point out. One way is to put four fender washers between the CF and the metal seatpost bracket. Another option is to use a piece of thick rubber from a thick tube and place the rubber between the CF and the metal seatpost bracket.

The work itself is easy. The tedious part is making sure everything stays in alignment and that the holes get filed square.

The file

I did it with a triangular file, and it just worked fine. That’s all my neighbour had that fitted the holes.