So this is probably going to be considered one of those dumb questions from a newbie to bike building, but I got a great price on a cannondale supersix hi-mod and by great price I mean free. The seatpost clamp was overtightened and it cracked the carbon just under the clamp. It looks like it stops maybe 1cm under the clamp and I was just wondering if it was stupid to just cut off the cracked part of the seatpost? I know you can cut down the carbon on the steerer tube of a fork and I was just wondering if you could just do a similar cut under the crack to stop it and then just start your seatpost from there. Like I said its probably a completely ridiculous question, just wondering.

the picture on the Cdale site doesn't show that much room to move the clamp down, but it does appear to be possible. If it will still grip the seatpost, it's worth a try. Do you have to cut before you see if it grips the seatpost?

Cutting is so permanent. Have you considered just riding it (if the post is able to be held tightly) or contacting Calfee about repairs? Andy.

Show us a pic. Carbon is very easy to repair despite what you may hear. You can sand the area down and open up the crack a bit to allow Epoxy to fill the gap and then wrap a few twists of wet out carbon tape over the clamp area. Wrap the area with electrical tape to compress the repair. When cured use a slightly larger seatpost clamp.
You should insert a seatpost of a slightly smaller size and cover it with clear tape until a slide fit is achieved. That will keep the interior clean and you'll be able to twist the post out even if traces of Epoxy leak into the seattube.
You will have to re-cut the slot and tidy the area up a bit and give it a final coat of Epoxy to blend everything.

I'm not clear on your question. You mention cutting the seatpost, but mention the top of the seat tube under the collar is cracked. Which part is damaged, the seatpost or the actual seat tube on the frame?

If the seatpost is cracked, don't cut it. Just replace it. A safer fix.

If the seat tube on the frame is cracked under the collar, still don't cut it! You're making a permanent solution to a temporary problem. There are a few different ways to handle the issue. The proper way would be to have it repaired.

Alternatively, although I don't necessarily recommend it, because this isn't a structurally critical area, a cheaper solution is to find the terminus of the crack and drill a tiny hole to prevent further cracking. If your seatpost holds and the crack does not propagate any further, then you're good to go. Otherwise, we can help if you need to have it repaired.

Sincerely,

I guess the OP has done whatever he was going to do and is not telling us.

My personal anti-stop drill jihad just isn't working. At least in metal there is the slightest glimmer of hope that stop drilling will reduce the likelihood of further crack growth. What conceivable good does a stop drill do in carbon? Any fibers you cut were capable of holding a considerable load.