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I've bought a giant TCR with ISP second hand. The previous owner had trimmed the seat, so I've added 7.5mm of spacers to make it fit me. Initially I had no issues. I moved the seat backwards slightly along the rails and since then my problems began! Basically one day when riding I noticed the spacers rattling together. When I stopped to check, I could see a gap between the top of the seat post and the first spacer, so they were able to move around. So what seems to be happening is that the clamp is able to move back and forth, even when tightened to 4Nm. I will tighten the clamp, however I assume that when I move my weight back on the seat, the clamp just slips. So here's what i've tried:
- Running just one 5mm spacer - the seat still moves, there is just no rattle heard
- Swapping clamps - the longer clamp seems to hold a little better, but still slips
- Friction paste on the post - no difference

So basically I want to know - is this a common, known problem with the giant ISP clamps? Has anyone else had this problem? What can be done to fix it? Are there alternative clamp designs that will fit my seat post? Any help/advice is much appreciated!
 

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1st thing to check is the flatness of the cut that was done on the ISP. Hopefully you got the cutting guide with your frame cause that makes this much more accurate to check. The cut needs to be perpendicular to the seatpost, not horizontal to the ground. Also, after cutting, I used 400grit sandpaper on a sanding block to make it perfectly flat with the cutting guide. It ended up lowering the seat clamp by ~2mm more than the initial cut, by the time I got it perfectly level, but then the seatclamp has even pressure on the spacers and shouldn't ever rattle loose.
 
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