I just took out my Coker from its winter sleep and the first thing I did was to add a T7 handle. The first rides of the year was great, but now the seatpost has started turning really easily. So every time I do a hard turn or when I freemount the seatpost turns and it is really annoying and maybe dangerous. I have never had this problem previous years
I have a GB frame with a KH 25.4mm seat post and a Nimbus double-bolted seat post clamp.
I have tried the obvious like tightening the seat post clamp and taking it all apart and putting it back together again, but it still turns.
Check and see if the clamp is tightening enough. One of the problems with the two bolt Alu Clamp is that the bolts hit the other side of the clamp and you do not get enough tork on the seatpost. You may need to get a adapter so that it completely tights down.
I have a GB frame with double-bolt clamp also. I notice the clamp seems to be a tad big for the frame, as the sides appear to be touching when it’s tight. Fortunately it’s tight enough, so I don’t have twisting seat problems. If your clamp is closing down to where the two sides are touching, you may have a smililar problem. I’d start by shopping for clamps (at bike shops). Try them out and see if they are a snug fit on your frame, or a little loose.
I suppose if that doesn’t work you might be able to file away some of the metal from the inside of the clamp, at the back where the sides are touching. This will allow it to close more, but this is not the best solution because when that happens the bolt holes are no longer aligned and it puts lots of stress on the bolt, and the bolt may break (especially if you change seat height a lot, like I do because I keep changing cranks).