Problem: I've got a vintage Ross MTB from the late 80s that I'm flipping. I've got the bike 90% ready for sale but I ran into a problem with the seatpost. The seat tube takes a 26.4 post, but I don't have any 26.4 seatposts. I've got several 26.0 seatposts and I tried two of those. They seem to be firm enough, but if I grab the saddle and try to turn it, it will turn. Riding on this setup, I know it will not hold position well. I also have one very nice 26.2 seatpost that I'd rather keep for one of my own bikes. The other alternative is to buy a cheapo 26.4 seatpost on ebay for about $10. Before I put more money into a flipper that's not going to sell for very much I thought I'd post here to see if anyone has good advice how to get a 26.0 to fit well into a 26.4 seat tube. Is there a way to fabricate a shim that will stay in place? I'm looking for something reliable for the next owner. Any ideas?