Here is a trick I have used on safety bicycles that are going to be ridden to stop seat posts and handlebar stems from slipping down. Basically it is an internal spacer.
(I realize that makes/models/years/designs of antique bicycles vary greatly but read the steps below to see how to fix a current or possible problem, especially on a restored antique bicycle with shiny nickel plated parts).
1) Set up your comfortable heights of seat and handlebars.
2) Place some tape to mark the junction at the seat post and seat post frame tube OR at the handlebar stem and front fork stem/steering head.
3) Remove the seat post and handlebar stem and measure from their ends to your tape mark. Write down measurements.
4) Using a tape measure or stick, internally measure from the top of the seat post clamp/top down to the crank bracket. Do the same from the handlebar clamp/steering head down to the fork crown and write down measurements.
5) Subtract the measurements you got in (3) above from measurements in (4) above to get length(s) for your new internal spacers and write down measurements.
6) Get a piece of plastic sprinkler pipe, wooden dowel, etc that will be strong enough and the outside diameter should just barely slide into the seat post frame tube or front fork stem/steering head. You want a sliding fit "if" you want to easily remove it in the future. If internal diameters of seat post frame tube or front fork stem are odd internal diameters not matching the diameters of the plastic pipe or wooden dowel, you can file/machine down the outside diameter of a slightly larger diameter flat washer that will slide into this odd inside diameter and use a smaller diameter pipe or wooden dowel that you slide in first and then place the washer at the top so the seat post or handlebar stem will bottom out on it.
7) Cut pipe, wooden dowel, etc to the measurements you got in (5) above and slide in your new cut to length internal spacers until they bottom out.
Lastly, slide in seat post and/or handlebar stem until they bottom on the internal spacer, align them and tighten in place.
You now have stopped the sliding down problem you may have been having or could eventually experience. The good part is you don't see this "fix" from outside the bicycle!
Mike Cates, CA.
