I stopped offering this modification in 2019. Lack of time to keep on doing everything I do, so I had (and still have) to make choices. But what I can do is provide instructions for doing this yourself or with the help of a skilled craftsman, frame builder or a bike mechanic.
See the drawing below of the shim I no longer produce. You can have it machined on a lathe using straight PVC sewage couplers cut in half and glued onto a 32mm pvc pipe just like I used to do (see photo at the bottom of this page. Clamp it with a light press fit on a piece of (old seatpost) tube for machining the outside diameters after having done the inside directly clamped into the chuck. Or have it machined from a solid bar of PVC or another plastic such as HDPE, but no POM, Nylon or Teflon due to the low friction these materials have. After all, it has to clamp the stem. I do have the 27.2 mm stems for the top part in stock with a 25.4 mm clamp section for the original Brompton handlebars. Add a high quality quick release seatpost clamp in 34.9mm like the ones from Hope Tech UK for instance
Make the cut right at the end of the 35mm Brompton stem tube , just under the handlebar clamp section. So you basically just cut the clamp off.
Note: This only applies to post 2017 Bromptons. Before that date stems were shaped differently. It's still possible to make the conversion on older Bromptons, but you have to look carefully where to cut the stem tube. It might be that same spot just under the clamp head or just under the curved section, which basically removes the curve from the stem and brings the handlebar closer to the rider as a result. Just keep in mind that there should be a straight section for the shim and some room for the stem to go down below the bottom of the shim. Think twice and cut once!
Deburr and drill a 6.5mm hole at 35mm below the end of the stem (at the back side) and make two cuts with a hacksaw to the hole to get a stress relief slot just like any seat tube has. Deburr again, add the shim using some carbon assembly paste on the inside of the steel tube for added friction (so the shim doesn't rotate during use) Also add some carbon assembly paste to the inside of the shim. And to the inside of the 34.9mm clamp. Add the clamp , the 27.2mm stem and the handlebar. Adjust the clamp. so it's snuck. And start rotating the handlebar alternating to the left and right to settle the assembly paste, while holding the front wheel between your legs. This way the assembly paste is really activated and you will notice the friction gets higher. Readjust the clamp if necessary and check if the stem slides up and down fairly easily. Make sure to never ever put the stem higher than fully engaging the shim.
Regularly check the friction of the 27.2 stem inside the original Brompton stem. If it's too low it will be very dangerous as you can lose control over the steering. So the quick release will have to be adjusted every once in a while to compensate for wear.
Not mentioned on the sketch: the shim has to be cut with a hacksaw twice, two mm apart, creating an open shape that can be clamped down.
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