I left sinking the seatpost for last for better or worse.
Start with some kind of seatpost or seat tube shim. Sinking a few inches of a real seat post (not the seat tube that attaches to) will increase the strength of your bamboo seat post (a shim will not provide much support). I went with a steel seat tube 28 x 9/6 x 650 mm from Nova Cycles. This fits the extra seat post I had laying around.
When you are cutting your poles make sure that you have enough room between the top of your bamboo seat tube and the nearest node to sink your metal seat tube. The requirement here is that you need to meet the minimum height requirement for your seat post (as shown above). Keep in mind that you don’t need to be able to fit the whole seat post in. You can cut it to length if need be (just make sure you use the same minimum height rule so the seatpost doesn’t snap off on you).
NOTE: I would not recommend sinking the actual seat post directly into the bamboo and then clamping it. This would likely crack you bamboo… I’ve seen a picture of a cracked bamboo seat tube online. Not a good idea.
The real pain here is reaming out the inside of your bamboo pole to allow your metal seat tube to fit inside. I used a curved course wood rasp (above) and a lot of sweat (not shown). Tape some cardboard around the pointy handle of your wood rasp to keep your skin on your fingers.
I would recommend going slowly and making sure you don’t screw anything up here. It is easy to over-ream any particular area or side. My first goal was to fit the seat post (smaller diameter than my metal seat tube) first.
After this I put a piece of 80 grit sandpaper around the seat post and rotated it until the bamboo would accept the metal seat tube (also shown above). This worked very well and I highly recommend it.
Make sure to cut it a little longer than needed because you will have to leave room for the seat tube clamp (binder) to clasp onto (shown above). Make sure it all fits.
Then you need to cut a small slit vertically down the metal seat tube so that the clamp can compress the metal around your seatpost. Widen it a bit and try clamping your seatpost down. Also sand down the rough edges. Now slowly cut away the bottom of your metal seat tube (non slit side) until your seatpost clamp fits just right.
Keep it together
Now glue away. I would recommend using some 404 west systems high density filler to fix it in place. I made mine a watery mayonaise consistency. This allowed me to pour some into the bamboo tube creating a 1/4″ pool of epoxy at the top of the bamboo node (ie as deep as I could get my metal seatpost in). In addition to the pool of epoxy at the bottom, I found that building up a thick layer of epoxy on the inside of the bamboo seat tube and a light layer on the outside of the metal seat tube worked best.
Make sure that you follow the instructions for proper bonding with metals (found under the “metal bits” page). Also as with all of your sanded bamboo bits (the inside of your bamboo seat tube this time) make sure that you ‘wet it out’ first (ie use a brush and cover all sanded bamboo with some epoxy before you start).