SUCCESS! Caustic Soda to remove seat-post-my experience


Dirt Disciple
I recently acquired a steel Billato built Battaglin with the holy trinity of “stuck-ness” - stem, BB and seatpost all stuck fast.

The rather nice 3TTT stem came out eventually with judicious application of various penetrating oils, boiling water and a bit of careful muscle power, the Campagnolo BB was the same after I invested in the correct Campag tool, however, the Kona aluminium seat-post resisted all attempts so after giving up on Physics, I decided Chemistry in the form of caustic soda would provide the answer!

Firstly, Health and Safety (it’s no laughing matter), you will notice regular usage of the words “care”, “carefully” and “safe” in this write up as caustic soda/lye/sodium hydroxide has the potential to really screw you up so treat it with respect and caution, plan what you are going to do and think things through before you crash ahead. Do not leave it lying around for children or wildlife to investigate and make sure other members of your family know what’s going on so that if there is an accident they are able to inform doctors etc. and so that they themselves don’t stumble upon the process in the garage, shed, workshop (keep these all locked). Keep a bucket of clean cold water to hand in case you need it to pour over yourself in the event of an accident!


I read accounts and watched loads of videos online and was horrified at the reckless approach taken by many. This put me off for ages until I established that these people were simply using far too high a concentration of caustic soda – they were impatient and wanted instant results, it doesn’t need to be like that. Your seat-post has been stuck for (probably) years, what difference is another week going to make?

I had already cut off the seat-post about 2” above the top-tube so had to plug the bottom of the seat-tube with a large slug of ordinary bathroom/kitchen mastic applied via the BB in order to contain the reaction – leave this to cure for at least 48 hours in a warm environment. I initially tried Blu-tack but this was not watertight.

After further research on the most excellent website, I settled on the recommended concentration of 200g of caustic soda to 1000ml of COLD water. The reaction is exothermic so be aware that the solution will become hot itself.
I purchased 500g of soda crystals from a traditional ironmongers for £3.50 (high street DIY sheds are unlikely to stock) and used about 1.5 litres of the solution, so each refill of the post was only approximately 225ml.

Set your frame so that the seat-tube is vertical in a large low sided plastic container, so that it can’t topple over, and that any spillages are caught and can be dealt with after the seat-post has gone.

Wear sensible clothing and footwear, always use long thick rubber gloves and a full face plastic visor (preferably) or goggles. Weigh the soda carefully in a separate disposable container so that you do not ruin the kitchen scales and use a disposable 1 litre bottle to measure the water. Immediately replace the cap on the caustic soda crystals container to prevent inadvertent hygroscopy and so that the crystals do not get spilt.

I transferred the cold water into a 2 litre plastic bottle then poured in the soda carefully using a DRY plastic funnel. ALWAYS ADD THE SODA TO THE WATER so that the solution gradually increases in strength NOT the other way around which may cause spitting.
Replace the cap tightly and gently invert the bottle back and forth a dozen or so times until the soda has dissolved, there is no need to shake it vigorously. The bottle will become hot as stated earlier but not uncomfortably – in some accounts I watched the bottle became so hot that the individual could barely hold it! Open the bottle slowly and carefully, you will hear gas being released.


CAUTION - NO NAKED FLAMES hydrogen will explode in high concentrations.


Now you are ready to start.

Use a separate funnel to carefully fill the seat-post to the top of the top-tube only with soda solution, not the top of the seat-post so that the reaction is contained and spillages are minimised. You will almost immediately see the reaction starting with a gentle fizz, rather like an Alka-Seltzer near the end of its reaction.

Replace the cap on the bottle containing your caustic soda solution, remove your gloves then your eye protection and leave the vicinity – open a window if you can and lock the doors. However, if this is not possible just ventilate the area when you return and follow earlier advice regarding naked flames and lights. Be aware that after a few days the caustic soda had attacked and completely dissolved the threaded top part of my solution bottle – glass would have been safer.

NOTE: the reason for removing your gloves first is so that you do not transfer any spillages of soda which may be on your gloves anywhere near your face and eyes!

After 24 hours you will see that the reaction has died down. Carefully dispose of the solution into the foul drain, it will be clear at the top but with a sludgy grey bottom section which is the remains of the seat-post, flush with some cold water and refill the seat-post with soda solution. After 5 days on a typical thickness seat-post you will notice that there will not be much post remaining but resist the urge to use physics again. Keep refilling, flushing and waiting – you are winning.

After 7 days it was pretty obvious the battle was over as when I emptied the seat-post there wasn’t much grey sludge and the remaining stump sticking out the top of the seat-tube just twisted out with my fingers. In your face seat-post!

I thoroughly rinsed down and flushed the frame out for 20 minutes using a fast running garden tap and a short length of hose pipe. Then I removed the mastic slug using a long length of 22mm copper tube as it is a perfect diameter and is softer than steel so wont damage the BB threading and just pushed it out firmly, any remaining mastic can be removed with a small wire brush. The frame received a further flushing then was hung up to drain out and dry. All gloves, containers etc were disposed of in the recycling.

Note that unless you are really careful spillages will likely cause damage to your paintwork. My paintwork was shot as the bike had been well used and needs repainting, so I was not too bothered but in retrospect no additional damage was caused as I was careful.

Finally, I would also like to thank my Physics and Chemistry teachers from my school-life during the 1980s who passed on their considerable expertise to a teenager and who still had the understanding to anticipate the likely outcomes of his efforts 30 years later.

Good luck, Ray


Senior Retro Guru

Interesting and thanks
I was however interested in you saying you cut the seatpost off 2ins above the frame. In the past, I've left as much post as possible in situ,to clamp in a vice and give the whole thing some muscle power. I've yet to fail doing this with wd40 and plusgas. I've ended up with round seatposts being flattened to look like aero,and feel good grip of post in vice and twisting the frame is the way to go


Dirt Disciple

Indeed, I did try various levers, oils, heat/cold and used the frame itself as a level with a vice but with no success. The embedded post was probably 300mm and I forgot to say in my last post that the crux of the problem was a 27.2mm post in a 27mm seat-tube, so it was well and truly stuck with likely bi-metallic corrosion thrown in the mix.



rBotM Winner
PoTM Winner
Removed a seatpost once with use of a cemented concrete floor based clamping device for drill pipe & a 16T overhead gantry crane....... comes out very easily with added +gas....... :)


rBotM Winner
Great write-up Ray - especially the safety notes (my neighbour once poured CS down a sink full of hot water and ended up in A&E). It's a tough job indeed freeing a seatpost, nice one ;-)


Dirt Disciple

Glad to add something back to the forum.

I confess to not knowing as much as some of the people on here, but help where I can!