Tips For Dissolving Your Bike's Stuck Seatpost Lye NaOH

I didn’t need to be doing this, I told myself, putting long rubber gloves back on, and two masks, and a hat, and eye protection. Sure the aluminum seatpost on this steel 1986 Fuji Sundance was seized solid, but it was in a good spot. The bike was rideable. I didn’t have to be working with a chemical solution that would dissolve my skin to the bone.

Here is the rundown on this bike. I stared at it every day when I was walking home in my neighborhood, getting rustier and rustier every winter. Eventually the owners put it out for trash pickup, I knocked on the door, and they told me “take it. Take it. It’s junk.”

How right they were!

I made it through replacing the rusted-solid bottom bracket, I taught myself how to get the obsolete roller-cam brakes working, and got the bike back on the road. I looked forward to riding this thing, but I never expected it to be such a treat as a town bike. It fits relatively large tires for an ‘80s mountain bike, and on some 2.3-inch wide dirt jump tires, the only thing I own that’s more comfortable is my sofa. This was Fuji’s next-to-top mountain bike of the era, made in Japan with Ishiwata tubing and neatly brazed lugs. It’s tough, and solid, and relaxing, and only marginally shorter than a Chevy Suburban.

This is all to say, I fell in love with this big, old, heavy bike. I was forming a relationship. I didn’t just want to sell it off to the next person, I wanted to leave it better than when I started it. For that, I needed the seatpost free.

I hoped this would be an easy win for me. Every day I sprayed the seatpost with PB Blaster and rode it unclamped. I hoped that eventually a bump in the road would knock it free. After a couple months of no success, I realized I needed sturdier measures.

Though the seatpost needs to move up and down, you can sort of break it free by twisting it. That is, theoretically, a method that works. I held the seat and twisted. Nothing. I held the post in a plumbers wrench and twisted. Nothing. I took the bike to a little bike shop (the lovely Nomad Cycle in Queens) and had the shop owner, Damon, stick the post in his vise and twist the entire frame around it. The bike snapped the vise.

A $500 vise, cast iron, shattered. Damon said that he’d cut it free, and I could come by a couple days later.

A couple days later what I got was a text from Damon that he tried to cut it, and even welded up a copy of this incredible jig but the post wouldn’t budge. I’d have to pick it up. The post was now sliced in half, so I couldn’t ride it. I had only one option ahead of me.

I took a trip to my local hardware store’s Crime Aisle and picked up some drain opener crystals. This is crystalized lye, aka sodium hydroxide, aka NaOH. Lye is nasty stuff, and it chemically dissolves the aluminum while it does not react to the steel of the bike frame. The idea is to seal up your bike’s seat tube, fill it with a caustic soda solution, and the mix will eat away what’s fused the seatpost to the frame without eating the frame itself. The trick is that in addition to dissolving aluminum, lye also dissolves human beings, as you’ll find in any old forum post about someone asking how to dissolve a stuck seatpost with lye. Every time someone asks, someone like this pops up. From Bikeforums:

“If it gets on your skin, your skin will feel slippery like soap. Why? because it turns people/animals INTO SOAP!”

And again from Bikeforums:

“From 1973 to 76, I worked summers at a chemical tank farm filling 45 gallon drums of this stuff from railroad tank cars. We wore gloves and a visor, no respirator. I can’t believe I’m still alive.”

As well as:

“Stepped in a puddle of sodium hydroxide leaking out of a tank at a chemical plant in the late ‘70s. It destroyed the boot leather and burned a small part of my foot to the bone.”

With this in mind, I bought myself some thick rubber gloves. (I already have plenty of masks thanks to COVID, and I had some eye protection lying around for other general safety-first reasons.) For sealing up the frame itself, I was lucky that my corner hardware store stocked drain stoppers in 7/8" and 1 1/8" diameters, which are generally the size of the tubes used to make bikes. I sealed them around the edges with some putty you use to mount posters on a dorm wall, and stuffed a rag where the bottom bracket used to be. I was reasonably happy that it only took me 30 minutes to strip the bike to the frame.

Here it is stripped, and then the process of getting plugged:

Armed with the knowledge of watching several YouTube videos and reading through a variety of forum posts, I decided I was ready. At least, I was ready enough to start mixing and get this over with. My mixture was, at times, rather more active than I hoped, in part because it was hard to know just how much crystal to mix into my water:

After one afternoon and a morning of filling, draining, and refilling the seat tube with drain crystals dissolved in water, the seat post finally came free, first in one and then in two more parts, wiggled out of the frame with an adjustable wrench.

If I had any advice to give, I will say that I found it was easy pouring the solution down the seat tube from the top and plugging the tube down at the bottom bracket. Others suggest flipping the bike over, plugging the seat tube around the post, and pouring in your solution via a funnel through the bottom bracket area. I guess if you aren’t sawing your seatpost in half, this would work fine. For me it was easy to keep the bike right-side-up to drain out the old solution and refresh it when I stopped seeing gases emitting from my little stovepipe of a bicycle.

I will also add that I initially bought crystalized Drano, as it uses NaOH. The problem is there is other stuff in crystal Drano, and it dyes stuff blue and leaves little beads of god-knows-what behind. I later found some actual only-lye-crystals and that was much easier to accurately mix and work with.

It’s hard for me to say that I was filled with joy as it finally came free. I knew it would come free. I had done the nuclear option. At what cost had come my victory?

The paint. People use lye as a paint stripper, and I see why. As NaOH interacts with the aluminum, it releases hydrogen in a rather active reaction, and when the solution is too strong, it bubbles free like a science class volcano. Enough of this stripped the frame down to the bare metal in a few places, and I’ll have to paint over it to make sure it doesn’t rust. Well, I’ll probably clear coat it. Patina like this doesn’t come easy.