Cycling UK Forum

As per title, this theory is put forward every so often, always (as far as I can tell) in the context of a stuck seat post. It seems to have originated with Sheldon Brown. But on the same site somebody says they don't think it will work because the ammonia can't get to the oxide, unless the seatpost isn't very stuck (or words to that effect).

I haven't got a stuck seatpost. What I have got is a folding Dahon Jack with an extremely stiff hinge. An hour of penetrating oil and Tri Flow has freed it up so there's a complete range of movement, but it's a real feat of strength.

I'm fairly sure it's a steel pin inside an aluminium cylinder, and now that I've found the open end, underneath, I was able to drip some Tri-Flow in. It was held in the reservoir made by the hole, but half an hour later it had gone, and there was some seeping out lower down. So there's enough space for Tri Flow to get through by the looks of things.

Despite Sheldon's proposal, I can't find a single example of anybody saying anything like - "hey, the old ammonia-eating-aluminium-oxide trick worked folks".

So do any chemists or engineers here think that it will work? Given that I have got some movement there, so can pour some in the top and "work it" to encourage it through.

And just as importantly, do I risk trashing something else, as it won't be possible to rinse it out completely?

Thanks.

Yes ammonia worked for me on a steel frame with alloy seat post. I had to plug various holes, position the frame upside down and leave it with the seat tube full of ammonia overnight. This was after Plusgas A had not done the trick. Still using the bike 20 years on.

fausto99 wrote: ↑17 Aug 2021, 1:45pm Yes ammonia worked for me on a steel frame with alloy seat post. I had to plug various holes, position the frame upside down and leave it with the seat tube full of ammonia overnight. This was after Plusgas A had not done the trick. Still using the bike 20 years on.

Great, thanks for the reply. So it does work!

A couple of questions if you don't mind.

1. What did you do once you'd got the seatpost out? Did you rinse out the seat tube or anything like that? I'm just concerned about leaving caustic chemicals in there, as this isn't like a seat post, so I might end up with some "trapped" ammonia.

2. Is the ammonia you used just the ordinary household stuff you can get from the local Homebase.?

It worked for me, although I hadn't had the seatpost in a big vice before either. So, it's impossible to say whether it would have come out without me injecting ammonia down all the flutes of the post for days on end. But... I treated a stuck seatpost with ammonia and managed to get it out!

biketips666 wrote: ↑17 Aug 2021, 1:55pm1. What did you do once you'd got the seatpost out? Did you rinse out the seat tube or anything like that? I'm just concerned about leaving caustic chemicals in there, as this isn't like a seat post, so I might end up with some "trapped" ammonia.

I'd rinse well with tap water, and then dry it all out.

Jonathan

Dahon hinges are removeable, so I think you'd be best completely removing it for cleaning. If you need replacements, you can get them at C H White and Sons (foldingbike.biz).

Jdsk wrote: ↑17 Aug 2021, 2:36pm

biketips666 wrote: ↑17 Aug 2021, 1:55pm1. What did you do once you'd got the seatpost out? Did you rinse out the seat tube or anything like that? I'm just concerned about leaving caustic chemicals in there, as this isn't like a seat post, so I might end up with some "trapped" ammonia.

I'd rinse well with tap water, and then dry it all out.

Jonathan

Thanks. You're a chemist of some sort aren't you? So I'll take that as reliable advice.

It's so stiff at the moment that I think the best thing will be to strip the frame and knock up some sort of "rig". Also knows as a black and decker workmate. Then I can spend a couple of happy hours listening to the radio while I waggle the thing back and forth. It's awkward as hell with the wheels on.

emleyman wrote: ↑17 Aug 2021, 2:42pm Dahon hinges are removeable, so I think you'd be best completely removing it for cleaning. If you need replacements, you can get them at C H White and Sons (foldingbike.biz).

I saw replacement hinges advertised, so that's what I thought. I'm pretty sure this isn't removable though. This is a Dahon Jack, the 26" folder they made for a couple of years, not one of the small wheeled ones. The hinge hasn't got an bolt with a hex head like some of the examples I've seen on the internet.

But I'll investigate and maybe even give C H White a call.

Edit. Just called C H White. No replacement hinge kit for this bike. Just to be clear, it's not the latch thing that's the problem, it's the actual hinge pin which is driven into the holes in the frame, as an interference fit.

The ammonia should dissolve the aluminium oxide but I'd be wary of using this method as there's a possibility you may dissolve some of the aluminium. I would try gentle heat from a hair dryer or hot air gun as a first line of attack.

The idea is to drive off the water of crystallisation from the aluminium oxide layer and this should loosen the hinge pin sufficient to drive it out with a parallel punch. You only need gentle heat so it shouldn't damage the paintwork provided you're careful with the hot air gun.

Norman H wrote: ↑17 Aug 2021, 4:50pm The ammonia should dissolve the aluminium oxide but I'd be wary of using this method as there's a possibility you may dissolve some of the aluminium. I would try gentle heat from a hair dryer or hot air gun as a first line of attack.

The idea is to drive off the water of crystallisation from the aluminium oxide layer and this should loosen the hinge pin sufficient to drive it out with a parallel punch. You only need gentle heat so it shouldn't damage the paintwork provided you're careful with the hot air gun.

Thanks. I can't drive the hinge pin out. It's in a blind hole.

In that case removing the water of crystallisation from the oxide layer should allow sufficient room to get some lubricant to penetrate. As you apply the heat you will often see the water driven off appearing as steam.

Norman H wrote: ↑17 Aug 2021, 5:34pm In that case removing the water of crystallisation from the oxide layer should allow sufficient room to get some lubricant to penetrate. As you apply the heat you will often see the water driven off appearing as steam.

Thanks. As I said upthread, I've added various lubricants, and I'm fairly sure that at least some of it has flowed through the hinge, as per description. The hinge isn't seized, it's just very tight. Will applying heat now reduce the volume of the aluminium oxide, freeing up the hinge?

Norman H wrote: ↑17 Aug 2021, 5:34pm In that case removing the water of crystallisation from the oxide layer should allow sufficient room to get some lubricant to penetrate. As you apply the heat you will often see the water driven off appearing as steam.

There is actually a very small grub screw across the underside of the hinge pin, presumably to stop the hinge pin actually falling out, if it was loose. It might just be a fail safe, but it shows that Dahon at least thought it was a possibility.

The chap at C H White said that these were often a bit tight at new, whereas my memory was that it was a little too free, and I ended up bashing myself when the thing swung round.

Thinking about it, heat might not be a bad idea. Reading this:

https://www.velonews.com/gear/mountain- ... osts-suck/

tells me that it should free the joint up (temporarily) as it's a steel pin inside an aluminium body, and aluminium expands more than steel when heat is applied. So with any luck it'll give a bit more room to get ammonia and/or lubricant in there.

This:

https://acim.nidec.com/motors/usmotors/ ... 0aluminum.

Doesn't seem to suggest that Ammonia will corrode the aluminium:

Ammonia: gas (dry) has no action on aluminum, even at elevated temperatures. When ammonia is moist or in solution, the rate of attack also is low for all concentrations at temperatures up to at least 1200F. Aluminum handling and process equipment are widely used.

But I'm not a chemist, so perhaps I've not understood properly. Please tell me if that that's the case.

Thanks.

Don't let me discourage you, I was merely sounding a note of caution and I doubt that ammonia will have dire consequences. Others have used it successfully to remove stuck aluminium seat posts from steel frames, although preservation of the seat post was never the main objective. I would try the gentle heat method first and if that fails give the ammonia solution a go. I'd suggest successive heating and cooling cycles with penetrating oil applied as it cools. I've generally found it's best to begin with gentle persuasion before resorting to the 4lb lump hammer.

My chemistry is more than a bit rusty these days but since nobody else has ventured an opinion, here's what I think might happen.

Aluminium is quite a reactive metal but most aluminium or aluminium alloy surfaces are protected from further oxidation by a thin surface layer of aluminium oxide. The ammonia will strip away this protective layer and expose a fresh reactive aluminium surface. One could then speculate that further oxidation of this fresh aluminium surface (either by reaction with dissolved Oxygen or by direct reaction with water) would occur, followed by further oxide stripping and so on and so forth. This may not be be a bad thing, provided the reaction doesn't go too far, as ultimately your goal is to free up the hinge. Jonathan's idea of thoroughly rinsing with water is probably a good idea.

Norman H wrote: ↑18 Aug 2021, 6:41amMy chemistry is more than a bit rusty these days...

Surely you mean "more than a bit oxidised"?