I'm renovating an old mountain bike. I'm not up on some of the tech, having learnt my bike mechanics back in the 70s.

The headset is grinding so I want to disassemble and replace the bearings.
See photo.
There is no bolt in the gooseneck -- I assume this is correct? I guess there should be a plug or stopper, but it isn't required mechanically?

How is the headset locked to the fork then?

To disassemble do I just loosen the big locknut?

This may seem obvious, but I've destroyed hardware before by trying to force it to do something it wasn't designed to do.

There is a bolt and wedge to secure the stem (gooseneck). Did you remove the bolt? If you did, you should screw it back in a few turns, and tap the end with a mallet. That will break free the wedge so you can remove the stem. If you did not, look down inside the stem. There may be a shoulder and you will see a 6mm hex bolt. Loosen it a few turns and again, tap it with a mallet to break free the wedge, and remove the stem. Then you can loosen the headset locknut and proceed with the bearing service.

That is a threaded headset and the stem should have a stem bolt.

My guess is that that wedge in the stem is stuck (or the stem is seized) and one would normally use the stem bolt to tap out the wedge, I'd start by locating a stem bolt and seeing if it can be threaded back in and from there one can tap on that to loosen the wedge.

Technique is to thread the stem bolt in until it is snug and then back it out 4-5 turns and give it a firm tap with a hammer to free the wedge... if it is not seized the stem should then be freed up for removal and then you can address the headset.

Correct, the above is not "new tech" but rather an old style threaded headset. Follow the instructions above to remove the stem then simply Google threaded headset overhaul, with the sheldonbrown and parktool sites offering some of the best guidance.

I'm probably wrong, but taking the photo at face value, it looks like some kludge of a threadless stem somehow stuck onto a threaded steerer. It would be interesting to see more photos and a description of how this was assembled. Anything I can conjecture would seem to be completely unsafe.

I suspect it's the stuck seatpost effect, in the corrosion and rust of the steel and aluminum bonding them together* .

Though it's an aluminum quill stem, and steel steerer fork, look up stuck seat post threads from the past.

*Galvanic Corrosion

Really looks like an odd quill stem and conventional threaded headset (no apparent split in the upper cylinder for clamping). Just free the wedge and stem as others have posted and proceed with servicing the headset bearings . . . see online tutorials.

Unfortunately, this seems to be the case.
There is no sign of a stem bolt or wedge, but the stem is locked in regardless.

Should I just try to use more and more leverage on twisting the stem till it releases (or breaks)?

I live on a tropical island, which is very unhealthy for bicycles. Warm, salty, humid air corrodes everything.

Have you confirmed that the wedge deep inside the steerer is free? Usually the wedge bolt (the really long bolt running inside the stem and steerer) is loosened a few turns then a hammer strike on the bolt's head breaks loose the wedge. Only then can you remove the stem if all is right. If the stem is frozen in the steerer from galvanic corrosion then the wedge will be freely rattling around and yet still the stem is stuck.

If this is the case then search for "stuck seat post" threads as this is the more common situation of stuck Alu in steel tubes. If worse come to be then a SawZaw or hack saw slice down the stem's quill will split it and let it come free. But try the chemical attempts first. Andy

Really, there isn't any bolt or wedge.
I know how they work, not seeing them is why I posted initially.

Here's a shot from the bottom.



As may be obvious, I scavenged this bike. Maybe this fork problem is why it was abandoned. But it was built by a highly regarded local bike shop, so I thought it was worth resurrecting.

I squirted WD-40 inside the stem and upturned it, hoping some will seep between the stem and fork.
I'll give it another try in a few hours.
I've never seen any of these other chemicals in shops here, and I have looked.
Since the stem wall is between the frame and the fork tube, how can cutting it loosen the bond?

Are you sure that what you see from the bottom is not a well corroded wedge stuck in place? You can stick a magnet up inside to see if the thing in the steerer is steel, or not. If it's steel it's likely the wedge. I have seen wedges that are so badly "rusted' that their threaded bolt hole is pretty stripped out. Wasn't there a bolt with an allen head inside the top of the stem? If not then the bolt was likely removed before, but since the stem was corroded in place the bolt no longer did any function.

You could try ATF or Kroll as they have a greater penetrating capibility then WD-40. You might also try some moderate impact force from either end to help shock things loose. You should consider the stem a loss anyways.

The stem goes inside the steerer, given the threaded steerer and headset design. Andy.

A further thought- If you were to cut/grind away the upper headset nut and cone you'd free up the steerer inside the head tube. Then you's really find out how the stem was placed in/over the steerer. Really, don't do this until you're out of any other options. You're likely to find out that you have a traditional quill stem after all. Andy.

Searching this forum is hopeless.

So I have to pick human brains. What are some of these unsticking agents?
I'll see if I can find them.
But I'm not in the US and many things you can find in the corner hardware store just aren't available.

-- update: eventually I stumbled on a few "stuck parts" threads.
Mentions of "PB penetrating oil" and "Kroil".
No idea if I can find these here, but I'll give it a try next time I'm in town.

Looks *exactly* like the bottom of a wedge.

That is indeed an expander. PB Blaster is another agent useful for loosening corroded parts, but anything labeled "penetrating oil" is better than nothing. Put some in from above, let it sit for a day, then flip the bike and put some in from below for another day. I cannot tell from the photo if the wedge is sitting on an internal ledge in the steerer. Hopefully not, as that is going to make your task extremely difficult.

If the steerer is not stopping the wedge from traveling further down just get a rod long enough to reach it from the top of the sem, put the fork blades on a block of wood on the floor, and give the expander several good whacks. Hopefully that will knock it down enough to release the pressure on the stem, but the stem may still be stuck from corrosion. if so the best way to get the leverage necessary to free it is to find a mechanic's vise large enough to hold the fork crown (padded) in place, clamp it in, and then twist the stem back and forth to loosen it. You can usually also place a block of wood on top of the stem and give it a few whacks once the expander is loosened, but yours appears to have almost no space between stem and locknut.

If the expander cannot move down further you are indeed "stuck." If you try to move it from the bottom you just move it further into the stem. If the stem merely has a recessed area for a bolt, with a small hole, you could drill out the expander and then try to use a rod to pound the stem out (LOTS of work). For me at that point it would not be worth it except for a high-end frame, which is not the case.

That thought did pass thorough my mind. It's shiny and perhaps aluminium, clearly contrasting with the rusty steel of the interior of the fork wall, and I think it might have a thread in the central hole. But it just fits so perfectly all around, no gaps visible, it looks like it's literally welded, not corrosion welded.

Here's a zoom in.



I gave it a few whacks with a long rod from the top to see if it was movable, but not being sure I was a bit half hearted. But now I think it must be a wedge after all. So, I'm going in to town later in the week and will search for some penetrating oil and soak it and then either get it loose or, if it isn't a wedge, destroy the fork.

So stay tuned for the exciting denouement.

Well, as above, I'll try to thump it down.

If that fails I might try a rat tail file and cut a slice from the hole to the edge.
The wedge should be relatively soft alloy, so it deforms rather than damage the fork.
But it may take about as long as the guy in Shawshank.

I've done similar in removing a jammed seatpost, but there I could do most of the dissection with a hacksaw.

"But it just fits so perfectly all around, no gaps visible, it looks like it's literally welded, not corrosion welded." AlanHK

That's exactly what galvanic corrosion is. The two dissimilar metals merge to a degree with electrons migrating from one to the other.

The alu. oxide can be "dissolved' away with ammonia, Drano and Easy Off oven cleaner also attack alu.

When i goggled "Removing stuck seat posts" this was the top link. https://sheldonbrown.com/stuck-seatposts.html Andy.