So I'm overhauling a commuting bike belonging to one of my friends. Mainly going to install a new headset, because the bike's steering has been bad for awhile, and getting worse.
Now it looks like the bike needs a whole new fork.

The threads are totally worn down at one point around the steerer tube, probably from a mishandled lock-washer sometime in the bike's past. There is actually a hole on the front of the steerer tube! And the part of the tube above the band of worn-down threads isn't perfectly parallel to the part below.
Even when my friend first got the bike (built up at a local recyclery), it wasn't possible to cleanly adjust the headset - it would be tight at some points and loose at others when turning the front wheel.
My guess is that riding the bike with a headset that isn't perfectly tight at all angles of the front wheel, caused stress at that already-weakened spot on the fork, along with continued friction on that spot.

I can't confidently reconstruct the life course of this bike to explain the damage - there are multiple plausible stories - but this is crazy enough to be worth posting. And I'm interested to hear alternate possible explanations.

edit: I noticed that the steerer tube itself is bent, about 1/4 of the way up from the crown race. again, this may have been there before (and been the reason that the headset couldn't be adjusted ideally, causing all the rest of the carnage) or it could itself be a result of riding with a loose-at-some-angles headset.

Also, just to show where the thin section of the steerer tube is... the last picture shows that it starts just beneath the threads of the headset cup itself - everything (including both washers and the locknut) is screwed on as normal, locknut close to bottoming out. Makes it almost appear that the steerer tube was drawn (like a wire when put under tension and it gets thinner while lengthening).

edit 2: as a couple of people have suggested, the most likely cause of the thinned/grooved area on the steerer tube threads, is that some loose bearings got stuck against the inside edge of the bearing cup (not the part against which bearings are supposed to sit) and over time, wore down the steerer tube. check out the last picture (#7) that I posted. It looks just like a badly pitted bearing track.

Note that this doesn't explain the crazily bent steerer tube. It looks a whole lot worse when holding and handling the fork, than it does in the pictures.

Yeesh.

it's still good! it's still good!

Holy effing cow! I've seen similar wear, but never to that extent. It usually happens on cheap bikes where the headset is loose and is left loose for a long time. Weird that the wear is on the front of the steerer- I'd expect it to be worn on the backside. Perhaps a ball bearing got loose, was stuck between the steerer and the headset and left there for a long, long time.

In any case, it's time for a new fork. Be thankful that didn't let go while you were riding it.

Let me guess? keyed washer?

Dang...You'd have trouble making a groove that clean with a dremel tool and a cutting wheel. Face plant waiting to happen.

That was my first thought as well. But the wear or shape is over such a long segment of the threading. If it was just from a keyed washer held by a loose headset I'd expect it to be a far more narrow side to side damage channel with a more or less normal looking key groove above and below. I suspect in this case the "key" is more of a flat segment or a semi round segment and that most of the groove is as it came from the maker. But that sure doesn't explain the segment of U groove that extends down onto the head tube unless the fork and headtube were cut together. Hmmmmmm... seems like this'll be one of life's great mysteries.

But that obvious rip in the threading and hole you can see light through is the kicker. Now THAT seems like the keyed washer could have caused that damage. There even appears to be some form of stretch damage that finally snapped or wore through where the hole is located. If I'm right then no wonder the headset is feeling a little coggy and less than fresh thanks to the angle the fork is holding the uppper bearing.

That kind of damage is usually the result of riding on a loose headset. The threaded cup wobbles on the steer tube, eventually eroding through the threads and even the steer tube itself. If you catch it early enough, you can fill the damaged threads with brass and cut new threads into the brass. This one looks too far gone for that. Unless it's a high-end bike, I'd just replace the fork. If you really want to save the fork, the whole steer tube will need to be replaced.

No. The damaged area is well below where the keyed washer would fit.

Just edited my original post to add two pictures of the bent steerer tube (which I just noticed) and a bit of text about it. The steerer tube itself is bent, about 1/4 of the way up from the crown race. again, this may have been there before (and been the reason that the headset couldn't be adjusted ideally, causing all the rest of the carnage) or it could itself be a result of riding with a loose-at-some-angles headset.

Also, it's not a keyed washer, it's the kind of washer that's mostly a circle and then has a flattened portion. You can see the flat part on the steerer tube designed to work with this. This design can cause damage, like a keyed washer, but didn't cause the damage here. The washer slightly flattened out the threads 6-8 threads from the top. I added one more picture to show where the thin section of the steerer tube is... it starts just beneath the threads of the headset cup itself - everything (including both washers and the locknut) is screwed on as normal, locknut close to bottoming out. Makes it almost appear that the steerer tube was drawn (like a wire when put under tension and it gets thinner while lengthening).

I expect the bike was in a front end collision of some sort early in it's life that bent the steerer tube. From then on the headet would never adjust properly.

That's my current best guess, too. It's kinda weird that the steerer tube would bend 1/4 of the way up from the crown race, instead of at the joint of steerer tube and crown (which is more typical).

In addition to the asymmetrical bearing forces as a result of the bent steerer tube, the threads above the "drawn" section aren't perfectly parallel to those below. But that second asymmetry is consistent with the tube being drawn thinner on the backside (where the hole is).

The Stronglight P3 headset, the flat on the steer tube and Mafac brakes indicate that it's probably a '70s French bike. That means that you'll have to find a fork with a metric steer tube or change the headset and stem.

The steering tube is bent forward. How did that happen? Time for a new fork.

The hole probably just eroded through the keyway, which is thinner than the surrounding metal. Many fork manufacturers tried to save money by building only a couple sizes of forks. The threaded section and keyway would be much longer than necessary, allowing the steer tube to be trimmed to accommodate a range of sizes.

Bike frame is a 1970's Mercier. I checked out Sheldon Brown's Headset Cribsheet, and the good news is that the cup diameter of the head tube is 30.2mm (same as ISO 1" headset standard).

So I can try to find a 1" threaded fork with similar steerer tube length, hopefully in ISO dimensions although I could get a JIS crown race if I need to.
I'll need to replace the stem (which is 22mm instead of the ISO standard of 22.2mm for 1" quill stems). But the bike needed a new headset anyway, so if I can find a new fork I'll happily build it back up together.
If I can't easily find a fork, the bike as-is currently is probably only worth $50-$60 anyway, so it's time to look for a replacement for my buddy.

I hadn't thought about the flat style of "key". In the electronics industry that sort of key is called a "D" key for the flat that gives it the look and name. The "flat" in the pictures looked like it was more of a semi circle. Hence my confusion.

And ya, I'd say this steerer was subjected to something epic during its life to bend that way and wear a hole of that sort which likely started from a tear in the metal.

That steerer's a mess!

It kinda looks like a ball or two has dropped out of the race and got caught...

And I can't imagine how that bend could be in the direction it is by any other cause than a very hard landing... and the upper bend in the steerer kind of points to hard landings... I bet the crappy old stem is bent too, right?

You'll have to modify the Mafac hanger with a file to work with a standard slotted fork. It's no big deal.

By the looks of the fork crown, it's not a high end Mercier, but it may be worth more than you think. Some of us are crazy about French bikes.

Yes, you can keep the old cups when you replace the headset, but why would you want to?

You know, a couple of balls getting stuck there is the only plausible explanation for how a nice intent like that could have been formed right around the steerer tube, directly below the top adjustable cup. That makes more sense than my "drawn like a wire" explanation - although I suspect the drawn-like-a-wire stuff happened as well, but only after the groove was already worn in there.

I have no idea on the lower bend. The paint is cracked on the backside of the steerer tube at the point where the tube is bent forward (1/4 of the way above the crown), so the bend ain't original.
As for the upper bend, I suspect that happened because of asymmetrical forces (normal to the area) and the bending happened after the groove had already been worn-in by the hypothesized stray ball bearings.
The stem isn't bent - it was a bit of work getting it out of the steerer tube, but that's only b/c I had to pull it past the indented section.

I was just planning to use a hanger designed for whatever style is the replacement fork (likely slotted). I'm also planning to replace the headset cups, it's just nice to know that they're a normal size (30.2mm) to press into the head tube.

So is there a market where someone would be willing to pay for the main frame? It's in good shape other than some surface rust, and I've always been impressed by how light the bike is (for an old-road-bike-commuter, retrofitted with flat bars).

Tim,
SOoooo, will I be hearin' from you soon? I got a couple of forks with big boy steertubes. One inch of course. Both are from low end LBS bikes. You're welcome to them if they help. Call me with dimensions and I'll check them out for ya'.

I don't suppose there's a tubing sticker? What size is the seat post?

Looks like the groove on top is from the fork tube riding on the bearing cup.Which means the cups were so loose that it most likely dropped a bearing causing the big vertical groove.

Since the bearings were not riding well in the cups,most likely the fork tube took the brunt of an accident at some point and bent it.See how the paint is missing from the top/front---lower/rear of the tube? It's missing because the tube stretched and popped the paint off.

Since the fork was so loose in the head tube when it got hit,that saved the top tube and down tube from being bent.The fork tube absorbed most of the impact and didn't transfer most of it through the headset bearings.

Look at the paint on the down/top tubes,right behind the head tube and see if any paint is flaking off.A sure sign of stress and bent tubes.

That's my guess.