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I've used WD-40 on my bike's headset as it produced some cracking noises. The result is a headset that gets loose after every ride.
I've tried to use a wrench to tighten it, alas, it went loose after a couple of days.
Any recommendation of how I can fix my mistake?
UPDATE - image of the bike. It has a threadded headset and quill stem.
Using WD40 on a headset should not cause it to loosen. Something else is wrong. You need to find out what that is.
First - don't ride the bicycle until the problem is identified and fixed. Doing so might be dangerous - both to you and anyone near, as you may lose control.
If your bicycle has a new-style "threadless" headset, the stem bolts may be undertorqued. Or, they may have been overtorqued in the past and crushed the steer tube that the stem clamps on to so that it no longer stays tight. Those are by far the two most likely problems with a headset that won't stay tight.
Park Tool has a series of good videos on bicycle maintenance. This one is for troubleshooting a headset:
What you do to fix the problem depends on what the problem is.
Update:
Given the posted photo, this video appears more relevant:
Once I had a bike that suddenly was having the headset coming loose everyday. I had this headset/stem (the one on the left):
Then I disassembled the fork and I discovered that the wedge was broken. Practically, I was only hours away from a crash. Important point: the wedge was all oxidated (inside) and rusty (outside, where in contact with the headtube).
Since you see a correlation between WD40 use and the headset loosening itself, I guess that using WD40 removed some of the oxidation/rust/whatever effect was keeping the nut in place by "static friction".
The WD40 removed that layer and now everything can move.
Go to a bikeshop and have them check the fork, unless you can do it by yourself, asap.
EDIT: sorry for the error, with "head tube" I obviously mean the "steerer tube" of the fork. And thanks to wikipedia user https://en.wikipedia.org/wiki/User:AndrewDressel for the stems picture.
did you remove the bearings? if so are you sure you installed them correctly? Installing the bearing ball cages upside down can result in the bearing becoming loose prematurely. Make sure the 2 nuts with which you tighten the headset (I am assuming it's threaded) are properly seated against each other (to lock them in place on the steerer tube). The lock nut (top nut on the steerer tube) usually has a ridge on the top (a flange with a piece of rubber keeping water out). If you acidentally removed a spacer ring in between the bottom nut (the one that has te bearing race) and the lock nut it could cause said flange to touchh the top of the steerer tube before it touches the bottom nut, which will prevent you from locking the two nuts together which would result in the headset loosening very easily.
You could indeed also try locktite but if the two nuts are properly locked/seated together it shouldn't be needed. The WD40 is indeed not a great lubricant, it also should not cause the headset to loosen on it's own (without an underlying problem being present).
If the lock nut is indeed touching the top of the steerer tube you could either shorten the steerer tube a bit or add a ring/spacer in between the two nuts such that the flange is not/barely touching the top of the steerer tube.
I'm guessing that what you did was sprayed WD-40 on the area where the stem clamps to the steerer tube, because you were getting some creaks from that area.
WD-40 is for water dispersal and consists of a light oil in a solvent. The solvent disperses the oil over whatever it's sprayed on and leaves a film that repels water. It also makes a sticky mess that dirt adheres to which is why many people don't recommend getting it on a bike.
The solvent also allows oil to wick into tight spaces so WD-40 can be used as a penetrating oil (although other products are better in this regard). The solvent has wicked oil between the stem and steerer and now the stem cannot grip the steerer properly.
Remove the top cap, take the stem off the steerer, clean everything, reassemble setting bearing preload and stem bolt torque properly. Refer to the Park Tool video in Andrew Henle's answer. This will probably fix the creaking you were getting in the first place as well.
Update:
Question now shows a bike with threaded steerer, so my answer is not completely applicable. It's possible the WD-40 is lubing the quill stem wedge but I'm more inclined to think that the wedge is broken, and onset of coming loose is coincidental with WD-40 application.
As a general advice, do not use WD-40 on parts that are close to any ball bearings or other places that rely on grease to work poperly. WD-40 is excellent to use if you want to degrease some part as long as you wash away the WD-40 leftover with a real degreaser. It seems to be fairly easy to make the misstake to think WD—40 does lubricate and therefore it could be used anywere that needs some lubrication. After applying WD-40 on a ball bearing for example, you probably made it less lubricated since the grease are now starting to drip out of the bearings and it is imossible to have any initial possitive effect to last. Typically you do the WD-40 again to have it even more lubricated and you do not realise it but you are killing the bearings.
To lubricate ballbearings or other parts that have grease applied, ideally you first wipe away grease and dirt and clean it with a degreaser and wipe it dry. Then when really clean and dry you reapply new grease to appropriate amount. For a cartridge ball bearen appropriate amount migth be to fill the volume to 1/3 with grease. As soon you have the bearings contaminated with some particles or dirt or something your bearings are being hurt and you clean and degrease and apply new grease to fix that problem.
WD-40 are not good if mixed with grease and are initself not long lasting. It can be used as a chain lube but need to be checked and reapplied often. In wet conditions it is not a good lubricant.
WD-40 is a very good cleaning agent and can often be applied to get rid of dirt, grease, rust and loosens stuff that are stuck, such as theaded parts that hasn’t been loosend for a long time. Remember to wipe of the WD-40 in the end.
