So sometimes when I unscrew my yoyo apart to either remove a nasty string knot, or to swap out a bearing, I notice that the axle is not very tight in the end that retained it during the unscrewing. That leads me to wonder how centered the axle will be after I put the yoyo back together. Is it possible for the axle to be screwed in further on one half than the other? And if so, wouldn’t that leave the yoyo unbalanced with extra weight shifted to one side? And if so, what’s the best way to make sure the axle is centered when screwing the yoyo back together?
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I tend to use blue loctite on the non-hex side of the axle to keep it in. Then I lightly lube the other side, the hex key side, before reassembling. This way the axle always stays in and the yo-yo unscrews from the correct hex key side every time.
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But that doesn’t center the axle. 
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I just copied this from a post made by Bosk from yoyonation. I used this method years ago and it works to center an axle. Not really certain how necessary centering is anymore but here’s how to do it.
From Yoyonation Forum:
Author: Bosk
Items needed:-
A BvM/PEAK/Bape/Dif ect…
PTFE tape.
A hex/Allen key to fit the axle.
Scissors
Take your axle out and wrap the last 6-7mm with loads of PTFE tape, at least 6-7 times
Then trim off the excess tape hanging off the end. (if needed)
Quote from: yoking on April 09, 2009, 10:16:24 PM
Use enough so you can not turn it without the hex key
(i’ve used loads and i’ve never done anything to the threading)
Stick the axle in the same side the bearing tends to stay in PTFE end first, twist it until it stops.
(remember its got lots of PTFE tape on it so it might only go in 2 turns)
You will then have a “non bearing half” and a “bearing half” (with bearing and the axle in it)
Put it together slowly.
When the non bearing half hits the end of it’s thread, it will start to screw the axle into the bearing half.
Close the yoyo tight.
(sometimes the tape will move up the axle and not go into the yoyo threading, if this happens just try again, after a few times of you’ll start to coat the inside with PTFE and it will start getting stiffer, but remember it needs to be really stiff or the axle will move over time)
Then open it again, spin the non bearing half off so you don’t move the axle.
Now, get the hex key and stick it in the top of the axle, hold the yoyo up with the hex key pointing upwards (like you’re looking at a clock).
Turn the hex key clock wise, counting how many rotations you get, until the axle end hits the other end of the thread.
Say (for example) you get 2 and a half turns before the stop.
now, wind the hex key back anticlockwise half the number you counted, (in this case one and a quarter times)
Put it together again, you now should have a perfectly centered and secured axle, and hopefully a smoother yoyo 
If there still a small vibe you might need to turn it one way or another a little bit to find that “sweet spot”,
start with a quarter turn one way, see if its better than before.
If its not you’ll need to turn it the other way and try again, eventually you’ll find that spot where its smooth, and if you used enough tape/locktite it should be stiff and won’t move when you unscrew the yoyo
EDIT:
You can also use locktight.
let it dry, crack the axle open with the hexkey and then tune it the same way when its stiff and tight ^^
(this way works better tbh^^)
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I have never detected one iota of “vibe” across literally hundreds of yo-yos based on axle centering.
I also feel axle centering only made much sense on yo-yos that had axle holes … virtually none do today. With an axle hole / axle nut you could thread the axle all the way out each side of the yoyo, in theory!
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Not to be argumentative but have you noticed vibe at all on any of those yoyos? If so, how do you know it wasn’t from axle centering?
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Almost all metals are ridiculously glassy smooth these days. I find it hard to believe that the axle was somehow magically perfectly centered on 200 different metal yo-yos?
This sort of axle centering obsessiveness might have made more sense in the days of axle holes and axle nuts where the axle could be threaded all the way out either side of the yoyo, rather than having a proper machined depth in the axle well, as it is on virtually all of today’s yo-yos.
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So, in other words, if a modern yoyo is screwed together tightly, then due to the space provided for the axle inside the yoyo, it is going to be almost perfectly centered?
How does that work when you put in a shorter axle, like when turning an unresponsive yoyo into a responsive one?
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Well that doesn’t really answer my question. 
As stated in my earlier post though I’m not sure either how necessary axle centering is on newer yoyos anyway.
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I do have a topyo elimination with super bad pulse vibe as I indicated in earlier replies elsewhere. And some metals are 9/10 instead of 10/10 air hockey puck gliding across a perfectly smooth table glass smooth. But in general, yeah, I’d say out of the last 200 modern metals I’ve bought, 90% of them were 10/10 perfectly smooth. The rest are basically 9/10.
I did get a yoyoofficer apex that was surprisingly vibey, another thrower at a local yoyo club meetup confirmed that feeling. It was such an outlier, you get used to no vibe at all, when you do get some it is kind of a shock (for modern monometals anyway).
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