Howdy,
I've learned something new today. A year ago I've replaced the idler pulley. I bought a replacement (cannot recall who made it), that was available at my supplier shop. After replacing, I found the serpentine not laid in the middle, it was just a bit off. The distance in between front of the bearing and front of the pulley was different. So, year after, it started whining again. I've took it out, and the bearing plays. So, here is a photo of the crap to avoid:

...luckily I didn't throw the original one, so I decided to rebuild it. The bearing that match the idler is 6203 2RS or 203P (outer diameter 40mm, inner 17, width 12mm). A new bearing cost $3. I got it pressed in, all perfectly match - the serpentine is in position.

 

Nice! Good to know!

 

Good to know, ty kind sir for being a Guinea pig

 

Thanks for the info... Way to take 1 for the team as they say

 

Time for an update... I owe You. Pressing the bearing in didn't went very centric, and now the idler is wobbling. Some used to say: the devil is in the details.
I am not giving up yet, I'll keep You updated.

 

Really confused here; the idler pulley is basically a skateboard wheel and it's not like it's hard to engineer. I've been using a cheapie aftermarket one for about 4+ years now without issue.

The wobble in your current self-rebuilt one is evident, but if the belt was riding in the middle of the previous one it likely means one of your other pulleys is out of alignment/spec. You might be looking in the wrong place.

 

Yes, cheap aftermarket was the only available option at my supplier, and it was a crap. The belt wasn't centered, because the idler was not manufactured within specs. The bearing was gone within a few thousand miles, and it didn't last a year. This is the one from a photo.

The one You see at the movie is a genuine one, that ideally match the serpentine belt, but the bearing didn't fit centric.

 

Did the same a couple years ago when the bearing went bad on my 1st gen.

Make sure you measure how deep the bearing is seated. I believe that on the back it's flush with the edge of the inner diameter.
This determines where it will run.

I had it pressed in on a press but the force is minimal. With a press you can properly align the bearing.
If you have access to a lathe you can turn a thorn which has a shoulder wide enough to properly sit the pulley and act as a stop for the bearing.
If it has to be set other than flush you can add a step for the bearing, that would nicely center everything.

Just for the fun of it and because we had a whole crate of stuff to sent for QPQ https://en.wikipedia.org/wiki/Quench_polish_quench treatment I had dropped my pulley in there. (Mind you without the bearing - of course).
Running ever since

 

The bearing was pressed in into a press. Although a friend of mine used a socket key between a press piston and the bearing (as an adapter), that probably wasn't flat. He should have use the old bearing instead. But whatever... Today I've taken out the pulley with a mounting pin, mounted into a lathe, found the deflection using dial indicator and Ive improved the seating to 0.3mm (+/-0.15mm) wobbling (measured at about the maximum pulley diameter). But once I've started the lathe, I discovered it is no longer circular, but resembles an oval and rotates totally eccentric. Of course, it was not the problem, I've machined the surface a bit and it is OK.

Yet, after assembling it back it runs perfectly, but I found the tensioner roller whining too Give me a break! They were both replaced a year ago and they're gone now. Eventually I gave up and placed an order at RockAuto for both of them, Delco (God bless the person posting me their link!).

If I understand Your idea with lathe well, I'd rather avoid pressing the bearing in with a thorn, that would actually press the inner race bearing, not the outer one.

I've read a bit on QPQ processing. Nearest owner of a QPQ processing plant is a firearm factory.

 

We have a lot of old and used aluminium and brass, even steel rods in a cupboard next to the press.
If nothing fits the application you just grab one of the most suitable pieces put it into the lathe and turn it to your specs.
You are correct you have to make sure it sits on the outer cage of the bearing. Just give it a little clearance in the bore that the bearings sits in (0.20 - 0.50 mm).
That's the way I have changed my bearings on the idler pulleys.

That it became ovate is most probably somebody putting it into a vise.

Good that you have fixed the problem. It's quite annoying having them screech or wobble.