Miserable brake swap experience (rotor run out)

[jbourne]

So I just spent the day changing my pads and rotors. I expected this to be a fun little exercise, and it's not the first time I'm doing brakes, so I started slow and careful, but then all the wheel hub rust started eating away at my time, and next thing you know I had to change targets to finish just the fronts, no hope for the rears.

One thing is very weird, though. I got a set of Pagid discs, and I feel like one doesn't fit right. The rust obviously made it much harder to fit than it should, but I wire-brushed the hub and sandpapered it to a point where I thought it was about as smooth as I could get it (and hell, there's supposed to be SOME tolerance for this, right? This isn't a space shuttle!), but while one of the Pagid rotors fit more or less what I would consider "well", the second one, is just ever slightly off. So there's obviously vibration at speed.

What's most strange is that the part #s for the two rotors are different. The one that fits "well" has A19 6.43.8 stamped on it. Still some issues with rust and fitment but overall I'd say it's about as good as I can get it. The one I am having trouble with is stamped A19 4.48.8. Older version? Old stock? This was ordered from Carparts4less.

The pics show how much rotor run out there was, I got it lower by filing and sandpapering, but it's still there. In the rotor pic, you can see the gap at 6 o'clock.

Am I crazy and I just need to work harder at cleaning the hub (also picture attached) or is this rotor badly machined? What should I do, get CP4L to reship a new one or go machine it somewhere? Or just get a good hardcore wire brush drill bit and try to make the rotor fit?

[welwynnick]

At the risk of generalising, discs are usually good, and run-out vibration is usually a fitting issue, and large single piece discs are more vulnerable.

The hubs have to be spotlessly clean, with caveats. They don't have to look like you could lick them, but you must be able to see the circumferential turning / machining marks all over. It doesn't matter if there's microscopic rust in between the machined ridges, but the tops of the ridges mustn't have any visible contaminant on them.

What I do is grind a good screwdriver clean and sharp so that it has a really good straight sharp edge, and use that to scrape everything off the hub.

I can't really tell from your picture whether your hub is clean enough, but I would guess not. You see that area around 2 o'clock where you can see some shiny circumferential lines? It should look like that all over. Good luck. I'm going to do some rears today.

Nick

[jbourne]

I see. I’ve never had this before - all the rotors I’ve done before (even on Porsches!) were somewhat forgiving of some rust. I guess I have to take everything apart and really go at it again before crying wolf, eh.

Should I go at it with a drill and a wire brush? I’m guessing it’ll happily withstand that?

[jbourne]

Although one thing that I find odd is that the other rotor sort of fit OK, whereas the problematic one won't (on both sides). So I'll allow for rust on the more .. "collaborative" rotor, but I'm wondering if this other one is just weird. Hrm.

[welwynnick]

Did your Porsche have floating discs? They're quite forgiving, more than single piece.

I never found wire brushing got more than 99% of the muck off. It's just not hard enough, you need to get 100% off.

I haven't read what professional mechanics use - they have to get it right first time, but I always use a sharp edge these days.

Nick

[welwynnick]

It's possible that one disc is warped and the other one isn't, but this is Pagid you're talking about, and my money says not.

Nick

[welwynnick]

By the way - those part numbers - are the discs handed?

[jbourne]

By the way - those part numbers - are the discs handed?

I don’t believe them to be. The OEM ones aren’t. I don’t feel like it’s warped, though. It’s as if the opening is either not large enough to go around the hub, or something’s behind it ... I dunno. I feel like a n00b, lol.

The Porsches also had one-piece discs (they were cheaper Porsches ).

[Jim Haseltine]

No, they aren't handed. Have you tried swapping the discs across the car? If the problem moves with the disc then it's the disc at fault.

Basic stuff - don't rely on the disc retaining screw to hold the disc tight to the hub when checking for run out, put some wheel bolts in. You may need to pack them a bit.

[jbourne]

No, they aren't handed. Have you tried swapping the discs across the car? If the problem moves with the disc then it's the disc at fault.

Basic stuff - don't rely on the disc retaining screw to hold the disc tight to the hub when checking for run out, put some wheel bolts in. You may need to pack them a bit.

Yeah, I did try the disc on the other wheel, and I felt that it's also a little bit off compared to the other one (I actually put the 4xx part one onto the left hub first, had a lot of trouble, gave up, put the 6xx, it fit well, I figured the hub was rusty or something so it might work better on the right, and the right was the same thing - which is why I am suspecting the disc from the beginning). CP4L is stalling now, though, and telling me some bs stories about no longer shipping international because of coronavirus and having to escalate to management, etc, so I have no traction there for now. So I'm going to take everything apart once again, and really go hardcore at it with a drill, a wire brush head, and sandpaper. Nothing to lose at this point. Also throw some grease on it so it doesn't stick in the future, and paint the center bell to avoid it looking like a rust bucket if I am stuck with it.

I was considering trying it with the bolts, but I wasn't sure how deep they can go before they hit the shield, so I didn't try it - but you're right, I should give that a go as well. I re-rented the lift on Thursday, so going to give it a try again.

[welwynnick]

I changed a couple of discs this morning and thought I'd try to capture the cleaned hub flange, though it didn't work out very well. The hub still looks rusty where the holes in the disc were, but what you can't see is that the ridge tops on the turning marks are clean and continuously visible, and the whole surface feels smooth to the touch. So it doesn't have to be completely clean, but that's an example of the level of prep that I aim for.

Nick

[jbourne]

I changed a couple of discs this morning and thought I'd try to capture the cleaned hub flange, though it didn't work out very well. The hub still looks rusty where the holes in the disc were, but what you can't see is that the ridge tops on the turning marks are clean and continuously visible, and the whole surface feels smooth to the touch. So it doesn't have to be completely clean, but that's an example of the level of prep that I aim for.


WP_20200401_12_43_54_Pro.jpg


Nick

Woah. That’s dedication. I got a drill wire brush and a lot of 180 grain sandpaper, tomorrow I’m taking it all apart again and sanding the bastards down. I hope this will fix it!

[jbourne]

Ok. So I brushed it (you were right, it’s only good to make you feel like you’re getting somewhere), then sandpapered it. I think it’s as clean as I’m going to get it... and yet it’s basically unchanged.
https://youtu.be/ky9RlA4_Ak8

Also, this is the hub I’m mounting on. I can’t get it cleaner than this, lol.

[jbourne]

Update for my own sanity. I rotated wheels, and did a brake test, and the wobble is basically gone - and the steering wheel does NOT wobble when braking, so I'm going to say that I think the rotor and hub is OK at this point.

Nick, you were right on the wire drill bit - it's borderline useless, only good to make you feel like you're achieving something. I did your way with the screwdriver and sandpaper, and had much better results, much faster.

The rears are done as well now, and FOR GOD'S SAKES, DON'T READ ELSAWIN AND DO NOT TRY TO REMOVE THE REAR CALIPER BRACKET!!!!!! I actually started, got to the bottom bolt, had a brief panic attack because it's so impossible to get to, started considering if I should remove struts and all...... then had a flash of brilliance to try and, you know, JUST REMOVE THE ROTOR, and it worked fine, no need to remove the caliper bracket.

Things you learn. Thanks for the help, all. At least the brakes on this money pit are OK now.

[welwynnick]

Good job on every front!

You were in luck on the caliper bracket. Some cars you can do that, some you can't. There had to be SOME job on the RS6 that's straightforward.

I assumed I could do it on my Passat this week, but no, the disc wouldn't quite come out. Although it's not as difficult as the A6, which has a broad based lower wishbone, there still wasn't enough room to swing the breaker bar a sixth of a turn, so it was a bit of a pig. Worse than that, it used internal spline head bolts, and my spline bits only went up to M12, and these were M14. And I thought I had every tool under the sun....

Cars!

Nick