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MG MGB Technical - MGB disc brake runout

Hello, does anyone know what the maximum runout can be on MGB front discs, I seem to have 0.08mm, which I believe is just over 3 thou. I've tried turning the discs round on the hubs, but doesn't make much difference, also made sure everything was clean on the mating surfaces.
Graham
G Cherry

Here's a snip from the workshop manual.

It sounds like you're on the limit.


Dave O'Neill 2

Best to work on .03mm as your max but can be stretched to .05mm as an absolute
It depends on rotor thickness really
If you had for example nice even rotor thickness and .05 runout you might still get away with it but if the rotor thickness varied say .02mm and you only had .02mm runout you would feel it for sure
Generally speaking brake shudder is caused by uneven rotor thickness which in turn was worn that way by running out of true
OR seized / partly seized calipers

willy
William Revit

I have just fitted new discs and the runout is 0.08mm, but I'm getting vibration only when braking from high speed, not from low speed. I've tried everything I can think of, turning the discs on the hubs, swopping from right to left. I'm now about to renew the callipers to see if this helps.

Graham
G Cherry

If your discs have a runout of 0.08 and the WSM says 0.03 is the maximum, then I don't see how new calipers will help that. I'd say you should get the runout within limits first, and if you still have the vibration then perhaps look at the calipers. What was the runout with the old discs? I'm not surprised runout causes vibration at higher speeds and not lower.
paulh4

I think you should take the new discs off and check the runout of the hubs, and if they are not close to perfect then you need to get them machined.Then see what the runout is like on the discs.
john wright

The runout is 0.08mm 0.03" which is just at the maximum. I didn't check the old discs and now wish I'd kept them. It's also a little more intriguing since the first discs I fitted were Brembo ones with EBC V8 pads. This was the first time I had vibration, so I though it may be the bigger pads causing the problem. So new EBC std pads were fitted, still had the Vibes, then I first checked the runout and saw it was 0.09mm, thought it must be the discs, so fitted new std OEM discs, still had the vibration, which is where I am now with 0.08mm runout.
Incidentally I did plan to renew the braided flexis this winter as they are now 20 years old, as are the callipers.


Graham
G Cherry

Have you got wire wheels or steel wheels, if you have steel wheels getting them skimmed (which will also true up any hub problems) will be no problem, most main dealers have brake laths, also some smaller garages have them,i don't know what they will charge, but to skim 2 discs can be done in an hour,another point to consider is the hub endfloat when checking discs runout, if you have to much will this affect your run out reading, A.T
Andy Tilney

First off make completely sure that the faces of the hub and disc are flat with no lumps or bumps due to corrosion or muck. The disc must bolt on to the hub clean metal to clean metal.
Mike Howlett

I have steel wheels, hub end float is 0.02" and all mating surfaces are clean and flat, I've tried everything I can think of, even tried a spare hub and still had the same runout.

Graham
G Cherry

Graham, do you mean 0.002"?
Dave O'Neill 2

Graham, I have been thinking about your problem. I not only have a mgb roadster but aso have a Transit camper. The transit suffered fairly bad brake judder from 50 mph, I couldn't see any problems with the discs, to cut a long story short the problem was worn front suspension ball joints. So it maybe worth having a good look around the front end of your car.
John
john wright

John-Graham
You can't compare with a Transit--nothing compares to a Transit
It might have ended up being the back doors causing it on one of them, and then what would Graham do--- lol -also worn struts on Transits will do the trick as well
ps. I work on Transits a fair bit of the time and you are quite correct saying worn ball joints will cause brake shudder
Seriously though, the idea of having an on car disc machine has merit - seems a shame to have to skim new rotors but it would be a very small amount that has to come off them - make sure they are done on-car though otherwise you will still be in the same position you are in now-
Actually, now to think of it someone on here went through this exact same problem a few months ago and sorted it
Maybe a trip into the arcives might save some angst
willy
William Revit

"The runout is 0.08mm 0.03" ..." - of course.

In which case as it's not beyond the max I wouldn't expect a problem. If two hubs are the same then it's looking more like the discs.

Did you bin the old discs? Shame on you if you did.
paulh4

Having worked in a service station, for over 7 years, where at least 2 brake jobs were performed everyday, having to cut new rotors was a commonplace occurrance. They were quite often more than .010" out of round. We had a disc brake lathe as well as one for drum brakes. New components rarely were true right out of the box. I replaced the discs on my '67 B with Brembos. They, too, were out of round. RAY
rjm RAY

As I said in my last post Main dealers usually have a brake lath (on the car disc machining) as this is not an uncommon fault
Andy Tilney

Hi All,

Sorry for the lack of noughts, the hub end float should read 0.002" and the runout 0.08mm or 0.003"
On the suspension question, the bushes are fairly new poly bushes from Polyflex, probably a year old now.
I've now sent the callipers for rebuilding and if this doesn't fix it, I'll have to find an on the car brake lathe as a final solution.

Thanks Guys for all the help and comments, I'll try and keep you posted with any news

Graham
G Cherry

Graham, with only 0.003" runout I wouldn't think that would cause your judder. The only other thing is DTV (disc thickness variation) it only needs a very small amount to cause judder from high speeds. Getting the discs machined on the car should cure the problem.
I am surprised you are sending the calipers away for rebuilding, Lockheed calipers are easy to do yourself, the only difficult bit is getting the metal seal retainer back in place.
regards John
john wright

Hi again,

Update
Now fitted the rebuilt callipers and new Goodridge flexis, so far all seems OK, nice and smooth from low speed braking. I haven't tried braking from high speed yet, that will come next week when I go to Alfreton.

Graham
G Cherry

Grahame,If judder persists, don't discount a problem with the rear brakes. When i built my V8 roadster, I had a violent judder under braking when the brakes had warmed up. It taxed me, and others, for weeks involving replacements, higher temp brake fluid, heat shielding flexibles, bracing the rack mountings and investigating possible bump steer. The problem was solved when I swapped the drums. On examination there were polished areas, which I assumed were corrosion. Just the "grip, less grip" effect causing the problem which everyone thought was a front issue.
Allan Reeling

Hi again,

Completed some higher speed braking now and all seems OK, the vibration has gone, but I'm still not really sure what caused it. I've now fitted rebuilt callipers, Goodridge flexis, Mintex 1144 pads and new bearings. Hopefully now trouble free for a while.

Thanks everybody for all the replies.
Graham
G Cherry

This thread was discussed between 06/10/2016 and 03/11/2016

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.