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MG MGB Technical - Replacing brake discs - followup

Well, thanks in no small part to the advice from this messageboard my GT passed its MOT this morning! Hurrah! Just in time for her 40th birthday next month.

It failed last time on one excessively scored brake disc, so I changed both discs, switched to new V8 Greenstuff pads and ended up having to replace one outer wheel bearing as the cage had broken up. As with all these things it turned into a rather longer process than anticipated because I had to keep stopping to order parts - first hub nut split pins, then the bearing kit, then a new retaining washer (locating tab was corroded and broke off), then a shim kit... so the advice is make sure you have all the sundries you need before starting!

One slight issue thought - obviously the new discs and pads are much thicker than the old worn ones and the right hand wheel in particular is binding slightly. Just enough to make it very hard to turn the wheel by hand if the car's in the air. It doesn't pull to one side and drives/stops fine so I assume it will free off after bedding in - the caliper pistons are as far back as they'll go. Other than the usual bed-in advice of avoiding harsh braking for a few hundred miles I plan to stick to short slow journeys and to keep an eye on them in case they overheat whilst they're rubbing - is there anything else I can do to help them bed in? Is this tightness normal?

Thanks again for the previous help and any more you can give!!

Cheers,
Tim
Tim Jenner

I have never had excssive binding with new pads It will stop as they bed, however watch the temperature, dont set fire to your new bearing grease. Is the caliper located symetrically to the disc? If it is I am not aware of anything you can do about this if the pistons are right back.
Stan Best

Tim,

Did you shim the new bearing properly? Did the wheel turn easily by hand before you installed the brake pads? The best procedure is to use oil rather than grease on the bearings and experiment with the right number and thickness of shims such that with the hub nut firmly tightened, the wheel turns freely but there is no end-play, or maybe just the slightest amount.

Good luck!
Allen
Allen Bachelder

I forgot to add that once you get it right, of course you disassemble, and repack the bearing with high temperature grease before final assembly.

Allen
Allen Bachelder

Tim - are both rubbing with the pistons fully back? Or only one? If the latter which is rubbing, inner or outer? If the outer only then I'd suspect incorrect assembly holding the hub a bit too far out. As Allen implies insuffient shims can cause wheel binding, but this is with the pads clear of the disc or not even fitted yet. The correct shims are obtained by juggling so you progressively add one thou at a time. When you end up with one set that gives no free play and another set that gives just perceptible play, use the lower set plus an additional thinnest shim to get the correct 2 to 4 thou end-float.
Paul Hunt 2

' Just thought of something else. When you spin the rotors by hand (before installing caliper) do theys run absolutely true? If not, you may have failed to adequately clean the hub surface to which the rotors bolt. Even minute particles of crud on this surface can throw your rotors off enough to cause excessive wear and drag, let alone VERY unpleasant braking after a short while. If the quite-new set of disgarded rotors I have here could talk, you'd know how I found this out!

FWIW,
Allen
Allen Bachelder

Thanks for the replies, all. I did reshim the hub according to Paul's excellent web site notes and the hub was free to turn before I installed the brake pads. In the abscence of a dial gauge I was as sure as I could be that the disc was mounted true and there's no judder in use. The pads were a tight fit when I installed them, both as tight between the pistons and the disc as each other. The outer might have been a bit tighter but really not much in it.

One additional thought I did have - I used repro discs but afterwards I read an article that said to always use genuine discs. Why is this? Could it be that the repro discs are slightly too thick? I appreciate it's far more likely that I've messed something up but I'm clutching at straws!

Thanks,
Tim
Tim Jenner

Tim, A new disc should be .340" to .350" thick. Information is from the Haynes mgb manual. If the repro discs are within these limits they should be OK. Maybe the replacement pads are thicker than standard pads.

Clifton
Clifton Gordon

There is specail spacer the goes between the inner bearing and the stub axle. It has a chamfer on one side to clear the stress relieving ridius where the stub axle meets the hub. Is that on the right way round?
Stan Best

Stan beat me to it, if the tightness had been on the outer pad only. Because it seems to be on both, as long as both pistons *are* fully retracted, then it sounds like oversize discs/pads. Are the brake hoses OK? These can delaminate inside and act as a check-valve preventing fluid going back into the master. With the bleed nipple slackened can you retract the pistons any further?
Paul Hunt 2

I found the same thing when I put the Greenstuff pads in my GT last year. Could obviously not check against old components and with pressure of time due to imminent MOT test decided to reduce the thickness. I used sandpaper held down on a piece of plate glass and rubbed the pads down by 35 thou. Could probably have achieved the clearance by taking the green paint off the back of the pads, but didn't know this at the time. Brakes didn't bind, wheels turned, car stopped in straight line MOT passed.

Vic
V Todman

Tim,

One thing I have seen with hub mounted replacement rotors is that some are .010 thou oversize. The reason for this is to allow machining to standard when they are mounted on the hub. This ensures the rotor runs true to the tapered bearing cups in the hub.

Usually there is an instruction sheet or sticker packed with the new rotor when this is the case.

Dennis
D. J. Broad

I had the same tightness with my new rotors (Moss stock rotors) and EBC Greenstuff pads earlier this year. Pistons were all the way back into the calipers, and it was a very tight squeeze to get the pads in. The wheels then wouldn't turn by hand, but all was well once it was on the ground and driving (no binding or pulling, and brakes work just fine). They certainly make the greenstuffs to the limit of what'll fit with new rotors in standard MGB calipers!!

Curtis Walker

Tim,

I seem to remember Greenstuff pads being a bit of a tight fit. If you can turn the wheel by hand then the pad will soon wear down.

Neil
Neil Lock

Thanks for the comments everyone. I'll take it easy and keep an eye on them - if they haven't freed up in a hundred or so miles I'll disassemble and make sure I didn't muck anything up!

Thanks,
Tim
Tim Jenner

This thread was discussed between 14/10/2006 and 19/10/2006

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