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MG MGB Technical - How thick should brake discs be?

My wheels were well hot when I got back from a drive today.

Checked the pads, they are fine. Could smell them though from inside the car.

The discs are about 4mm thick and the surface looks ok.

Any ideas?

1973 BGT by the way.

Matt
Matt

Matt. In normal usage, the wheels should not get too hot to put you hand on and leave it there for a few seconds. (Much depends on how much you are using the brakes. This may not apply if you have just driven down a twisty mountain road, but does apply in general driving.) If the hub area of the wheel is too hot to put your hand on, the brakes are dragging. This is not a good thing as it can cause both brake and wheel bearing problems due to the overheating. (The worst case of brake lock I saw was with a trailer back in my military days. The truck in front of us had a trailer that had locked the left rear brake which had overheated sufficiently to cause the left rear tire to blow out.)

Sources of dragging brakes are a bad master cylinder, blockage in a hard line which does not allow the pressure to come off when the master cylinder retracts, bad flex lines which are failing internally and do not allow the pressure to be released or bad pistons which do not retract when the pressure is released. Minimum pad thickness is 1/16", or about 1.5mm. I replace mine somewhat sooner, with about 1/8" or 4mm of pad thickness remaining.

Jack up the front end of the car, support it on jack stands, step on the brake pedal, then, get out and try to turn the front wheels. There should be no more than a light drag. If they are hard to turn, there is a problem which needs to be corrected. If the brakes are sticking and have become very hot, make sure you clean, inspect and either replace or re-grease the front wheel bearings depending on what you find. Les
Les Bengtson

Matt,

Discs should be no thinner than 7.6mm - looks like you need new ones!
Chris Betson

Chris. Where did you get this figure? My workshop manual (Bentley reprint) states,
"Before the lining material has worn down to a minimum permissible thickness of 1/16 in. (1.59mm) the friction pads must be renewed.". My personal minimum is 1/8" remaining because I tend to drive fairly hard. Has the composition of the pads changed or has the factory (pad maker's) recommendations changed? I ask this because, earlier this year, David DuBois sent an off line e-mail to me with some information that contradicted the fuel pump information that was available to me. It seems that things had changed since the reference books I had at hand had been written. The final result is that Dave wrote what is, today, the definitive article on fuel pump pressures and flow rates. So, if the factory manuals are no longer the definitive reference for the thickness of modern brake pads, I, and most of the rest of us, would very much like to know this. Thanks, Les
Les Bengtson

I think you're talking cross purposes guys. Chris is talking about the brake discs (rotors), not the pads.

Tim.
Tim Jenner

Tim. You are exactly correct. When Matt mentioned 4mm I took it for the pad lining thickness that he was speaking of. Chris caught that mistake and realized that the thickness of the disc, or rotors as we refer to them, should be of "the original thickness or .350" to .340" (8.89 to 8.63mm)" and that they may be ground no more than .040" or 1.016mm. "Good save" as they say in baseball. Thanks. Now it makes sense. Les
Les Bengtson

I'd imagine that if the discs are down to 4mm there would be a very large ridge at the edge where the pads don't make contact? That's normally how I guage wear because it's easier than measuring the thickness of the disc.

Tim.
Tim Jenner

Tim,

You can't always go by the ridge because if the DPO has let the pads wear right out then the ridge is worn away by the steel backing plate.

It is usually bad maintenance like that that causes the excessive wear to the disc in the first place!
Chris Betson

This thread was discussed between 24/06/2003 and 25/06/2003

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