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I've just installed a pair of new front brake discs purchased from one of the main large TR parts suppliers.

When rotated there was a visually obvious wobble on both discs. Checked with a dial gauge this run out is 30 thou on one side and 37 thou on the other. 

What run out would be regarded as acceptable for both safety and to avoid being able to feel it at the pedal ?

Dave McD

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Dave I wouldn’t expect any on new discs. Surely modern manufacturing would be more precise. especially on a safety item. 
Are they seated ok?

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Dave , tighten up the wheel bearings to remove any slop and then check the run out, at the moment you are measuring running clearances. I believe 5 thou is acceptable run out when the bearing slop is removed.

Mick Richards

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Just a couple of thou is what you would expect. 30/37 thou (0.7-1.0 mm) is certainly not acceptable.

Waldi

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I'd be more worried about the difference between the two sides than the total runout. Does the thickness of the disc vary around the circumference?

Pete

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If after tightening up the bearings your still getting appreciable runout I would be slinging them back at the suppliers.

Stuart.

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As suggested I checked wheel bearings. I found that they were very loose, obviously contributing to the measured disc run out.

When I installed them I was sceptical about the inner felt seals. Brand new from one of the main suppliers, they seemed to be very thick and hard and I wasn't completely happy when I set the bearings. I seem to remember a thread about these in the not too distant past. Anyway, now they have absorbed some grease and I've pushed the car about the workshop many times (this is a restoration which hasn't yet been on the road) they seem to have softened and compressed, hence the loose bearings.

I tightened those up to the point where I could only just rotate the hubs without the road wheels on and checked the disc run out again. Much better but still around 10 thou so still a bit concerned. When I refitted the road wheels they rotated relatively easily so I've left the bearings as they are and will recheck everything again in a few weeks to see if the seals soften further.

I'm busy on another project at the moment but when I get back to it I'll remove the discs and lay them back to back as suggested by Neil to see if that shows anything up.

Dave McD

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Did you ever measure the run out on the old discs?

My TR used to consume about 3 pairs of discs for every set of pads. The run out was dreadful and every few thousand miles the judder would be back.

The fundamental problem was the outer races of the bearings were not a good fit in the hubs. When I moved to expensive vented discs I had to fix the problem so sourced a good pair of second hand hubs. This fixed it.

These days I have moved on to the CDD hubs with the uprated bearings, and I am still using my original pair of vented discs. They are getting a bit thin now though.

Neil

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