www.jec.org.uk

Like many of us older members we think in feet and inches, so metric measurements don't come naturally. So what, you may ask, Well, I've been following one or two threads in regard to brake disc fitment and the maximum limits of disc run out.

Did you know:- One inch = 24500 microns

1/1000th of an inch = 24.5 microns

Recommended Maximum Disc Run out is 40 microns

2/1000th of an inch = 49 microns

With a dial test indicator calibrated in inches we are looking for a MAXIMUM run out of just under 0.002thou. Zero run out is the ideal to aim for and can be achieved. Regrettably, failure to achieve this will inevitably lead to brake judder in about a thousand miles.This condition can arise with virtually no brake applications because the pads are in light contact with the disc and on each revolution of the disc the high points of opposing sides of the disc and 180degrees apart wipe the disc causing unequal wear.

Just a thought to sleep on

Oldtimer

Sent me straight to sleep!

Did you have a nightmare

Hi Oldtimer,

I always worked on 4 thou run out as acceptable, once filled a new set of discs to a TR6 and got 10 thou run out, a lot of time later and swopping the discs to other bolt holes and sides I got it down to 4 thou.

A great car, it wouldn't go, wouldn't stop and didn't like corners, oil in the dashpots, new discs and pads and the callipers freed up and the steering column re-secured properly and poly bushes made it a nice car again. It had previously just passed an MOT but the owner wasn't happy!

Brian