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Post by p5andrew on Feb 4, 2023 11:22:57 GMT
I have acquired a pair of second-hand brake discs which, as is very common, are a little worn/corroded on the outer parts of their diameters. I am hopeful that there is sufficient sound material to allow me to regrind them and I am fortunate to have the necessary skills and equipment to tackle this. Can anyone tell me the minimum permissible disc thickness? I have been through the workshop manuals but for the life of me cannot see the dimension anywhere.
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Post by enigmas on Feb 4, 2023 12:51:05 GMT
Both discs should ideally be .500" (1/2") thick. I'm sure that you're aware of the following if you are going to machine the disc surfaces but other enthusiasts may not be. P5s run old disc technology, ie, solid discs (non ventilated.) If you are to resurface the discs ensure that it is absolutely minimal. If one disc is slightly thinner than the other it will overheat and the car will pull to the opposite side.
Solid discs run much hotter than modern ventilated discs, hence the original disc compound (used by Rover for the P5) is both scintered, comprised of asbestos fibre, very resistant to heat, hard and crush resistant. Modern P5 disc pad equivalents do not produce the same standard of braking quality.
The thinner the disc the sooner it will overheat, fade, smoke and cause erratic braking.
When machining the discs and fitting them to the car ensure there is no more than .003" runout, otherwise pad knockback is induced. Check the runout with a dial indicator when the disc is fitted back on the car. Knockback can be felt as a pulsating pedal during braking.
Been there, done that with the above.
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Post by p5andrew on Feb 5, 2023 9:43:57 GMT
Thanks Vince. All noted, understood and agreed. What I am hoping is that someone can tell me the regrind/reface limit. Otherwise, I will need to fall back on my own common sense and experience! As you say, standard new discs were 1/2" thick with a maximum runout of 0.003".
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Post by lagain on Feb 5, 2023 14:52:30 GMT
Perhaps just change the pads. Many years ago I noticed the edges of my discs were ribbed, when I changed the pads they wore the edges down and made them shiny again.
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tonys
Rover Fanatic
Posts: 419
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Post by tonys on Feb 5, 2023 19:26:42 GMT
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Post by enigmas on Feb 5, 2023 22:22:33 GMT
Below is a direct repost of the specs provided by p5tgc.
Interesting to note, a new P5 front disc based on the info below is over 1/2" thick (.568") Removing .030" from each disc face (that's a massive clean-up to the friction surfaces) still leaves the disc at .508" thick.
I'd run a pair of "matched" discs at .508" thickness without any concern but would certainly go no lower, given the specs below.
* Keep in mind that these are heavy cars and that the front brakes provide something in the order of 70% of the car's braking effort. The front discs absorb a tremendous amount of heat during braking. The thinner the disc the less efficient the brakes become.
As an aside when I first recommissioned my P5 coupe I did run thinner discs (as new replacements weren't available at the time) but the braking quality was never up to standard, and I drove the car accordingly until an alternative option became available.
Nominal thickness of a new disc is quoted as 14.4mm (0.568inches); Minimum thickness of worn disc is 13.16mm (0.518inches); according to Service News Letter dated December 1970 (Vol 3, item 246).
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Post by p5andrew on Feb 8, 2023 18:51:18 GMT
I have just clocked the discs I have. The main part of the friction area measures 0.549" (13.95mm) at the thickest part. Measuring at the thinnest part at the periphery and estimating how much material would need to be removed to get them back to a uniform surface produces a minimum thickness of 0.495" (12.57mm). The two discs are within a 'thou or two of each other.
These figures are clearly below quoted tolerances. I shall therefore abandon my plans to machine them and save up the pennies for new replacements. Not worth taking risks with brake components. Blast...!
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Post by enigmas on Feb 8, 2023 22:04:12 GMT
Hi Andrew, I can't see that running the discs at .495" (thickness) would be detrimental. The .005" reduction in thickness from .500 is about the thickness of a sheet of computer printer paper.
Yes, if new disc sets are available and you have the spare funds buy a pair for future use, or swap them when the time comes, but I'd run those discs (at .495") without any concern. You're not about to go racing with the car are you? 😎👍
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Post by p5andrew on Feb 9, 2023 16:30:15 GMT
Definitely no racing to be seen here! I guess I will get on and machine the discs and see where they actually end up.
I just have in the back of my mind the experience of a friend of mine more than 40 years ago. He machined the rear discs of his P6 down to what my calibrated eyeball deemed far too thin. He reassembled everything, roared up the road, slammed on the brakes and sheared both discs. OK, his driving style did not help! The result was quite dramatic, though...
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Post by enigmas on Feb 9, 2023 21:26:25 GMT
Andrew why not have the discs machined at a brake shop if you're not confident with facing them safely. It's not expensive and the brake shop have the appropriate equipment, knowledge and expertise. Tell them the minimum thickness you want them cleaned up to (.500"), and that's what you'll get. Note that when machined correctly the disc faces will have a distinctive surface texture. It's very odd to have brake discs shear even if very thin and undersized. They'll fade, overheat, glow, smoke, but shear off(!)... as the rear brakes only do about 30% of the car's braking. The machining must have been suspect(?) inducing a stress riser or fatigue fracture at a critical point (generally where the disc meets the hat). That particular point should have a generous fillet radius. The calipers on a P5 are a "fixed head style". As such they will not centralize over a disc that is machined or surface ground on an offset. An equal amount must be removed from either side of the disc face so the disc does not become offset. If the disc is offset the caliper will apply more pressure to one face of the disc. A pad wearing more on one side is evidence of this. A "sliding head caliper" will accommodate an offset disc but that's a different style of caliper altogether. If you're not confident with your machining process then best not to go there. This is a PBR sliding head caliper.
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