Brake Disc Minimum Thickness?

[ninjarrr]

Changing the rear pads on my R3 and thought I'd look at the discs too. There's a really good guide by diddyman that I will follow for the process. But first I thought I'd check the discs too....

They are currently 17.7mm thickness.

EuroCarParts replacements discs by Mintex and Pagid both have a minimum thickness of 16mm.

I have no idea what the current discs are but likely wont be OEM given the car has 90k+ on the odo.

So I'm wondering:
1) Should I replace the discs or are they good for another set of pads?

2) Is the thickness measured across both surfaces? i.e. here>>||=||<<to here

Also for any other noobs doing this, this video was excellent - https://www.youtube.com/watch?v=hOqwwkoG0Y8 (1st of 2)

[warpc0il]

The minimum thickness is at the point of greatest wear, which makes it impossible to measure with a ruler or calipers, since the edge will be at least unworn or even grown with rust.

There are three ways of measuring the thickness:
- a deep-framed micrometer that can reach to the worn section; this is the official method.
- if there's no rust on the edge of the disc you could use either a dial gauge or straight edge and feeler gauge to measure the depth of the wear.
- if there's a lot of rust on the outer edge then you can take this, and slightly more off with an angle grinder, creating a shallow chamfer to the worn section, enabling you to check with calipers.

I've done the latter a few times and in most cases it has proved that the disc were below the limit.

In one case the thickness of the rust was deceiving and the discs weren't that bad.

Any signs of cracks and the discs are toast, regardless of any measurements.

Sent from my SM-G800F using Tapatalk

[ninjarrr]

Thanks Dave. I used a digital gauage as there wasn't any rust on the disc.

I also managed to find min thickness 16mm etched on the existing discs. Given that brand new the discs are 18mm thick, I decided they should be well within the wear limit.

Visually discs looks fine. One has a slight lip in the middle of the disc but it's very minor so have decided it doesn't merit brand new discs.

Unofrtunately the piston surface is corroded a little and the rubber seal has a tear but the piston inside is in good shape, shines nice and clean with a wipe. I've put rubber grease on the edge of the piston where it slides in and out to create a seal and will get a caliper refurb next time I do the brake pads if there are any signs of issues.

[Dr. FrankenRex]

If your disc doesn't have that much rust on it, I'd suggest it's fine regardless of measurement

Kidding aside, it certainly sounds like those discs have plenty of life left in them

[ninjarrr]

Thanks Dan!

All was well on the first side. But on the other side, I keep winding the piston in, it rotates smoothly but Ive been doing this for over 20 mins and it's barely moved back a few mm. Any ideas what could be the issue?

[warpc0il]

You have to apply pressure while rotating.

Sent from my SM-G800F using Tapatalk

[ninjarrr]

Yes I have been, I think! I did the other side fine. Took maybe 10-15 mins of winding back. I'm at risk of rounding off the piston grooves as I've been trying this side for almost 30 mins.

Brake fluid is near max in the reservoir but car is on a slope so I can't read it properly, the reservoir cap is off. There is also very little fluid in the reservoir above the little net insert so I am hesitant to take more brake fluid out in case air goes in.

Handbrake is off. I have not disconnected the hand brake cable for the caliper. I have also only disconnected one of the caliper bolts so it is pivoting off the bottom bolt (unlike the other side where I took the entire caliper (not the bracket) off with both bolts undone.

[PeteH]

Loosen the bleed nipple, and be prepared for pushing the fluid out of it. The piston doesn't actually screw in, it's just that you can't push it in without screwing (if you see what I mean). So you do need a fair bit of push to get it to go in. Are you using a proper wind-in tool? Personally I wouldn't try to get an RX-8 rear piston in without one.

[ninjarrr]

Thanks Pete, I did and it seemed to recede a few mm with the bleed nipple loosened but still is taking a lot of turning. I can see liquid (brake fluid) comes out as I turn in the piston so it seems to be doing the trick.

I shall do more of that in the morning. I'm using the wind in tool recommended in the other thread (the Draper 52334 block).

Now that I've loosened the nipple will enough air enter so that I would need to do a brake bleed?

[PeteH]

I use this type. It gives the piston no option but to go in!
https://www.screwfix.com/p/laser-adjust ... lsrc=aw.ds

Yes, you will need a quick bleed, but it should only take a couple of pedal pushes to clear the small amount of air that might have got in.

[ninjarrr]

Thanks again Pete. Unfortunately I've admitted defeat and going to take it into the garage.

I put the old pads back in because I just couldn't get the piston to retract hte final 5mm

Sounds like the caliper is no good?

[Dr. FrankenRex]

I would be worried about the piston being corroded, personally...

[warpc0il]

Something definitely not right, which is not what you want with brakes.

[ninjarrr]

Caliper/piston both fine. The mechanic used his wind back kit (one of the fancier ones) and pushed it in straight away. I'm also very jealous of the electric gun they use on wheel nuts and trolley jacks.

My most likely guess is the Draper block had rounded off or I have got weaker since I did the other side...

A few more things I learned for any other noobs reading:
* Copper grease is not recommended for alumininum calipers like these. Brake grease is newer & better. This was in the original guide but I must have missed it.
* The Brembo brake pads from ECP did not fit on the upper end so filing the paint coating is fine to get it to fit better.

[qwakers]

the lazer tools one is great. highly recommended

[Dr. FrankenRex]

* Copper grease is not recommended for alumininum calipers like these. Brake grease is newer & better. This was in the original guide but I must have missed it.

Ceratec is the recommended one that I've heard methinks