This is an interesting failure. Please click for larger picture. Photo is compliments of John Foster. Notice this carbon puck is not just worn down smooth, but it is fractured with a large chunk of carbon broken out. The part was purchased from Moss Motors July 2017, installed Sept 2018, failed Aug 2021."Powertune" is printed on the defective bearing.

All carbon bearings will wear out eventually, only a matter of when. A carbon release bearing normally lasts as long as a friction disc, If you treat it well, likely over 100,000 miles. If you rest your foot on the clutch pedal regularly, all bets are off, as that can wear out a carbon bearing in very short order.

If you drop the part on a cement floor, it may knock a chip out of the corner, but should not fracture out large pieces. The material is fine grained, soft, very homogeneous. Not real high in fracture strength, but relatively tough, meaning it can absorb quite a lot of shock without fracturing. It deforms a bit, then returns like a spring. If forces exceed the yield point, you end up with a dent, but not necessarily a fracture.

When a carbon bearing fractures like that it was probably a manufacturing defect, not properly compacted, implying not enough pressure force applied while pressing the powder together to make the cake. Insufficient compaction pressure leaves a void inside that is the beginning of a fracture line when the part is hot off the press. When it is later bumped with mild force or continuing vibration in use, it just falls apart, with not much force required.

In recent years, like in the 2005 to 2015 time frame, failed carbon bearings were fairly common. I suppose there may have been one bad production lot that got wide spread around inventory so multiple vendors were selling the parts. If you knew about the problem in advance, and know what to look for, you can do a scratch test on one edge to tell if it is a hard cake or a soft cake. If you can shave a little wedge off the corner with a knife, it's a soft part. If firm application of a knife breaks off a little chip, it's a hard part, good to use.

I have not heard of problems with these parts much in the past five years. There would of course be some older problem parts stuck in inventory, slow sellers, that still sneak out occasionally. There may or may not be a date on the package (which of course will be long gone after installation). Most likely not any date on the part itself. Best time to test it is before installation, but if there is still enough material remaining you might still get a good test of the material that's left. Try digging at it with a sharp point of a box cutter knife. If you can dig powder out to make a crater, it's soft. If it is properly compacted and hard, it should be difficult to dig, and material would come out in flakes or small chunks.

Bottom line is, if you were unlucky and got stuck with one of the sub-standard parts from past production time, such is life. Install a new part now, and just drive it without worrying. I haven't heard of anyone having two such failures in short mileage. Maybe if you had one failure you check the next part before installation.

P.S.
Recent scuttlebutt is that quality of carbon release bearings from Borg & Beck has gone down some since they changed sourcing to China, so some people are recommending avoiding this part. I have no personal opinion (yet), and I can't personally recommend a better source either. However, I have had good service with the release bearings I buy from Moss Motors( USA). They come in a generic looking white box tagged "Made in India" and have a neat plastic cover to protect the carbon puck during storage and shipping. So far they pass my personal scratch test for firm integrity of the carbon material. If it ain't broke don't fix it? I have no reason (yet) to change my source for this part. --Barney

Addendum:
On 2/11/2022, Peter Ryle wrote:
"Skiptune UK, who are rebuilding my 1500 engine, told me about the problems with B&B clutch kits. They will rebuild my original clutch cover plate and use a Powertune release bearing, that they have good experience of:
https://www.psautoparts.co.uk/product/cl-mga-clutch-parts-flywheels/grb103-clutch-release-bearing-hd1069/HD1069. I believe Moss also sell the Powertune clutch parts, and in the UK they also offer an OE spec graphite bearing (possibly AP ?) albeit at a high price (53 pounds or about 70 USD).
When I rebuilt my MGB drive train two years ago, I bought a B&B clutch kit – the cover and driven plate were fine, but the release bearing carrier had been made too fat and did not fit in the release arm fork. I managed to source an AP release bearing that fitted and works fine with the B&B cover/plate. I think this became a common problem at the time, as many UK MG parts suppliers stopped selling the B&B kits due to ‘quality issues’. I am not sure if they are back on sale here in the UK as yet". -- Peter