It's a bit of a minefield with many variables.
A graphite release bearing _should_ last the life of the clutch friction
plate, apart from my roadster described below I've never had one fail, the
clutch has started slipping from friction plate wear before that. But if
the main driver rides the clutch and sits at traffic lights with the pedal
down then its life will be reduced.
A few years ago someone started making them with the graphite ring pinned
into the casting instead of being glued and they tended to explode in a
short time. Hopefully they will be out of circulation now.
Suppliers want you to buy roller bearing release bearings at extra expense
(clutch kits normally come with the graphite type) but they have their own
problems. John Twist describes how they aren't the complete answer by
demonstrating how they move in an arc as the pedal is operated and released,
so sliding across the face of the release ring on the cover plate, which
causes excessive wear. Graphite bearings are designed to cope with that,
roller bearings aren't. Whilst what he says is true the arc they actually
move in use is very limited, the real problem is there is nothing to
centralise the bearing to the cover plate in the first place. Depending on
how far your release arm is off-centre - and I found mine a good 3/16" - as
well as sliding back an fore all the time the load will be on one side
significantly more than the other, something else they are not designed for
http://www.mgb-stuff.org.uk/clutchr3a.htm#5 I've been told by someone who
has worked on these cars for decades that "They are all like that".
Proponents say all modern cars have roller bearings so they must be good, so
does the MGB GT V8 and the Midget 1500, and they don't fail. But that is
because they have a guide tube over the first-motion shaft to keep the
bearing centralised to the cover plate.
On the roadster I've had a graphite bearing break up because the casting
wore through on one side, and a roller bearing get destroyed when the
friction plate was barely half worn. At that point I noticed how off-centre
the release arm was, so brought it back into line. The most immediate
effect is the biting point is now much higher than it has ever been - almost
too high. It was always lower than my other cars before and extending the
'throw' of the hydraulics made little difference. I'm sure that is because
before it was pushing one side of the pressure plate down less than the
other side because of the offset. There is a graphite bearing in now, time
will tell.
As for the gunge, that is normal as the flex hose ages. That will have no
effect on release bearing wear.
PaulH.
----- Original Message -----
I have a 71 MGB and Iâ??m wondering how many miles people are getting from the
carbon release bearing.
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