This page is about the /5 BMW motorcycles and later.
Is your clutch OK when cold and gets really grabby after warming up? It isn't
that you are clumsy. A grabby clutch usually means that the input shaft of the
transmission has excessive end play. It can grab even when cold, on a bike with
a really loose input shaft. The shaft is moving and engaging the clutch by
itself.
I recently learned of another reason for a grabby clutch from Dave. His bike
had a grabby clutch and it was due to a broken clutch rod. It was broken at the
felt and the edges were at an angle. They would alternately make the rod longer
and shorter as they "turned" against one another. Replacing the clutch pushrod
fixed the symptom. Thanks to Dave.
It is also possible for the spines to need to be lubed, but that is far less
likely. It also does not vary with temp.
To reduce this end play means removing the transmission, opening it up and
re-spacing the shaft. While one is in there, several other things should be
checked and replaced. This job is not trivial and should be left to an
experienced /5 BMW mechanic. In my opinion, very few persons exist today that
are capable of properly rebuilding an old BMW tranny. Some will say that only
the splines need greasing. It may be true that they need grease, but greasing
the splines to allow the clutch plate to float, and therefore not grab, is the
wrong solution. The shaft shouldn't be moving in the first place, fix it.
The splines should be greased, but only so that they don't rust and wear
prematurely.
The end play may be checked without opening the transmission. It takes a dial
indicator and a way to hold the transmission down. Get it all set up ahead of
time. Heat the transmission up to about 180-220 F and put the dial on the input
shaft. Grab the shaft and push and pull on it. The end play can be measured more
accurately than with the cover off. BMW calls for the end play to be .004," but
I like to set it at .001 or .002 when cold. The end play increases when
warm and you may well find .004" when the whole case is heated up.
To correct the end play one must open it up. That means several special tools
and is not for the faint of heart. To measure the end play I use a homemade
tool. It is an old ruined rear cover. It has been machined off until the bearing
surfaces are revealed. Then the bearing holes are opened up by hand sanding a bit to allow it to
drop over the bearings without heat. The "tool" cover surface must
be very good and clean. It holds the shafts in perfect alignment
and allows the depth gauge to reach the bearings. A bearing that isn't "held"
will tend to cock to one side and that is why the method shown in the BMW
workshop manual won't work well. It uses parallel bars and a depth gauge.
The number stamped into the cover is the resulting thickness of the cover. That number is used in the math for calculating the shims required. One must
then measure the depth of the bearing cavities in the cover to be used. I
suggest that one also check the surfaces of the case and the "real" cover for
dings that may cause a leak. After that it is all addition and
subtraction.
While this input shaft spacing needs to be done, one should strongly consider
replacing and servicing other parts while one is in there. Whenever the tranny
is out, I suggest that you check and service the clutch, rear main seal, oil
pump cover O-ring and the tranny input shaft seal. Learn more about the
clutch.
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