Five years of hydraulic clutches on KTM's left me with a minor addiction for the smooth feel
when I bought my KX. Plus, the pull of the KX cable-actuated clutch was making my hand
sore. So I ordered up a Hebo universal hydraulic clutch kit from the good guys at
eRider.
The kit is designed to fit most motorcycles and comes with a bag full of assorted parts
that will probably work for just about any application. Here's what I got from eRider:
Master cylinder, lever, and slave cylinder.
The master cylinder looks a lot like what
you'd see in a brake system, except the
lever is on the opposite side. It even uses
DOT4 brake fluid.
Parts and accessories. Note the two bags on
the left. These are pop rivets that are used to
adjust the action of the slave cylinder.
The system came set up
with a cable end that
wasn't compatible with
the KX's actuator arm.
No problem, there was a
clamp-on cable end in
the parts bag that fit
perfectly in the actuator
arm. To get the
incompatible cable out of
the slave cylinder, I
removed the circlip...
Instructions - English or
Spanish (your choice).
...then pulled out the cable
end stopper thingy...
...and then the cable could be pulled out of the cylinder.
I used a different
cable from the parts
kit. It had one of the
same cable ends for
use in the cylinder,
but the other end of
the cable had
nothing on its end.
That end was
inserted into the
cable end clamp that
fit on the KX's clutch
actuator arm.
Here's the incompatible cable
assembly removed from the cylinder.
The hook tool helped pull out the
cable end stoppie thingy.
Here's another view. I wrapped part of the cylinder with
an inner tube because the slave cylinder was touching
the corner of the engine. Hebo recommends that the
cylinder be kept away from heat sources. Note that I
haven't installed the rubber boots that come with the
parts kit. You can see the cable end clamp that fits on
the clutch actuator arm. The cable end goes through a
hole in the fitting and a screw clamps everything in place.

See the rubber boot on the left end of the slave cylinder?
Here's where the pop rivet comes in. The end of the rivet
happens to be about 30mm long, which is how deep
Hebo recommends the piston inside the cylinder should
be from the outside end of the cylinder. I used the pop
rivet to push in the piston 30mm, then clamped down the
cable end clamp in the actuator so there wasn't any
slack in the cable. After that, I cut off the excess cable. It
would be nice if they included some cable end caps like
bicycles use (the cable is too thick for bike cable ends).
Now, the fun part. The general idea is to
mount the slave cylinder somewhere
where the cable has a fairly straight shot
to the clutch actuator arm. The picture
doesn't show it, but I found a flat piece of
metal with holes drilled in it; one end of
the metal piece was attached to the
cylinder mounting hole and the other end
was attached to one of the power valve
cover bolts. You can see that I haven't
yet attached the cable end to the
actuator arm.
Does it Work?
Oh yeah, it works. Beautifully. I tried it out at the
Lebanon opening round of the Missouri Hare
Scrambles Championship. For the first time since
I've owned the KX, I rode without a hint of arm
pump in my clutch hand. The clutch action is
typically smooth, as with all hydraulic clutches. To
be honest, I was just a bit skeptical that the pull
could be as light as advertised. While it's not an
astronomical difference in pull, it is just lighter
enough that my clutch hand was never sore. In
fact, I never even noticed the lever at all. The AJP
lever is a different shape than most, and it's a
shorty-style lever, which I haven't used in many
years. The action is pretty much two-finger, and it
feels the same throughout the whole pull and
never feels any different no matter how much you
abuse the clutch.

The other concern I had with the Hebo slave
cylinder was its size in comparison to the Magura
units. But as the pictures show, it tucks out of the
way and is protected from frontal impacts by the
pipe and side impacts by my boot. The fittings on
both ends of the system are standard banjo
connections, just like hydraulic brakes (it uses
DOT4 brake fluid; the Magura's use oil).

All in all, the Hebo kit is a great add-on. While not
exactly inexpensive, any mod that makes me
forget about the bike (or a sore hand) and
concentrate on riding is a good mod. I'd
recommend it for any bike.
Here's the lever and master cylinder. The
lever is a little funky, but adjustable like
most. The lever and master cylinder are  
made by AJP, which is pretty well known in
the trials bike world. I had to disconnect
the line from the master cylinder to get it
routed up to the handlebars, then bleed
the system. The bleeder screw requires a
7mm wrench.
Update January 2005:
Had a little problem with the Hebo's
cable...it broke. Here's the culprit:
Since the cable doesn't have a
perfectly straight line from this
housing to the clutch actuator arm, it
gradually rubbed a hole in one side of
the housing. The hole itself wasn't
necessarily a bad thing, but the sharp
metal left behind was enough to cut
the cable. In the pictures above you
can see that the cable end is a bit
funky, so finding a replacement was a
concern. But the good guys at
Donelson Cycles in St. Louis were able
to find me a $15 clutch cable with
ends that matched pretty well with the
original (eRider also sells the
replacement parts, but I didn't need
all the housings and other parts that
come with it). I have no idea what kind
of motorcycle this cable came from,
but Donelson's had it hanging
amongst a bunch of spares. And the
best part is, the replacement cable
has identical ends at each end of the
cable (does that make sense?), so I
have a spare in case it ever breaks
again.

On the KX, I'm not sure there's a way
to prevent the cable from wearing a
hole in the housing. But the life of the
housing can be prolonged if it's
rotated every so often. That way, the
cable's not always wearing in the same
spot. In the picture at right you can
see where the cable is going to wear
on the new housing. The housing can
be rotated without taking anything
apart.

Also, the stock cable housing can be
Left: original cable
Right: replacement from Donelson Cycles
Master Cylinder: Check the banjo bolt washers
One other thing that happened was during a cold spell in December '04 (the Hebo's first
exposure to significantly below-freezing weather), the banjo bolt leaked out the brake
fluid. Not sure exactly why, but I'd guess that maybe some moisture got in there and
froze, creating a small gap in the sealing washers. I replaced the washers with new ones
and it seems to be O.K. Just another thing to watch out for.

Update October 2007
After the Upland, Indiana enduro, I noticed the slave cylinder had developed some
"incontinence". Brake fluid was leaking out the cable housing, due to the age and abuse of
the internal seals. Naturally, the seals are odd-sized metric o-rings, hard to find anywhere
this side of McMaster-Carr. A repair kit does exist (see photo below), but good luck
finding it. Your best bet is ordering up 14.5x1.5 Viton o-rings for the piston and
14.1x1.6 for the cylinder. McMaster-Carr has both - the part numbers are 9263K576 for
the piston o-rings and 9263K625 for the internal cylinder o-rings. Unfortunately, these
come in packages of 25 (only two of each o-ring are required).
cut off and used in place of the supplied set of cable housings. It appears to be a bit
stronger, less prone to wearing from the cable, and the rubber boot fits perfectly around
the housing to seal out more dirt and grit.

One final note: just about any clutch cable can be used as a replacement, even if the end
isn't exactly like the original. It just needs an end that's bigger that the hole inside the
slave cylinder that the cable fits through. I would have used a regular clutch cable if
Donelson's hadn't found me such a good match.
These things are remarkably simple.
This is the Hebo repair kit, part number HR8910961
Update April 2008
Those of you with more knowledge than me on the science of o-rings (which is
most of the human population) probably raised an eyebrow or two when I used
Viton o-rings in an application involving DOT4 brake fluid. Turns out Viton and
DOT4 don't play well together. The o-rings tend to melt and stretch when exposed
to polyethylene glycol-based fluid (Wikipedia, makin' me sound smart).

As always, there are options to solve this predicament. The first is to locate a Hebo
slave cylinder repair kit. Rising Sun Imports (a/k/a
TrialsPartsUSA.com), located in
a small Indiana town remarkably close to the farm where I grew up, carries all
things Hebo, including the repair kit. As you can see from
this photo, there's not a
lot to it. In fact, all I really needed was the 4 o-rings. Unfortunately, the retail price
for the kit is $32.95.

Instead of spending over $8 per o-ring, I did what any other cheap bastard would
do: I switched to DOT5 brake fluid. It's silicone-based and compatible with Viton
o-rings. Many auto parts stores carry DOT5 - I found a 12-oz container for about
$7 at Murray's Discount Auto Store. It is a bit of a chore to clean out the DOT4
brake fluid from the whole system. I took apart everything - the AJP master
cylinder (including the plunger), the braided steel line, and of course the slave
cylinder. The two fluids are not compatible, so the DOT4 must go. Now, the o-rings
and the brake fluid live in harmony.
Click on
pictures for
larger image
HEBO HYDRAULIC CLUTCH CONVERSION
March 2004
Update January 2009
After nearly a full season using DOT5 fluid, I decided to suck it up and spend
$32.95 to buy the rebuild kit. One of the downsides to DOT5 is that it doesn't have
the lubricity of DOT3/4. This probably doesn't matter as much in braking
applications, but for the moving parts inside the slave cylinder, a more slippery fluid
was needed around the seals. The clutch action wasn't as smooth with DOT5. It felt
like the seals were sticking slightly at a couple points within the range of cylinder
movement. Upon inspection, the seals themselves looked just fine. They just
needed to slide more easily across the metal surface of the cylinder.

Other notes from the 2008 season:

  • The AJP master cylinder cap is apparently very sensitive to the tightness of
    the cap screws. At the Rock Biter Enduro, I had problems with the clutch
    fading, which were solved when I only tightened the cap screws just past
    finger tight. The master cylinder wasn't able to vent properly with tight screws.
  • Parts Unlimited makes a clutch cable for the KX250 that is perfect for use as
    the Hebo cable actuator (mfg. part no. K282156; Dennis Kirk part no.
    193475). Although this might be considered an "el cheapo" clutch cable for
    standard use, it has the correct ends to fit into the cable housing inside the
    slave cylinder. Also, you can hack off the cable guide that slides into the
    housing on the engine case, and you've basically got the same cable actuation
    as the standard clutch cable.
Here's the pieces I cut off from the Parts Unlimited clutch cable. The metal cable
guide (short piece in bottom of picture) fits into a slot on the engine case, just like
it does with a standard clutch (refer to
this picture). The cable movement, over
time, will still wear a groove into the small end of the cable guide (right side)
because the angle isn't perfectly aligned with the clutch actuator arm. Eventually
this can cause the cable to break. However, if you rotate the guide in the engine
case housing every so often, the cable won't always be rubbing the same spot on
the small end of the guide and the cable will last longer
.
The cheaper-type cables have ends like this (the higher quality cables sold by
Motion Pro and others have just one rounded piece that's pressed into the cable).
In this case, cheap is A-ok. The larger rounded piece can be slid off the cable and
discarded. The other end of the cable is identical, so it's like you're getting two
clutch cables for the price of one.
The best I could do with the supplied parts. See the
January 2009 update (below) for a better alternative.
This is how the cable end fits into the guide
inside the slave cylinder: