Date: Fri, 14 May 1999 16:26:17 -0400
Subject: Scorpa Rotax Clutch
From: Chris L Johnson 

Here is a report on my `96 Scorpa 250 Rotax motor clutch.  It is highly
likely the clutch is the same up to 1999.  Last time I rode, it was a bit
hot and the clutch got a smidge grabby, so I assumed the plates needed to
be scratched up.  I think, however, I only need to cut the 400cc of
Dexron II ATF with about 30% of 10W40 motor oil for summer riding.  The
heavier oil will slow clutch engagement a bit.

I noticed the aluminum casting for the clutch cover contained the clutch
slave cylinder, and the bulge in the casting for the hydraulic circuit
indicated the slave cylinder was pushing against the clutch.  Most bikes
have the slave cylinder on the opposite side of the motor with the clutch
pressure plate actuation rod passing through the first transmission
shaft.  The Scorpa slave cylinder is on the clutch side.  This means the
clutch has to have a backwards pressure plate arrangement which pushes in
rather than gets raised.  I opened the Rotax clutch plate and found that
to be true.  No need to drain the oil, just laid the bike on its side.

I removed the 6 low-head height M6 socket head cap screws and the plate
popped right off.  Only a little projection on the slave cylinder piston
sticks out of the inside of the clutch cover and contacts a ball-bearing
style "throw-in bearing".  Hmm, very high quality look to the Austrian
components.  Hey, the clutch plates say Sure Flex!  Cool.  The cap screws
were a bit corroded, from the lack of grease on the threads and water
inclusion due to the screw being on the unsealed side of the o-ring for
the cover - will grease them.  The cover plate is sealed with an o-ring
that seals well but gets dirt and crud trapped on the outside border.  I
had to be careful to clean this off properly.

In the center of the clutch is a plate affixed by 6, M4 socket head cap
screws.  You can't see the springs, which are under the plate.  Remove
the six screws and underneath find only 3 clutch springs used instead of
the full 6 used on the fast bikes.  This is similar to what you can do to
the Gas-Gas 160, and it is a smart motor designer that uses six instead
of five clutch springs so this can be done and retain balance on the
plate forces.  The Fantic has five, so you have no option but to use them
all.  I had wondered if I could take out 3 of the springs if there were
indeed six, but this was already done.

To remove the plates you must take off the what is normally the clutch
basket nut.  It is a 1-1/16" or 27mm hex nut.  This is because of the
"backward" configuration with the clutch pressure plate being inside the
basket and turned the other way.  Plates looked good, no surface
treatments required.  The heavier oil should take the grabbiness out of
the engagement.

Put the whole mess back together, greasing the o-ring and the screw
threads.  Very easy to work on as long as you don't need to pull out the
plates.

I measure the springs and calculated the values (hope I remembered the
formulas right).

Spring Free Length = 1.421 average (36.1 mm)
Spring Outside Diameter = 0.656 (16.66 mm)
Spring Inside Diameter = 0.490 (12.45 mm)
Spring Wire Diameter = 0.083 (2.11 mm)
Number of Active Coils Preloaded = 5.4
Spring Length Preloaded in the Assembly = 0.760 (19.3 mm)
Preload on each Spring, Assembled  = 0.660 (16.8 mm)
Calculated Spring Constant = 67 lbs. per inch (1.2 Kg/mm)
Spring Force Preloaded, One Spring = 44.3 lbs. (20 Kg)
All Three Springs Preloaded but Unactuated = 133 lbs. (60.4 Kg)

If you desire to change the factory pressure plate force, you can use the
above to find softer or firmer springs.  If I could, I'd try to find
slightly softer springs to try in there because the clutch does not slip
now and the lever pull is not quite a light as I'd like.  A future
experiment, perhaps?

As for changing slave cylinder diameter to change pull force, I wouldn't
advise that.  That would require remachining the cover and a new piston
and sea.  It is about as good as it gets, because even with 400cc of ATF
and only 3 clutch springs, the clutch will still drag a bit unless pulled
full to the hand grip.  With that thinner oil and fewer springs, drag is
acceptable under normal use with partial clutch lever pull-ins.  Not a
super clutch but a passable one.  At least it does not make that whirring
noise typical to the old rotary valve Rotax motor clutches and which is
also present in the new Bultaco to some extent.

Bored yet?

Chris Johnson, Director of Engineering
College Park Industries, Inc.
papazit "at" juno.com (home address)
chris "at" college-park.com
www.college-park.com