The Clutch Slave Cylinder Recall

The great clutch slave cylinder recall, one that had many throwing their hands in the air and shouting with joy. Unfortunately, most didn’t realise what it was actually about (and that it didn’t apply to them), and were a little let down as a result.

This recall applied to ’01 model year bikes, fitted with an updated clutch slave from the factory. The updated clutch slave bought about its own problem, however, which would have been very difficult to predict. As an engineer, I find the problem rather interesting, so I’ll explain it as best as I can. It might get a bit hard to understand.

First, a couple of photos. As stated above, this recall is for ’01 model bikes. Which all have silver engine covers and grey (silver-ish) belt covers. Not green, like all the pre ’01 models. If your bike has the type of clutch slave cylinder shown in the picture below, then your bike was part of the recall. By now, most would have been replaced. The slave in this picture is the updated, warranty replacement part. Some people have talked about using this slave as a replacement on the older bikes. However, it is not completely interchangeable with the old part, as the pushrod required is a different length (you can change both pushrod and slave) and (more importantly) the O-Ring will not seal against the older alternator cover, letting crap in just like before. You need a thicker O-Ring – something I know now, as I’ve tried it and had it fail.

The new slave, from the outside and from the inside. We’ll work thru the marked features.

This slave cylinder design is not meant to be serviceable, as I found out when I tried to pull apart one of the first ones that gave us a problem. At that point, the recall hadn’t yet been issued, so we just replaced it. A couple of our customers had this done more than once before we became aware of the factory fix, and we then modified a few before the recall was issued. We were then directed to replace all, even the ones we had modified, for the sake of official procedure and records.

The one I did pull apart (out of "official" curiosity – the man at NFI wanted to know what was inside, requiring a rough ‘custom’ tool to get the 12–ish point cap out) was nice and clean inside, so the problem wasn’t contamination like the older design. And the problem the fluid leak caused was also far different.

With the old design, the fluid leaks out past the piston seal to the inner side and down the inside of the sprocket cover, down the lower edge of the alternator cover and then onto the ground (via the fairing if fitted). This new slave is a cast piece with the screw in cap from the outside, so, from the back (see the above photo) the only inner opening is the hole for the pushrod to go into. This pushrod opening is sealed from the sprocket area by an O-Ring sitting in the marked recess, which gets squashed against the machined surface of the alternator cover. Thereby keeping any chain associated gunk out. The old design didn’t have this total seal, just a rubber bellows covering the piston.

The sealing of the inner side is completed by the O-Rings on the clutch pushrod inside the alternator cover, so that, theoretically, the inside of the clutch slave cylinder (the bore, piston and seal) is totally isolated. This total isolation, however, created something of a problem.

What happened was, under some conditions, a vacuum was created on the inner (right hand) side of the clutch piston – in the area the pushrod goes into. This vacuum was strong enough to pull the clutch fluid from the outer side of the piston (the left hand, or fluid side) past the seal on the piston and into the cavity on the inner side.

This cavity is where the piston slides as the clutch lever is pulled in. When the vacuum pulled the fluid past the seal, the fluid stayed in this cavity with no where else to go, as the inner side of the slave is sealed by the O-Ring against the alternator cover and by the O-Rings on the pushrod. As clutch fluid is a liquid, and not compressible, this would then cause a hydraulic lock on the inner side of the piston, stopping the piston moving. Which was felt by the rider as the lever going solid before the clutch was pulled all the way in. This would allow the clutch to disengage less than usual or not at all.

This was the bit that confused us at first. It wasn’t the usual clutch problem, so when it happened, we replaced the slaves without any real idea why, or belief it would work. The fact it happened to some owners more than once made me think that some characteristic of the way they used the bike or clutch was provoking the problem. I mentioned this to one owner, who (typically) interpreted this as me saying he was doing something wrong and he got all upset. Which ended any useful conversation we might have had. So I never got any closer to working out why it might be happening.

When the clutch is pulled in, the pressure in the system to overcome the force of the clutch springs and disengage the clutch works on the seal, forcing it out against the bore of the slave. This helps the seal seal, so to speak. There is no way the vacuum would overcome this sealing. But when the clutch is engaged (lever out), there is no pressure in the system, and no pressure helping the seal do its job. So this must be when the vacuum was generated. Although, the problem occurred to owners riding both in traffic (using the clutch a lot) and on the highway (not using it for long periods). The only common characteristic was that, if the slave was removed, fluid ran out from behind. Resulting in the problem going away. This became the "side of the road" fix if we had someone ring in when out riding.

The official solution to this turned out to be rather simple. Going back to the photo, the originally fitted slaves being replaced under warranty didn’t have the vent hole or the white paint mark. The vent hole goes thru to the inner side of the piston, and is there to stop the vacuum forming. Simple enough and effective. Its location is protected from any crap from the sprocket area, so doesn’t generate any other problems in that sense. We began drilling vent holes in slaves after being told of the factory solution before the recall was issued, but were later told to replace all, including ones we had drilled. Simply for the sake of official procedure, and the white paint mark.

The white paint mark is there to denote an "updated" slave, and all new bikes now have this slave fitted. To check if a bike needs the recall performed, you just need to look for this white dot. This is easy if you have a little mirror to look behind the banjo bolt boss. White dot means ok, no white dot means replacement required.

Because this is an official recall, carrying out the recall and then submitting the required warranty form is the only way to have this officially recorded with the importer.

This brings us to the end of the clutch slave recall story. Just something I found rather interesting. I’m sure many of you realise I need to get out more often.

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