V4 CNC 29mm Slave Cylinder Problem

Hello everyone! Long time reader, had to join up to give warning! My name is Sam, I bought Bill Schaffers clapped out Streetfighter, and own a Hyper 1100.
-Intro Over-

The motorcycle is a 2020 Streetfighter V4S.
Has the CNC clear clutch cover, CNC pressure plate and spring retainer.
Had the dreadful “Bleed the clutch every 500 miles” Ducati experience. Previous owner had already installed RCS masters, so I figured I would buy a new slave cylinder to see if it fixed the issue.

Decided on the CNC for its large piston size, hoping it would work will with the RCS master, it seams to move a lot of fluid.

Didn’t notice much of an issue during instal, it seamed a little hard to push the slave up to the case but that was about it.

First ride reveals the clutch now slips under heavy acceleration. I had several theories from spring pressure in the slave, to too much engine oil in the bike.

Long story short, the cylinder doesn’t come close to operating the in the same range as the OEM. Removing the internal spring reveals the push rod bottoms out the cylinder about 0.2mm (.008”) before touching the case.
Same measurements with OEM brembo reveal it is nearly dead center of its total travel. 3-4mm from bottomed out, and 4-5mm from fully extended.

Clutch pack measures 50.67mm, I believe it is 50.8mm when new, but not sure.

I have been back and forth emailing with CNC all week, while my bike sets in the garage lifeless during the last few weeks of my riding season.

I have sent them a drawing of most of the measurements of the piston and cylinder. Their response was that it was correct, no dimensional flaws compared to their drawings.

If you got a brand new clutch pack it will probably work for a while, until you wear a few tenths of a millimeter of friction material away and bottom out the cylinder.

I think the real kicker is, it comes with 2 4.5mm thick spacers they want you to instal in between the slave and the sprocket cover.
Why wasn’t that material left on the cylinder when it was machined? Maybe it was a mistake? Maybe it was the same Z-Axis mistake that caused the cylinder to be missing ~4.5mm of depth. Doesn’t take a genius.

had the same when installing a febur oil cooler replacement at a 1299.
these screw seat faces are not precise in depth.
Why they should? normally it doesn't matter.

All that chicken scratch. Those 2 extra spacers are if you have the chain guide shoe removed and re-using the OEM bolts. If you have the the chain guide installed use the spacers for OEM bolt. If guide is removed and and you have shorter proti bolts (2-4mm is the shoe width off top my head), the cylinder installs perfectly without spacers. I just installed a 29mm on a V4R without using the spacers and it steps through a worn dry clutch. I did the slave cylinder before replacing the clutch packs but it still extends the disks far enough to cycle gears.

I did have an issue with bleeding all air out. The cylinder had to be removed from the bike with clutch line still installed, depress the piston all the way in and bleed it a few times off the bike. Once its clear, install on bike and do a final bleed.

What does need to be improved is the quick shift mechanism, it needs to be damn near spurred out of 3rd or 4th, the missed shift lurches on the highway are always fun

Astral11 said:

All that chicken scratch. Those 2 extra spacers are if you have the chain guide shoe removed and re-using the OEM bolts.

Click to expand...

My issue is unrelated to the length of the bolts or existence of the sprocket guard.

The issue is that the clutch rod extends in to the cylinder leaving only 9.54mm of room. The piston is 9.75mm this.

Meaning, it unclamps, or it moves my pressure plate by 0.2mm when you bolt it up.

You could fix it by placing those spacers in between the cylinder and the engine case, but then engine oil would leak out. . .

How many miles on your V4R?
If you ever get the chance, post back here with some dimensions of your 29mm slave cylinder.

I’m 100% sure they sold a bad batch. There probably not all like mine.

Making of my titanium piston to offset their messed up cylinder dimensions. Note the reduced depth.

Making your own Ti parts is always cool. Nicely done.

It seems the piston may not of been fully compressed, or it was not fully seated from CNC, slightly out of round maybe that wouldn't let the piston fully return? If its bled correctly then maybe the return spring was out? 2mm of protrusion is odd, I haven't opened the piston myself but I assume you'd find any kind of hardware error from CNC during that inspection. When bleeding mine off the bike, I used both thumbs to press the piston in as far is it would seat, I wish I would have taken depth measurements to see if that spring wears out over time.

So you machined the titanium piston to be 2mm deeper compared to CNC, that is some fine work and now I'm jealous haha. Did it return your clutch function?

I'm in the process of making a titanium cowl from .035 sheet to replace the sprocket cover and prevent the lube slinging onto the engine.





She's a 2020 with 3300 miles on it. I'm waiting for the new clutch disks to come in that just went out, oddly I was in the middle of the slave cylinder build when the clutch worn out, so I'm basically rebuilding the left and right side of the clutch assembly at the same time. It would be nice to get a titanium Corse shifting rod but haven't been able to find one

I made the Piston this past winter, it fixed the slipping and I haven’t had to bleed it since then. My V4 Streetfighter has about 12k miles now, I think it had 10k at the start of the season.

In the middle of slave cylinder rebuild or in the middle of installing it?
The dry clutch is also quite different from the wet? Does it have the same number of plates and the same stack height? 50.8?
What did it measure when it started to slip?

Wet clutches last forever, I could probably still be using the OEM, but I went through a whole fiasco with an STM wet slipper that I was never able to make function. Bought 2 new clutch packs trying to make it work.

That's awesome Would be nice to replace the standard piston with Ti.

I was in the middle of slave cylinder/sprocket change project, then the clutch started slipping while out I'll take some depth measurements to see where it sits, I haven't opened it yet

Wet clutches are still great over dry, I guess it's how you prefer to work the bike, the 959 was solid, unfortunately I haven't rode a Streetfighter. Did you end up going with the dry clutch or able to make one of the STMs work? I'm interested in upgrading the dry plate materials and lightweight baskets for the V4. EVR has a really nice design that would help manage dust control

Check this out about the slave cylinder measurements being off. Were the first 2 steel-cupped steel plates re-installed correctly?

The Ducati Dry Clutch 101

Screenshot further into article:

@andyb can we receive your expert advice on these clutch slave cylinder measurements being off? I think we might of found some of that relevant info previously mentioned, must've been your good luck blessings lol

I don’t think I understand what the article is talking about.

My OEM clutch contained a a single dished, or cupped, steel plate. It was also a little thicker than the rest of the steel plates.
No one could verify that this was actually OEM. The people I asked claimed the V4 did not come with any dished plates.
I did not reinstall this plate.

Either way, I haven’t experienced any issues with not being able to unclamp the clutch sufficiently.
All my issues were the opposite. Struggling to get the clutch to clamp.