Pinasco’s 251 Bull Clutch system solves one of the problems of the Vespa PX clutch design for powerful engines. Instead of simply mounting to the crankshaft using a woodruff key and a silent prayer, it uses a splined fit.
As such, the Bull clutch is easy to assemble and dismount, but the chances of it spinning on the crankshaft are effectively zero.
And there’s no silly woodruff key to drop into the engine.
Need to recap?
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Vespa largeframe clutches: 6-spring, 7-spring and 8-spring (Cosa-2)
Construction
The Bull clutch broadly follows the early ’90s Piaggio Cosa 2 layout, simply known as the ‘Cosa Clutch’ in the UK. This design is sturdier and heavier than earlier Piaggio clutches, but given an increase in friction plate numbers (7 friction surfaces versus 5) it could utilise less spring pressure. This meant that the clutch lever action was much easier; reducing rider strain.
There are a lot of positives of the Cosa 2 layout; which is why many of the performance options (such as MMW, BGM and SIP) use it as the basis of their clutches.
Pinasco Bull Clutch (image: SIP Scootershop)
Cosa 2 clutch issues
One of the common problems of aftermarket replicas of the Cosa 2 clutch was inconsistencies of machining of the basket (‘bell’).
For the clutch to release correctly – so that the plain plates do not drag on the friction plates – there needs to be sufficient room for the sandwiched plates to separate. On many aftermarket clutches there wasn’t sufficient space for the plates to move apart resulting in the scooter trying to ride forwards even with the lever pulled in.
The standard solution to this was to get the clutch basket machined to lower the slots into which the ‘ears’ of the friction plates sit. On many aftermarket clutches the slots were between 19mm and 20mm deep. Extending this depth to 21mm by machining usually allowed the clutch to work as it should.
Measuring the slot depth in the basket with a digital tyre depth gauge. Several slots on Iggy’s clutch were less than 20mm deep
It’s a great deal of effort to extend these slots with a hand file, and tricky to do with a rotary tool without creating a rounded corner to the slots. What does work well – if you don’t have access to a milling machine – is a ‘powerfile’ with a sufficiently narrow sanding belt.
Which file?
Powerfiles are not to be confused with paedophiles. One you get from B&Q and the other from the BBC. At least you could in the 1970s. I’m not sure that they keep them in stock anymore.
Another frequent problem with Cosa 2 clutches is damage to the friction plate ears and basket. The problem of the standard steel friction plates is that the ears are very thin and this small contact area must therefore be subject to a great deal of force.
Burred ‘ears’ on the plates of a well-worn original Piaggio Cosa 2 clutch.
Pinasco solved this issue on the Bull clutch for the 251 engine by using motorcycle-style friction plates which have ears that are the same width as the full thickness of the friction material on each plate. Sharing the load over a wider contact surface reduces the damage to the ears and clutch basket.
Note that other versions of the Bull clutch come with normal cork-covered plates with standard ‘thin’ ears, but the 251 version comes with superior plates. (Image courtesy SIP Scootershop).
Spring checking
The Pinasco Bull clutch comes with 2 sets of springs: The silver ones are ‘strong’ and the gold ones are ‘super strong’. It is also possible to ‘mix-and-match’ to create an intermediate spring tension.
Another potential source of clutch drag problems is if the springs do not compress far enough to allow for full separation of the clutch plates.
We decided to check if this was likely to be an issue with the Bull clutch by measuring the spring length once it becomes coil-bound: i.e. when all the spring coils touch each other.
• For the gold springs we measured 13.4mm fully compressed
• For the silver springs we measured 7.8mm fully compressed
The next thing to check is the minimum distance between the spring seats with the clutch fully-compressed. When we checked the Bull clutch there was plenty of space for either set of springs, so this will never be an issue. Good news.
VIDEO: Bull clutch assembly and fettling
Assembly order
In the instructions – those silly bits of paper that ‘real men’ casually discard – Pinasco give an assembly order for the plates of the Bull clutch.
What they don’t include in the instructions is how to line up the clutch parts to ensure that all the springs correctly sit into their recesses. There are eight possible orientations of the clutch centre in the basket but 12 clutch springs, so it is important to get both parts of the clutch in line.
There is a dimple machined into one part to help align them.
Like the original Piaggio clutch, the Bull clutch uses several different types of plain steel plates in the stack.
- The thickest 2mm steel plate fits at the bottom
- After the 1st friction plate is one of the plain, thinner steel plates
- After the 2nd friction plate is a special steel plate signified by a notch in one of the inner teeth
- After the 3rd friction plate is another plain, thinner steel plate
- Last is the friction plate with cork on only one side
- Finally, the assembled clutch closes with a circlip (called ‘Seeger’ in Italian)
Position of the notched steel plate is important because it is not perfectly flat. It is slightly dished so that clutch take-up is smoother.
Note the notch in the tooth of this unique plain plate which requires a specific location.
Pre-fit testing
The previous Bull clutch I assembled went together without a hitch and works perfectly. Iggy was not so lucky.
With the clutch fully assembled and compressed using a compressor tool, you should be able to turn the primary pinion (the toothed gear) inside the basket by hand with only very slight resistance. Iggy’s however, had significant drag.
This shouldn’t be needed on a new clutch but it was on this one!
Using a powerfile to carefully lower the slots
Upon investigation, it proved that there was very little clearance for the plates to separate, so we measured the depth of the slots in the basket. Some of these were less than 20mm deep. With the slots evenly lowered to around 21mm with a powerfile we reassembled the clutch and tried again…
This time there was sufficient space for the plates to separate and the primary drive gear could now spin without undue effort. At last!
Re-checking the depth of the slot after filing
With the slots lowered we achieved 1mm clearance between the top plate and circlip when the clutch was fully compressed. This was enough clearance for the centre to be spun without excessive drag.
TOP TIP: For final assembly, the ends of the clutch circlip are better located in the groove of the basket rather than being unsupported.
Final fit
Beneath the Bull clutch, on the crankshaft, sits a special spacer washer supplied with the kit. What isn’t supplied is a suitable clutch cover, so you must purloin one from another engine.
ONE TO WATCH: Note that early covers from the PX125/150 will still fit but you may need to grind out a rib from the inside of the casting for clearance of the larger clutch basket.
Thankfully none of the Cosa 2 clutch designs use the dreadful castle-nut and tab washer arrangement of early clutches. Pinasco supply a wavy washer and a special self-locking nut. With the clutch nut torqued to 46Nm (34 ft-lbs) we could fit a new pressure plate and then the clutch cover.
Phew. Time for a brew…
Video and images: Iggy
Words: Sticky
Additional images: SIP/Pinasco
Next time
Next time we deal with the ignition and CDI location before Iggy finally gets his 251 on the road. You can read that here.
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