Racing crankshafts - Disc Valves for Vespa 50-125/PV/ET3/PK/XL
Racing Crankshafts are a simple and effective tuning step for every rotary valve engine of a Smallframe Vespa. A Racing Crankshaft enables higher engine speeds. The Engine becomes more lively and agile. The result is better acceleration and a higher top speed. In contrast to the original Crankshaft, a Racing Crankshaft almost always has a significantly longer intake time. The crank webs are aerodynamically optimised and often polished. Racing Crankshafts with a larger stroke than the original are referred to as Long Stroke Crankshafts. More stroke = more cubic capacity = more torque. With racing crankshafts - Disc Valves, the inlet on the Crankcase does not need to be adapted.
Racing Crankshaft Disc Valves have a cut-out in the crank cheek on the clutch side, which allows the original disc valve inlet to be retained. In most cases, the timing of the rotary valve inlet is extended compared to the original. This is intended to enable better filling, especially at higher engine speeds. On some types, the timing cheek is also optimised for better flow. Depending on the type and manufacturer, many of these Crankshafts have a Conrod with additional lubrication holes or higher-quality bearings or a forged connecting rod. A good Crankshaft is balanced in order to achieve smooth engine running. Particularly high-quality shafts use extra balancing weights inserted into the crank webs for this purpose. All Crankshafts are supplied with upper connecting rod bearings. If you want to convert to electronic Ignition, you should go straight for the 20/20 mm Racing Crankshafts. The thicker and therefore more stable Cone is also recommended in combination with a Racing Cylinder.
Long Stroke Crankshaftsare Racing Crankshafts with a longer stroke than the original. But a Racing Crankshaft for a 125cc Vespa would actually be a Long Stroke Crankshaft if it were fitted to a 50cc. But then the 50cc is no longer a 50cc, but at least a 125cc.
The next decision criterion is the choice of the right engine type.
Each engine type of a Smallframe has its own Crankshaft
As the Smallframe engines are compatible with each other, except for the automatic, we indicate the cubic centimetre class of the engine type here. The following engine types can be distinguished:
50 cc/43 mm: the small smallframes are short-stroke engines with 43 mm stroke and 87 mm Conrod. The Vespa 50/N/L/R/S/Special/SR/SS/PK50/S Models have a 19/20 mm Cone.
In the 80s, the Vespa PK50/S/SS/XL/XL2/FL/HP/N/Rush models were fitted with a thicker 20/20 mm fan wheel stub. All Racing Cylinders up to 115 ccm run with these shafts.
80-125 ccm/51 mm: the Crankshafts of the large smallframes have an original 51 mm stroke and a 97 mm Conrod. Here, too, there are differences in the cone dimensions: With 19/20 mm, the Vespa 90/R/SS/100/125/PV/ET3/PK80-100 S/PK125 are on the road.
On the Vespa 100/PK80-125 S/PK125, the stump grows to 20/20 mm and the PK 125 ETS/N/XL/XL2 Models have the most stable stump with 24/25 mm. Racing Cylinders from 125 ccm require these shafts.
Important: With a suitable Cylinder and Bearing, all Shafts can be fitted to any Smallframe. Except for the PK automatic. These Models have their own Crankshaft.
More stroke = more cc = more power
Standard stroke: In addition to the cylinder diameters, the original engine types also differ in terms of the stroke as listed in the engine classes.
Long-stroke: If you want more torque, you can opt for long-stroke variants. In the case of smallframe racing Crankshafts, we also refer to Crankshafts with a longer stroke as Racing Crankshafts. The most common stroke variants are
45 mma DRT speciality and particularly popular in Italy. The Long Stroke Crankshaft for the "small" 50cc Cylinders. This is interesting for all those who do not want to exceed 125cc but still want to get the maximum out of the Engine.
53 mm:for those who want a reed valve but do not want to sacrifice maximum displacement, the 53 mm stroke Crankshaft is the right choice - for performance fetishists! More stroke = even more power and torque for all engines over 130 cc. The Crankcase must be adapted to the larger diameter of the crank webs.
Note: Stronger cylinder base gasket or cylinder head gasket required. Additional stroke should only be compensated for by a head gasket with half the thickness of the additional stroke; e.g. when converting from a 51 ccm engine. E.g. when converting from 51 mm to 54 mm, a head gasket 1.5 mm thick. If a foot gasket is used for equalisation, the exhaust should also be machined. This only makes sense for higher tuning levels.
A longer Conrod does not change the stroke, but the Cylinder must still be raised accordingly. The longer Conrod has the advantage that it is less inclined at half the stroke and the Piston therefore exerts less lateral force on the cylinder wall. A longer Conrod also reduces the maximum piston speed and therefore also the change in speed of the Piston during one revolution. This reduces vibrations. Popular connecting rod lengths for smallframes are:
- 87 mm: original for 50 cc Models
- 97 mm: original for 125 cc Models
-105 mm: often used for Long Stroke Crankshafts. The original 97 mm Conrod is quite short. Especially in combination with a longer stroke, a longer Conrod ensures smoother running.
The Conrod makes all the difference
The Conrod is one of the most highly stressed parts in an Engine. It has to transmit the force of the explosion, withstand extreme acceleration and extreme heat. In addition, two bearings run in the running surfaces of the Conrod, one at engine speed. A high-quality Conrod is therefore suitable and important to prepare an Engine for Elevation loads or to increase its stability. But how can you recognise a good Conrod and what are the structural differences that make it higher quality and more resilient?
The best-known features of high-quality Conrods:
Forged Conrods: Compared to standard Conrods, they have a better strength-to-weight ratio. They are therefore more stable for the same weight or lighter for the same stability.
Silver bearings: The quality of the bearings used is of great importance for the service life of the Crankshaft. Bearing with a silver-plated cage are often used for connecting rod bearings. This is intended to improve emergency running properties and wear behaviour through better heat dissipation and adhesion of the oil film.
Additional lubrication holes/grooves: Additional holes or grooves allow more Oil to reach the bearings. This not only provides additional lubrication, it also helps with heat dissipation.
Polished connecting rod/blade connecting rod: A polished Conrod not only looks good, the smooth surface should also reduce turbulence. A surface treated in this way is also denser and more stable.
Blade connecting rod: A blade connecting rod is designed to minimise turbulence and the air resistance of the Conrod.
There are a large number of features and combinations of the implementation of the properties on a Conrod. To make the selection a little clearer, we differentiate between three types of Conrod:
Standard: a good Conrod, but manufactured to standard specifications.
Sport: An improved Conrod that has at least two of the features listed above.
Race: An elaborately manufactured Conrod that has at least four of the features listed above.
This simplified division into three categories is intended to help you find your way around the large selection of Crankshafts and Conrods.
A beautifully smooth-running Engine thanks to effective balancing of the Crankshaft
Strong vibrations not only have a negative effect on riding comfort, they also affect the durability of the individual components on a scooter and, last but not least, the performance of an engine. This can be remedied by "finely balanced" Crankshafts. "Finely balanced" is not actually a clear technical term. It is more of a colloquial term for a particularly well-balanced Crankshaft. Nevertheless, we have decided to use this term to provide information about a Crankshaft. We use the term "finely balanced" to describe Crankshafts that fulfil all three points mentioned above:
They are particularly precisely aligned.
They oscillate at 12 or 1 Clock.
The manufacturer has realised a certain balance factor in the design.
Unfortunately, this is no guarantee that the balancing of the respective Shaft will be optimally matched to the intended Engine. But it is a clear sign of quality, which greatly Elevates the likelihood of a smooth-running Engine. More on the subject of balancing in our Technology blogs.
The different types of crankshaft - the Cone. This is where the Flywheel sits correctly
Another important distinction is the alternator cone on which the Flywheel/Polwheel sits. There are also three classes here. They differ in the dimensions of the alternator (LIMA) shaft seal seat and the bearing seat of the LIMA bearing:
19/20 mm: Oil seal seat 19 mm, bearing seat 20 mm, thin/pointed Cone with M10x1.5 mm pole nut, fitted as standard on classic smallframes and the PKs of the 1° series. Externally recognisable in most cases by the 2-hole mounting of the Intake Manifold.
20/20 mm: Shaft seal seat 20 mm, bearing seat 20 mm, thick/blunt Cone with M12x1.25 mm pole nut, fitted as standard on PK/S of the 2° series and the PK50XL Models. Intake Manifolds with 3-hole mounting were almost exclusively installed on these.
24/25 mm: Shaft seal seat 24 mm, bearing seat 25 mm, thick/blunt Cone with M12x1.25 mm pole nut, standard on the PK125 ETS/N/XL/XL2.
Basically, the thicker the Cone, the more stable it is. This means that the 19/20 Cone is sufficient for simple tuning. If you want more, you should install at least the 20/20 Shaft. The more power and the higher the Engine revs, the greater the forces on the crankshaft stub and the greater the risk of it tearing off. The 20/20 Shafts can be installed in the 19/20 mm Casing with the appropriate Oil Seals. Conversion to an ETS Shaft with 24/25 mm Cone is easily possible on all gearshift models with the appropriate conversion shaft seal ring/bearing. This very stable Shaft can also withstand extreme loads. If you are converting from a thin/tapered to a thick/blunt Cone, you will also need the matching Flywheel/ Flywheel. It is then advisable to convert completely to an electronic Ignition, e.g. SIP PERFORMANCE VAPE, and save yourself the hassle of the old breaker ignitions. If you want to upgrade a 50cc to more than 125cc, you always need a 51-54 mm Shaft. This is also possible without any problems.
Our top brands for Crankshafts for Vespa 50-125/PV/ET3
MAZZUCCHELLI: Good and favourable standard and Racing Crankshaft for Smallframe. Made in Italy.
POLINI: For some years now, the Italians have also been offering Crankshafts for Smallframe Models again, some of them elaborately balanced and in unusual sizes
PINASCO: Beautiful Shafts for Smallframe Vespa Models, whether short or long stroke.
DRT: High-quality Crankshafts for Smallframe Models from the Italian forge. DRT even offers Long Stroke Crankshafts for 50 cc Engines.
Technology tips
If you want to delve even deeper into Crankshaft technology, we recommend our Technology blogs.
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