Racing crankshafts - Reed Valve
for Vespa 125/150/Rally from '57 onwards
Racing Crankshafts - Reed Valves are the ultimate in engine tuning for large-frame Vespas from the 60s and 70s and a must-have if you want to achieve outputs of 25 hp and more. A reed shaft enables higher engine speeds. The Engine becomes more lively and agile. The result is better acceleration and a higher top speed. The crank webs are aerodynamically optimised and often polished. Depending on the design of the crank webs, we distinguish between Bell-Shaped Crankshafts, Flowed Crankshafts and Full Circle Crankshafts. The intake on the Crankcase must be adapted for all diaphragm shafts. There is no going back to the Disc Valve. But who wants to go back?
Crankshafts whose design does not allow the use of the original rotary valve inlet are referred to as diaphragm shafts. A different type of intake control, usually a Reed Valve, must be used. A Reed Valve controls the fresh gas supply according to demand and the operating situation. The negative pressure in the Crankcase opens the diaphragm; once the pressure difference is equalised, the diaphragm closes again. In this way, the Reed Valve automatically regulates the gas supply at the ideal control time. The control time of a rotary valve inlet, on the other hand, is rigid and cannot adapt to the respective engine speeds or operating situations. This means that the intake timing is either too long or too short in many speed ranges. In the case of the Vespa, the rotary valve intake is also relatively small. If a large Carburettor is installed for tuning purposes and the Engine speed is increased, the cross-section quickly reaches its limits and acts as a throttle.
Diaphragm shafts are available in different versions.
TheBell-Shaped Crankshaftor mushroom shaft, is a Crankshaft whose shape resembles a bell or mushroom. It also offers the mixture as little resistance as possible. It is not quite as uncompromising as the Flowed Crankshaft, as its symmetrical shape is not quite as ideally suited to the asymmetrical intake times of the Engine. However, a very large cross-section is available, especially in the phases with the greatest vacuum. As a result, the difference in performance compared to the Flowed Crankshaft is minimal. On the other hand, the symmetrical shape has the advantage that uniform balancing with an ideal balancing factor can be achieved more easily and effectively.
FlowedCrankshafts are flow-optimised diaphragm shafts that harmonise very well with diaphragm intake manifolds. On most large-frame Engines, the intake sits above the crank cheek on the clutch side. A particularly flow-favourable machining of this crank web often resembles the shape of a lip.
Full Circle Crankshafts have , as the name suggests, two completely closed webs. A large diaphragm inlet is required here. It only makes sense to use a Full Circle Crankshaft in conjunction with a direct inlet into the Cylinder. These shafts are also available with a long stroke. The basic idea behind the Full Circle Crankshaft is better pre-compression, which is achieved via the completely closed crank webs. However, in engines controlled via the Casing, even large milled diaphragm inlets with boost ports cannot prevent the mixture from being unfavourably obstructed by the cheek standing in the way.
The next decision criterion is the choice of the right engine type.
Crankshafts with a reed valve only for the engines of the younger large-frame Vespas of the 60s and 70s
A distinction can be made between the following engine groups:
125/150 cc, 57 mm stroke, 105 mm Conrod: The first Vespa largeframes Vespa 125 VNB/GT/GTR 1°/Super/TS/150 VBA/VBB/T4/GL/Sprint/V 1°/Super 1° still come with a narrow Cone on the fan wheel side. These engines are not suitable for stronger tuning. It is therefore best to convert to a PX engine.
The thicker fan wheel stub of the later PX comes with the following models Vespa 125 GTR 2°/TS 2°/150 Sprint V 2°/Super 2°. All Crankshafts of the PX125/150 fit here. The range is large.
200 cc, 57 mm stroke, 110 mm Conrod: the queen among the sports models of the 70s. The Vespa 200 Rally VSE1T 33997 ->. The Crankshaft of the later PX200 is used here for the first time. The tuning possibilities are enormous and popular. The range is therefore also very large and varied.
Note: Installing a PX Engine in the older models is also popular. With a 200cc Engine in particular, every vintage bike becomes a powerful and reliable touring bomber or racer.
Bell or lip - creativity in the cheek moulds
Mushroom and Flowed Crankshaft are shapes that are ideal for use with a reed valve intake. If a Vespa Engine is converted from a rotary valve intake to a reed valve intake, the fresh gas is opposed by the control cheek of the Crankshaft, which regulates the timing of the rotary valve intake. In order to maximise the cross-section here, the timing cheek of a Flowed Crankshaft is machined so that the fresh gas can flow past it into the Crankcase. This is ideal in terms of performance, but so much material is removed in the process that compromises have to be made when balancing the Crankshaft.
The shape of the mushroom cam leaves only slightly less space for the fresh gas than the Flowed Crankshaft. On the other hand, its shape is symmetrical and ideally distributed for balancing purposes. It is therefore the ideal compromise for powerful everyday and touring engines with diaphragm intake.
More stroke - more torque - more fun
Standard stroke: In addition to the cylinder diameters, the original engine types also differ in terms of stroke, as we have listed in the engine classes.
Long stroke: If you want more torque, you can opt for long-stroke variants. When looking for long-stroke variants, you need to pay attention to the stroke lengths of the diaphragm shafts. The most common stroke variants in the large frame 125-200 cc class are
60 mm: Very popular for adding more stroke to an Engine. These shafts usually fit into the original Crankcase without modification.
62 mm: From 62 mm stroke, the Crankcase must be adapted to the larger diameter of the Crankshaft. However, a sufficient part of the centring lip of the small half of the housing can still remain. a stroke of 62 mm can also be realised with most Racing Cylinders, which are designed for a stroke of 57 mm. 62mm is therefore the last stage of expansion that keeps the conversion effort within manageable limits.
Note: A thicker cylinder base gasket or cylinder head gasket is required. More stroke is usually compensated for by a head gasket with half the thickness of the additional stroke. If a base gasket is used for equalisation, the exhaust should also be machined. This makes particular sense for higher tuning levels.
The subtle difference lies in the Conrod
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, resulting in less friction. A longer Conrod also reduces the maximum piston speed and therefore also the change in speed of the Piston during one revolution. The timing also changes with a long connecting rod. However, only slightly, because the piston elevation curve changes slightly. As the crank webs are held together by the Crankshaft Pin, it is desirable to have the highest possible pressing dimension to prevent the two webs from twisting. The connecting rod length is always measured from centre eye to centre eye.
The most common Conrods for Vespa 125/150/Rally from '57 onwards:
105 mm: the standard Conrod for all models in this group except the PX 200.
110 mm: The standard Conrod for the PX 200, often used as an upgrade for the Crankshafts of smaller engine blocks.
127 mm: QUATTRINI uses a 126 mm Conrod for its M232/M244 Cylinders. The Conrods of the SIP racing crankshafts for these Cylinders are 127mm to make it easier to compensate for timing differences. Other manufacturers such as BFA and VMC also offer large-diameter Cylinders for the connecting rod length.
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 endure great heat. In addition, two bearings run in the running surfaces of the connecting rod, one at engine speed, so a high-quality connecting rod is 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 distinguish between three types of Conrod:
Standard: A good Conrod that has been 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 orientate yourself in the large selection of Crankshafts and Conrods.
The better the Crankshaft, the more effective the balancing and the smoother the Engine will run
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 is optimised for the intended Engine. But it is a clear sign of quality, which greatly Elevates the probability of a smooth-running Engine.
The design of bell or mushroom crankshafts fulfils the ideal requirements for an optimally balanced crankshaft. Both sides of the crank webs are symmetrical. This is why this shape always balances at 12 Clock. The shape of the Shaft results in a material distribution that provides ideal positioning of the Counterweight for smooth running. Both cheeks are shaped exactly the same. This means that there is no wobbling or forces within the Shaft.
Our top brands for Crankshafts for Vespa 125/150/Rally Models
SIP PERFORMANCE:high-end Bell-Shaped Crankshafts, designed by the top tuners of the German-speaking scene and manufactured in Italy in a premium crankshaft forge. As SIP Performance version with polished forged Conrod or in the Premium version with a high-quality bronze Conrod.
SERIES PRO: Before the SIP PERFORMANCE Racing Crankshafts were available, worb5 updated the oldschool MAZZUCCHELLI Shafts for us. The Flowed Crankshafts are the result of many years of tuning experience.
MAZZUCCHELLI:more old-school with Full Circle Crankshafts. The Bell-Shaped Crankshaft was created in collaboration with SIP.
Technology tips
If you want to delve even deeper into Crankshaft technology, we recommend our Technology blogs.
Customers ask - SIP answers
"What other parts do I need for a crankshaft replacement?"
Often only cent items, but annoying if you don't replace them:
Woodruff Keys - don't put an old Woodruff Key in a new groove
Gasket Set Engine - if you don't change it, it will drip
Crankshaft bearings - always replace the LIMA and CULU sides as well
Oil Seal Crankshaft - replace both sides here too
NutLock Cap - if you have a Castle Nut on the Clutch, you need it:
