Vespa Small Frame Engine Rebuild Part 2

Small Bore Corps

8 chapters8 takeaways16 key terms5 questions

Overview

This video details the process of rebuilding a Vespa small frame engine, focusing on a mild build for a V50 frame. It covers cleaning engine cases, preparing and installing bearings and seals, converting a 3-speed transmission to a 4-speed, reassembling the clutch and kickstarter mechanisms, and installing the cylinder and cylinder head. The presenter emphasizes proper techniques for bearing installation using heat, the importance of correct parts for the 4-speed conversion, and careful reassembly of internal components. The video concludes with the reinstallation of the clutch cover, brake plate, and intake manifold, preparing the engine for an air leak test and eventual installation back into the scooter frame.

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Chapters

Engine cases were cleaned using a three-step process: degreasing, blasting with aluminum oxide, and glass beading for a vapor-honed finish.
Bearings are installed individually using a heat gun to expand the case, allowing bearings to drop in, rather than relying solely on oven heating.
The crankshaft's fly-side bearing inner race was installed with a 1mm gap for easier future removal.
Two-piece outer races are used for easier installation compared to one-piece races, which can be difficult to remove.
The weld method is used to remove stubborn one-piece outer races from the case.

Properly cleaned and prepared engine cases ensure a good seal and longevity of new components. Careful bearing installation prevents damage and ensures smooth operation.

Heating the case with a heat gun and having the bearing 'literally just dropped right on'.

The input shaft bearing is installed, often requiring a bearing and seal driver kit to be tapped into place.
The clutch side bearing for the crank is heated and lightly tapped into the case.
A snap ring secures the clutch side bearing.
The bearing for the clutch basket/primary drive is heated and tapped into place, secured by another snap ring.
The output shaft bearing is installed, noting the difference in inner diameter compared to the crank bearing.

Correctly installing these bearings and their retaining rings is crucial for the smooth rotation of the input and output shafts, which are fundamental to the transmission's function.

Using a bearing and seal driver kit from Harbor Freight to install the input shaft bearing.

Rubber engine mounts and the rear shock mount are pressed into the case.
The clutch side seal for the crank is installed with the spring facing inward.
The crankshaft is installed, ensuring it seats properly in the seal and bearing.
Kickstarter rubber bushings and the kickstart quadrant are installed, replacing worn-out components.
The shifter mechanism is reinstalled after replacing its O-ring and lubricating it.

These components are vital for starting the engine and selecting gears. Proper installation and lubrication ensure smooth engagement and prevent premature wear.

Replacing the O-ring in the hole for the kickstart quadrant and lubricating it before reinstallation.

A Pini 2360 primary drive is installed, requiring the bearing to be heated and forced into place.
A snap ring secures the primary drive.
The kickstart gear and spring are installed over a cone washer.
The conversion to a 4-speed transmission requires specific parts: an ET3 gear stack, an ET3 cruciform, and a 4-speed input shaft.
The cruciform differs between Primula and ET3 models, and the correct one must be matched to the gear set.

This section details the upgrade to a 4-speed transmission, which enhances the scooter's performance and versatility, and the installation of the primary drive which transfers power from the crank to the transmission.

Using an ET3 cruciform which has four notches (first, neutral, second, third, fourth) compared to the older Primula one which has three.

The output shaft gears are carefully stacked, ensuring the taller side of each gear faces upwards.
The cruciform is installed onto the output shaft using a special tool to compress the balls and spring.
Gear stack tolerances are checked using feeler gauges (1.5mm drag is acceptable).
The transmission is installed into the engine case, ensuring the primary drive's pawls engage with the cruciform.
The two engine case halves are joined, aligning the input shaft with the gears, and secured with bolts.

Correctly assembling the transmission gears and ensuring proper tolerances are critical for smooth shifting and preventing the gears from popping out of engagement.

Using a tool with an indent to hold the balls and spring while pushing the cruciform over them to install it on the output shaft.

The small sprocket for the primary drive is installed onto the crank, requiring heating for a snug fit.
The input shaft is installed into its bearing in the other case half.
A new gasket is used to seal the case halves, lubricated with grease for easier disassembly later.
A new crankshaft with a 51mm stroke is installed.
The piston is installed on the crankshaft, ensuring the arrow on the piston points towards the exhaust port (upwards on small frames).

These steps assemble the core rotating components of the engine, including the crankshaft and piston, which are essential for combustion and power generation.

Ensuring the arrow on the piston points upwards, towards the exhaust, when installing it onto the crankshaft.

New, longer studs are installed for the 90cc+ cylinder.
The cylinder base gasket is coated with sealant and the cylinder is slipped onto the studs.
Port edges are smoothed with a Dremel to remove sharp burrs.
The cylinder head is installed, with the spark plug facing forward, and torqued to specification.
The clutch is assembled inside the case due to space constraints, using a tool to compress the clutch spring and plates.

Properly installing the cylinder and head creates the combustion chamber, while assembling the clutch allows for power transmission to the rear wheel.

Smoothing the sharp edges of the cylinder ports with a Dremel tool before installing the cylinder.

The clutch nut and crank nut are torqued to specification and secured with lock washers.
A clutch cover modification (grinding) is necessary because the aftermarket primary drive interferes with the stock cover.
The clutch cover is reassembled with a new gasket and bolts torqued.
The brake plate is reassembled with new O-rings on the cam and larger Primula brake pads for a 5-lug hub conversion.
The rear hub is installed with a spacer and new nut, awaiting final torque and cotter pin installation.

Completing the assembly of external components like the clutch cover and brake plate, and performing functional checks, ensures the engine is ready for installation and operation.

Grinding material off the inside of the clutch cover to provide clearance for the aftermarket primary drive.

Key takeaways

1Engine case cleaning is a crucial first step for a successful rebuild, impacting seal integrity and component longevity.
2Using heat is an effective method for installing bearings, allowing them to drop in without excessive force.
3The 3-speed to 4-speed transmission conversion requires specific, correctly matched parts, particularly the cruciform and gear stack.
4Careful attention to gear stacking order and tolerances is essential for proper transmission function and to prevent popping out of gear.
5Lubrication is vital during assembly, especially for seals and moving parts, to prevent damage during initial startup.
6Aftermarket parts, like the Pini primary drive, may require modifications to fit with stock components, such as the clutch cover.
7Torquing fasteners to the correct specifications and securing them with lock washers or by bending tabs prevents loosening due to vibration.
8Even seemingly minor details, like the orientation of piston arrows or the placement of ring gaps, are critical for engine operation.

Key terms

Engine CasesBearingsSealsCrankshaftInput ShaftOutput ShaftKickstarter QuadrantShifter MechanismPrimary DriveCruciformGear StackClutchClutch CoverBrake PlatePiston RingsC-clips

Test your understanding

1What is the three-step process used to clean the Vespa engine cases, and why is each step important?
2How does the presenter recommend installing bearings, and why is this method preferred over solely using an oven?
3What specific parts are required for a 3-speed to 4-speed transmission conversion on a Vespa small frame, and what is a key difference between Primula and ET3 cruciforms?
4Why is it important to check gear stack tolerances after assembly, and what is the acceptable range mentioned in the video?
5What modifications might be necessary when installing aftermarket components like a Pini primary drive, and how are these typically addressed?