Replacing ball bearings on the Vespa engine

When the engine is open, such as during an engine overhaul, it is advisable to replace the ball bearings. This guide applies to Vespa PK and Vespa V50 engines. Of course, the workshop is also applicable to other engines such as LF engines of Vespa PX 80 to 200ccm, Cosa, Sprint, etc., and also relevant for the assembly and disassembly of bearings in other areas such as the steering tube swing arm. If you are disassembling the engine for the first time, then watch the revision of a Vespa PK 50 engine.

Removal of the old ball bearings

To protect the bearing seat, the area around the bearing is heated with a hot air gun. Our experience has shown that a hot air gun is more gentle on the engine block compared to a gas flame and also allows for more even and effective heating.
Some bearings can be easily knocked out, while others may not. The generator-side auxiliary shaft bearing of the Vespa V50 and PK 50 usually comes out together with the auxiliary shaft when the entire housing half is heated in the oven for a few minutes. Temperature recommendations vary, so no recommendation can be given. However, a little temperature is necessary. After all, pizza doesn't taste good at 50°C. Use only a special workshop oven and ensure good ventilation.

While the clutch-side ball bearing on the drive shaft can be relatively easily removed, the needle bearing on the rear wheel shaft of the Vespa PK 50 and V50 is not easy to replace. Even with heating, it doesn't just come out of its bearing seat. A professional method is to use an inner bearing puller. The mass of the slide hammer pulls the bearing out gently and without much effort. Another method of removal is by using a suitable expanding plug, available at well-stocked building supply stores. First, the bearing cage is cut open at the top and all needles are removed. Then, the plug is inserted and expanded. When the plug is pulled out, it should theoretically pull the bearing out with it. This method only makes sense with an additional screw welded to the plug for the slide hammer and is still not always successful, as the plug may slip out of the bearing more easily than the intended inner bearing puller.
The third method is grinding out. With some practice, the bearing housing can be weakened at two points so that the sorry rest can be spooled out with pliers without leaving any traces on the bearing seat.

Note

It is not recommended to push the bearing out by supporting it on the housing. We have seen many cases where housings were severely damaged by this method.

Installation of the new ball bearings

New ball bearings can be installed gently by heating the bearing seat evenly as during removal. The ball bearing goes easier into the bearing seat if it is additionally cooled down. Shrinking the bearing temporarily (only while it is cold) by several hours in the freezer makes the bearing go in easier. A good alternative is a freezing spray. While the freezer takes several hours for -18°C, the freezing spray reaches -40°C in a few seconds. Spray each ball bearing generously and then insert it into the heated bearing seat.

Sometimes the bearing will just fall in. Then, that's it. However, bearings can also resist falling in easily from time to time. There can be several reasons for this. Perhaps the bearing seat was not sufficiently heated, or the bearing was not sufficiently cooled down. Sometimes the bearing seat is slightly worn or distorted, which does not necessarily have negative effects during driving.
In that case, the bearing can be knocked in via the outer ring (Caution: never hit the inner ring). For this purpose, a drift punch in combination with a heavier hammer with a large striking surface (e.g., a sledgehammer) is suitable. On the one hand, not much swing is needed, and on the other hand, the surface of the sledgehammer is so large that you can focus on the tip of the drift punch instead of the striking surface of the hammer. If the drift punch lands between the inner and outer ring in the balls, then the bearing is only a case for the discard pile.

A revision kit for Vespa 50 and PK 50 can be found in the shop. Individually configurable.

Summary and Tips for High Mileages

Installation

Proper installation is a prerequisite for high engine mileage and protects bearings, engine housings, and nerves.

Bearing clearance

The use of ball bearings with recommended bearing clearance also optimizes mileage. Highly stressed bearings such as the auxiliary shaft bearing have normal or no additional bearing clearance. This heavily stressed ball bearing would quickly get more play with increased bearing clearance from the factory, which would lead to increased wear and premature bearing failure.
The two standard ball bearings on the crankshaft are exposed to higher speeds, but in the Vespa V50 and PK, they are not subject to radial loads (unlike the PX), and with increased bearing clearance, they have a longer service life.

The retrofit bearing NU204 brings slightly more play into the system due to its design. For lightly tuned engines to protect the bearing seat during repeated engine splits, the NU204 is fine.

For particularly high speeds, the NU204 with increased bearing clearance is advisable.

Encapsulation

An open design has the property that it is exposed to the environment, in this case, the fuel-oil mixture or gearbox oil. Oil is used for lubrication in an engine, not grease. Areas for the use of grease include, for example, the O-ring on the kickstarter shaft or the O-ring on the shift fork. The ball bearings mounted in the crankshaft area should be open to be fully flowed through and lubricated by the fuel-oil mixture in the crankshaft space.
On the drive shaft, a single-sided closed ball bearing is alternatively possible. The encapsulation is very easy to pry out.