The following how-to was borrowed fromVespa-ciao.nl with permission.
Written By: Herr Flick
Pictures by: Tonni
Dutch to English translation by: FonS
Of course we start by removing the engine from the frame, no further explanation needed.
Remove the flywheel cover, so you have access to the flywheel. Remove it with a flywheel-puller. If you don’t have one, you can press the rubber from the coil-wire inside the crankcase, put a screwdriver through the hole and gently tap against the inside of the flywheel. But always tap, turn it a bit, tap again and so on.
Now you remove the ignition.
Secure the engine by clamping it by the crank shaft in a vice (put some cloth between it to prevent damaging).
Use a torch to heat up the small half of the crankcase. If you don’t have one you can use a paint stripper(heat gun) or you’re mothers stove.
Remove the small half of the crankcase and repeat the procedure for the large half of the crankcase. Try not to heat up the crankshaft as well, and if you don’t have a new seal, be careful not to burn the seal
If everything went well, you now have two half’s of the crankcase and the crankshaft with the bearings on it (they stay in the crankcase as well sometimes).
Now would be a good time to drill the air inlet out to 13mm. How to drill the air inlet out to 13mm
Of course, remember to clean the crankcase well!
Now it’s time for the overhaul. First you remove the old bearings from the crankshaft. Secure it in a vice and use two screwdrivers to get underneath the bearing. Then you use them as a lever to remove the bearing. Start with small screwdrivers and then move to some bigger ones.
Crankcase, -shaft and overhaul kit:
There is more then one way to do this. Ideal is: place the crankshaft in a freezer for a few hours. You could heat the bearings a bit before mounting them (I never do this, cooling the crankshaft is enough). The crankshaft will shrink because it cools down and if you heat up the bearings, they will expand. Now you can easily mount the bearings (don’t forget the SHIM rings!).
Because this method is time-consuming, you can use a bushing and/or a plastic/rubber hammer to gently tap the bearings in place. It’s safer to use the cooling method, so if you have little to no experience it’s better to use the cooling/heating method.
The crankshaft is now ready to be mounted:
Heat the large half of the crankcase well
Put the crankshaft in place and position the gasket as well. You can use two bolts to hold it in place better, like you see in the picture.
Now heat the other half of the case.
When it’s hot enough, it’s easily placed around the bearing. Remember to let it cool for a bit, you can get burned easily.
Now you can fit the case’s bolts. Tighten them evenly and cross-wise. They don’t need to be extremely tight, it’s only M6.
Check if the gasket is sticking out, if necessary, trim it with a razor blade.
Now you place the new seal (don’t do this earlier, because you could burn it when you heat the case). Put it as straight as possible on the case and evenly press it down. No deeper then the edge, or it will touch the bearing.
Reassembling the ignition, preferably with a new condenser and breaker points.
The following pictures are for the assembly of the ignition. This it where everything should go.
When everything is back in place, you need to adjust the breaker points. Align the key (wedge) with the little hammer on the breaker points.
Adjust the distance between the points to 0.45mm using a feeler gauge. Tighten the screw and check the distance. The gauge should be able to slide through smoothly.
Now it’s back to mounting the cylinder and so on.
One last tip: put the flywheel in place and then mount the clutch to secure the flywheel. This prevents breaking the key (wedge) when you mount the clutch after placing the engine in the frame.