Puch VZ50 Engine Overhaul

This page shows some photos and provides some help to a basic strip-down and rebuild of the Puch VZ50 engine and gearbox.

VZ50 Gearbox internals

To remove the engine from the bike, the exhaust, and carburettor were removed.
Both the pedals were also removed so the chain could be disconnected.

The three engine bolts were then undone and the engine supported whilst the bolts were withdrawn from the chassis.

3 engine bolts (circled)

With the engine on the bench the side panel was removed to reveal the clutch mechanism.

The clutch thrust bearing was removed by first unclipping the circlip, to reveal the main clutch bolt.

Removing the clutch bearing

On the other side of the case, the housing over the flywheel was removed.

To undo the flywheel centre bolt, clutch centre bolt, primary gear bolt and extract the flywheel, the piston was locked inside the bore by passing some thick rope through the spark plug hole. This provided a secure but soft way to stop the engine turning.

Undoing the layshaft gear bolt

These photos show the clutch dis-assembly steps.

Removing clutch top

Removing clutch spring

Remove spring cage

Removing clutch plates

Lift clutch basket off shaft

Under the clutch assembly there was a thrust washer and circlip to remove.

Finally on this side of the engine, the washer and circlip from the pedal shaft were removed followed by the primary gear.

Pedal washer and circlip

On the flywheel side of the engine an extractor tool was used to pull the flywheel off the crank.
Behind the flywheel the stator plate position was marked to try and retain the engine timing and then the three screws were removed to lift the stator plate clear,

Stator plate and screws

Also on this side of the engine the circlip holding the main output sprocket onto the shaft  was removed followed by the gear and a second circlip behind.

Before splitting the crankcase, the foot change mechanism and cylinder head had to be removed.

Removing the foot change mechanism

Cylinder removed

To split the crankcase, the seven crankcase screws and two stand-off bolts were undone.

Then the crank case was eased apart.

Crank case split

The crank assembly was inspected for wear. The big end bearing should have no radial play at all, although some side-to-side movement is acceptable
Replacement of this bearing is a job for a two stroke specialist to press the crank apart and re-align it on assembly.

Any play on the little end bearing requires the bearing and gudgeon pin to be replaced as they are matched pairs.

On this crank there was pitting under one of the crank seals. This was repaired with a speedi-sleeve kit for a 17mm shaft.

Speedi-sleeve kit

image of crankshaft damage

Speedi-sleeve fitted

In addition new crank seals were fitted.

Crank seal orientation is different on the left and right sides of the engine.
In both cases the open side of the seal should face the clutch.

The following photo sequence shows the rebuild steps for the gearbox.

First the right side casing was prepared by sticking the 14 loose roller bearings in place with grease.

The crank was Inserted too.

Roller bearings in place

The output shaft parts were assembled as shown

Output shaft assembled

The 3rd speed gear was added.......

.....then the bronze bearing washer

The input shaft assembly was next taking care to ensure the second gear collars were meshed as shown.

Adding input. Note interlocking collars

The pedal shaft was inserted ensuring that the friction spring was located correctly in the housing features.

Adding the pedal shaft

The layshaft was installed next.

Layshaft location

The gasket was installed so the total gearbox cluster looked like this.

Gearbox fully assembled.

The left side of the case was prepared with another two sets of roller bearings, one with 14 rollers and one with 18.

Left side of the case

Patience was used to drop the left side case into place without loosing any of the rollers.
The case was then secured with the seven screws and two stand off bolts.

The foot change mechanism was set to the neutral position and the selector fork too, before refitting the foot changer with a new gasket.

The selector is in neutral when the detent shown is engage. It is the middle detent in the row of 3 that are close together.

Approximate fork position for neutral

If the piston has been removed it must be replaced with the arrow pointing towards the exhaust port of the cylinder.

Correct piston orientation

The cylinder was replaced in the correct orientation shown using a new base gasket and new cylinder head gasket.

Correct orientation should have the slightly longer fins positioned at the bottom when the engine was on the bike,

Torque for the cylinder head was 7.2 ftlb

Correct cylinder hear orientation

The main driving gear and clutch assembly were replaced.

Then using rope in the bore to lock the engine, the main driving gear and clutch nut were tightened.

Then the clutch thrust bearing was installed.
Primary gear torque = 36.2 ftlb
Clutch centre nut = 16.6 ftlb

Before replacing the engine side cover, the circlip and shims were placed on the pedal shaft.

As the side cover was lowered the clutch release bearing and cam mechanism were orientated to latch together.

On the other side of the engine, the stator, flywheel and flywheel cover were all replaced (not forgetting the woodruff key under the flywheel).

If the timing was not marked during re-assembly then the process here describes how to set up the timing,

With the engine back on the bike the gearbox was filled with SAE-30W gear oil until it came out the level plug.

Carburettor and exhaust we reattached and the pedals connected to the shaft with cotter pins.

Gearbox level plug