Puch VZ50 Fork Rebuild

One of the Puch rear shocks was leaking oil so they were both reconditioned using the following process.

Rebuilt shock with new spring

With the shock removed from the bike, a 22mm open ended spanner was pushed between the spring coils to access the flats on the top cap.

Accessing the top cap nut.

By rotating the top of the shock including the spring it was possible to lift the shock workings out from inside the coil spring.

Extracting the shock inner workings

The oil contents were collected form the main shock housing and the inner workings and measured to be 40ml

Oil from the rear shock.

This image below shows the contents of the piston assembly.

The impact driver was used to release the piston from the end of the plunger rod.

With the piston removed the parts were removed from the plunger rod and laid out to keep them in order.

Parts from the top of the shock absorber

On this bike the leak was due to a corroded plunger rod. So it was planned to replace it.

The rod was extra tight in the upper mounting but was unscrewed with a long bar in the mounting eye and the rod gripped in the vice.

Unscrewing the top mounting

The rod was slightly under 10mm diameter but the thread had been rolled on to make it large than the shaft.

Rod diameter 9.79mm

Thread diameter 10.54mm

The plan was so make a new rod from 10mm steel.
It would have a 10mm thread at the top which would necessitate a helicoil into the top mounting.

So the top mounting was drilled and helicoiled for a standard 10mm x 1.5mm metric thread.

Helicoiled top mount

The lathe was used to copy the plunger rod dimensions but using a 10mm diameter rod and M10 x 1.5mm external thread.

The internal thread the other end was a Metric M7 x 1mm thread, at least 29mm deep.

The small 1mm oil hole was only drilled on one side, through into the threaded cavity.

Old and new plunger rods

The dimensions used for the replacement rod are shown below.

The top cap and guide bush were both reamed out to 10mm for the new plunger rod.

The seal in the top cap was a tighter fit on the larger rod, but was found to slide OK.

Reaming the top cap

The parts were assembled on the new plunger rod in the original order.

The reassembled shock parts.

Replacement springs were unavailable but some substitute ones were found from some cheap motorcycle shocks.

The spring rate on the Puch shocks was measured using the bathroom scales and the drill press to push down on the soring.

The results was that the Puch spring rate was 2Kg.mm

The replacement spring (shown here) had a rate of 1.8Kg/mm

Measuring the spring rate.

The replacement springs were the correct length, wire gauge, coil count and ID.
So for a lightweight rider were judged to be acceptable.

To source similar shocks, these were made by Forsa and were part number 807E.

New and old spiring

the rebuilt Shock