Dismantling and assembling pistons, cylinders: VW (ACD, AS)

ACD
Cylinder
mark before removing
Check clearance between piston and cylinder => fig.
6
and
7

Piston rings
Offset gaps by 120 °
oil scraper ring gap must lie upwards
removing and installing with piston ring pliers => fig.
1
Installation position => fig.
2
.
measure ring gap => fig.
4
measure ring-to-groove clearance => fig.
5

Retaining ring
Piston pin
If difficult to remove, heat piston to approx. 60 °C

Piston
Weigth difference max. 10 g
Identification => fig.
3
.
check clearance between piston and cylinder => fig.
6
and
7

Fig.1
Removing and installing piston rings

Fig. 2
Checking piston ring seating

Top faces towards piston crown

a -
Upper piston ring
b -
Lower piston ring
c -
Oil scraper ring

Fig.3
Marking of the piston

A -
The character "A" next to the arrow corresponds to the spare part number index of the reepctive piston. It is used as distinguishing feature.
B -
Arrow (stamped). Install piston in direction of arrow to the flywheel.
C -
Pair size indication is made by a colour point (blue, pink, green).
D -
Weight class indication (+ or -) stamped.
E -
Weight class identification by a colour point (brown= - weight, grey = + weight).
F -
Piston size indication in mm

Fig. 4
Checking piston rings gap

Push ring squarely down to approx. 4 - 5 mm from cylinder wheel.

Gap clearance / in mm · Wear limit in mm
Piston ring top · 0.30 - 0.45 · 0.90
Piston ring bottom · 0.30 - 0.45 · 0.90
Oil scraper ring · 0.25 - 0.40 · 0.95

Fig. 5
Checking pistons rings to groove clearance

Clearance / in mm · Wear limit in mm
Piston ring top · 0.07 - 0.10 · 0.12
Piston ring bottom · 0.05 - 0.07 · 0.10
Oil scraper ring · 0.03 - 0.05 · 0.10

Check clearance between piston and cylinder

Fig. 6
Checking cylinder bores

Measure cylinder with internal measuring instrument. Adjust order of magnitude in a gauge (micrometer).

The measurement is carried out approx. 10 - 15 mm below the cylinder upper edge.

Fig. 7
Checking piston diameter

The nominal piston value is stamped on the piston crown top. The measurement is carried out on the lower shaft end vertically to the piston axis.

New installation clearance: 0.04 - 0.06 mm

Wear limit: 0.2 mm

The cylinder subdivision with the corresponding pistons is made in three different magnitudes according to the following table:

Magnitude · Colour · Cylinder / ø mm · Relevant piston / ø mm
Normal magnitude / Nominal dimension ø 85.5 mm · blue / pink · 85.492 - 85.508 / 85.502 - 85.518 · 85.45 / 85.46
1stoversize / Nominal dimension ø 86.0 mm · blue / pink · 85.992 - 86.008 / 86.002 - 86.018 · 85.95 / 85.96
2nd oversize / Nominal dimension ø 86.5 mm · blue / pink · 86.492 - 86.508 / 86.502 - 86.518 · 86.45 / 86.46

If the clearance is approaching the value of 0.2 mm when measuring the piston and the respective cylinder, pistons and cylinders must be replaced as a pair and a set of the same magnitude (normal magnitude or oversize) must be installed.

The weight difference of the pistons in one engine must not exceed 10 g.

Pistons whose cylinder is worn must not be renewed separately. If the respective cylinder of a damaged piston is not worn, it is often sufficient to install a new piston of the corresponding pair size.

As the compression ratio must be maintained when installing grounded cylinders, the assigned oversize pistons have smaller values (dimension piston crown/piston crown eye).

Note:

Cylinders and pistons of the same magnitude must be installed in one engine only.

Whether new pistons and cylinders are installed depends not only on the wear check but also on the engine oil consume.

If the consumption rate is more than 1 litre/1000 km it is in general necessary to look over the engine.