Skoda Workshop Service and Repair Manuals

Reworking valve seats Special tools and workshop equipment required t  Depth gauge t  NAC milling cutter for reworking valve seats   Note t  When carrying out repairs on engines with leaking valves, it is not sufficient to machine or replace the valve seats and valves. It is also necessary to inspect the valve guides for wear, particularly on engines with a high mileage → Chapter. t  Rework valve seats only sufficiently in order to obtain a proper contact pattern. Calculate the maximum permissible reworking dimension before commencing. If the reworking dimension is exceeded, the proper operation of the hydraulic valve clearance compensation is no longer ensured. If this is the case replace the cylinder head. Calculating maximum permissible reworking dimension –  Insert valve into the guide and press firmly against the valve seat.   Note If the valve is replaced when carrying out repair work, use a new valve for the measurement.

–  Measure distance between the valve stem end and the upper face of the cylinder head. –  Calculate max. permissible reworking dimension from the distance measured and the minimum dimension.

Minimum dimension: inlet valve	7.6 mm
Minimum dimension: outlet valve	7.6 mm

„Measured distance“ - „minimum dimension“ = „max. permissible reworking dimension“. Example:

Measured distance	8.0 mm
- Minimum dimension	7.6 mm
= max. permissible reworking dimension → Remark	0.4 mm

1)  The max. permissible reworking dimension is shown in the figure for reworking the valve seats as dimension „b“. reworking valve seats

Reworking inlet valve seat

a	= Ø 28,7 mm
b	= max. permissible reworking dimension
c	= 1.5…1.8 mm
Z	= Bottom edge of cylinder head
α	= 45 ° valve seat angle
β	= 30 ° top correction angle
γ	= 60 ° bottom correction angle

Reworking exhaust valve seat

a	= Ø 25,0 mm
b	= max. permissible reworking dimension
c	= approx. 1.8 mm
Z	= Bottom edge of cylinder head
α	= 45 ° valve seat angle
β	= 30 ° top correction angle
γ	= 60 ° bottom correction angle

Work procedure Reworking can be carried out by hand while complying with the following conditions: l  Wear limit of valve guides must not exceed the permissible dimension → Chapter. l  Use NAC milling cutter with carbide metal tips (min. 90 HRC). l  Press on the milling cutter using slight pressure and mill in such a way that an even removal of swarfs is ensured over the whole working surface.

Reworking valve seats with NAC milling cutter –  Place cylinder head on a felt base and secure to prevent it from turning. –  Match diameter of guide drift to diameter of valve guide.

Valve guide	Ø Guide drift in mm
Inlet valve	6,0 - 0,01
Exhaust valve

–  Match diameter of milling cutter to diameter of valve seat.

Valve seat		Ø Milling cutter 90° mm	Ø Milling cutter 120° mm	Ø Milling cutter 60° mm
Inlet valve		32	32	21/34
Exhaust valve		30	30	21/34

Milling sequence 1 -  Mill valve seat with 90° milling cutter until a perfect contact pattern is achieved. (Do not exceed maximum permissible reworking dimension!) 2 -  Chamfer top correction angle with 120° milling cutter until the valve seat diameter -a- ( → Anchor) is achieved. 3 -  Mill bottom correction angle with 60° milling cutter until valve seat width -c- ( → Anchor) is achieved.

–  Grind in valve/valve seat with fine grinding paste so as to achieve a perfect contact pattern -arrows-. –  Check contact pattern e.g. with water colour (perfect contact pattern over entire circumference). –  Install valve springs. –  Inspect valves for tightness. The tightness of the valves can be checked by filling petrol into the inlet and outlet canal (no petrol must flow out at the valve seat). After completing the repair measure once again the distances between the valve stem ends and the upper face of the cylinder head and calculate the maximum permissible reworking dimension → Anchor.   Note If the reworking dimension is exceeded, proper operation of the valve gear is no longer assured and the cylinder head must be replaced.