Practical tips – replacement and repair
What do I need to watch out for when replacing and repairing valves?
When replacing valves, it is very important to check the valve guides too. Consequential damage to valves can often be caused by old and worn valve guides. Also learn about standard values for valve guide clearance.
The following valve damage and sealing problems can be caused by a worn valve guide:
- Burnt through valve head
- Breaks in the fillet of the valves
- Breaks in the grooves for the valve cotters
- Excessive wear of the valve stem faces
- Worn rocker arms on one side
- Increased oil consumption due to worn valve stem seals
- Worn or burnt through valve seat inserts
|Stem diameter||Clearance: Intake valves||Clearance: Exhaust valves|
|5 to 7 mm||10 - 40 μm||25 - 55 μm|
|7 to 9 mm||20 - 50 μm||35 - 65 μm|
|9 to 12 mm||40 - 70 μm||55 - 85 μm|
For valves with a skirt diameter of less than 8 mm in particular, knocked out valve guides have a big impact. In case of a knocked out valve guide, the valve head rests eccentrically on one side of the valve seat and is pulled into the seat with the clamp force of the valve spring(s) (Fig. 2). This can cause the valve to break over a longer period of time. Due to the resulting alternating bending stress, the valve breaks at the intersection of the skirt and valve head.
For valves that are operated via rocker arm or finger type rocker, knocked out valve guides result in an increased radial application of force on the valve stem. As a result, the valves break in the groove area for the valve cotter halves.
Installing new valve guides
Due to the press-in procedure, the valve guides might be slightly constricted radially on the inside diameter. The extent of this distortion depends on the ratio between housing bore diameter and outside diameter of the guide. Furthermore, the rigidity of the two components can play a decisive role. If there are strong differences in the rigidity of the housing wall, the radial distortion can differ quite significantly along its length.
When installing and removing the valve guides, ensure correct warming up of the cylinder head (specified by engine manufacturer). Use suitable mounting mandrels. The additional cooling off of the valve guides facilitates assembly.