The demand for constant quality (Cpk) and precision for new cylinder head production highlights the inadequacy of the principle of tapered pilot as an acceptable solution for valve seat machining. This solution, long appreciated for its economical aspect, no longer responds to current technical requirements.

The axis/position taken by a tapered pilot within a valve guide is never the same as the one that would be determined by measurement and/or that taken by a valve while in movement (refer to drawing). This difference is further accentuated by the shape defect of a new or used valve guide (please refer to the graphical representation of a new valve guide by a Talyron measuring machine).

The random positioning of the pilot within the valve guide goes against the notion of Cpk so developed today within all OEMs.

Without forgetting that even in the case of a simple repair, the very dimension of the pilots required for current engines is not sufficient, in any case, to resist to the irregular cutting efforts of form tools.

NEWEN refutes energetically the position of the defenders of this obsolete technique.

Likewise, the form tool exhibits too long a cutting profile to guarantee a regular cut over 360 degrees (evenly around the seat).

The value of the inside relief can vary on a scale of 1 to 3 on one same seat and the radial efforts absorbed by the spindle will inevitably trigger the actual flexing of that latter and will translate into a form defect of the valve seat such as chatter marks, undulation and/or oval forms that will make the machining of the seat incorrect and out of tolerance.

Current valve seat materials and required tolerance are no longer compatible with this machining technique.