OEM subcontractors, high-performance specialists and engine rebuilders must take into account and respect precise shape and dimension tolerances while machining valve guides and valve seats.

With this "aide-memoire", NEWEN attempts to gather, within a short document meant for production technicians, the various symbols and tolerance signs typically used by OEMs to define their requirements in terms of cylinder head machining and, in particular, valve guide and valve seat machining.

NEWEN brings together each form and dimension tolerance with direct and indirect incidences on the functioning and reliability of the cylinder head, without attempting to over simplify or denature the combined effects of two or more parameters.

With this document, NEWEN is working on denouncing generally accepted ideas as well as false ideas, widespread ideas that, because of their simplism and inaccuracies, mislead engine rebuilders and alter their judgment.