Hylomar 'M' was originally sold as the 'Racing Formulation'. It uses an acetone based solvent, and it is a little tackier than the Hylomar Advanced Formulation, or 'AF' which has no solvents at all. Hylomar M will stiffen up slightly as the acetone evaporates, becoming a sticky, waxy material. The Hylomar AF uses no solvent, and it has the consistency of honey. Run a bead on an oil pan gasket, and it will flow out slightly over time. Neither one will harden. As a gasket dressing, the AF will allow something like a pan gasket to be used over and over again-perfect for parts that are removed for service regularly. If you have a joint where to finely machined surfaces are clamped together without a gasket, the M is better as it will form a thin film whereas the AF can be totally squeezed out from between the machined surfaces. Every shop should have a tube of each. After you have tried them, you are not likely to use anything else on your car.
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