Zddp In Motor Oil

ZDDP in Motor Oil

Talking about motor oils. Because of the ongoing changes to the formulation of oils intended for use in modern engines (and specifically those equipped with catalytic converters) it is more important than ever that the correct oil is used in classic car engines. Faced with achieving very challenging targets for emission reductions set by the U.S. EPA, motor manufacturers have been putting increasing pressure on oil suppliers over the past ten years to remove substances from engine oils that will lower the useful service life of catalytic converters. One of the substances they have pressured the oil suppliers to remove from engine oils is the additive ZDDP (zinc dialkyldithiophosphate) because ZDDP, along with some other metal additives like manganese, mixes with the exhaust gases and gradually clogs catalytic converters, significantly shortening their life and reducing their long term effectiveness. As a consequence the ZDDP level in ‘modern engines’ has been declining since the mid 1990’s - which is a major problem if these oils are used in older classic car engines.

Without ZDDP an older engine will wear out faster, especially in metal to metal contact areas such as the tappets and cam lifter interface. ZDDP has an excellent track record of protecting the sliding metal-to-metal cam lifter interface, and for more than 70 years has been added to engine oils in amounts approximately 0.15% phosphorous and 0.18% zinc. ZDDP protects by creating a film on cams and flat lifter contact points in response to the extreme pressure and the heat at the contact point. The film of Zinc and phosphorous compounds provides a ‘sacrificial wear’ surface that protects the metal of the cam and lifter from wear.In the course of normal service the conversion of ZDDP to zinc and phosphorous compounds depletes the ZDDP level in the oil and studies show that depending on the specific engine and severity of duty, after 2000/4000 miles of operation then level of ZDDP in the oil can drop below that considered adequate to provide wear protection to the cams and lifters. It is therefore extremely important for the longevity of classic engines that they are filled with an engine oil containing ZDDP and that the oil is changed at the recommended mileage intervals.There is however a potentially much more serious (and very expensive) problem with using a ‘modern’ engine oil in a classic engine that has just been rebuilt- as has been reported recently in the US on comparable engines.This is total cam/tappet failure after only about 500-1000 miles of opeation.When an engine is first re-assembled, even with the most careful machining of the cam and use of the best quality replacement tappets, the rubbing surfaces are still pretty rough (under a microscope you’d see high peaks and deep valleys in the metal surfaces).As the engine is run in, these high peaks wear down, but at the start, all of the pressure is concentrated on a very small part of the surface area.

If the rebuild process is done properly, a rebuilding paste containing high amounts of molybdenum is rubbed on the cam and tappet tips during re-assembly.This additive lubricates the surfaces until the high points can be removed, resulting in a surface that is a hundred times smoother than any machinist or metallurgist could possibly achieve. And this, is the potential problem.The ‘smoothing’ process, which takes place primarily during the first few hours of the running in period - but continues to take place for several thousand miles or hundreds of hours after that- requires one other ingredient. That ingredient is ZDDP.

ZDDP, in combination with the molybdenum-disulphide in the break-in lubricant paste, creates a continuously renewing metal surface in the low spots that help spread the pressure.
With no ZDDP in the running-in oil, or no lubricant paste, instead of a smooth surface after runnin-in, the peaks on the surface of the tappet can literally weld themselves to the cam, causing pieces of the tappet surface to flake off, and ruining the lobe surface of the cam.

Interesting reading indeed guys, and so apart from looking at what E numbers your food contains, in future it will be as well to check the additives in the motor oil you are using or your tappets may be ground down, not a nice way to go!

Melvyn Ford

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