Motor Oil and Your Healey (ZDDP)

Originally published in the Austin-Healey Magazine


The times, they are a’changing in automotive technology, but you already knew that.


Modern automobile engines differ from our old Healey lumps in two important ways. First, they have to interact with catalytic converters to remove nasty things from the exhaust so that we won’t run out of clean air to breath, and second, they have fewer points where metal actually rubs against metal anymore so that we can get more power and better mileage from them. What you might not know is that the American Petroleum Institute standards for additives in oils have been changed to make oil better suited for the new cars and the result affects our Healeys.


The major change is that an additive named zinc-dialkyl-dithiophosphate, ZDDP for short, has been reduced significantly in oils specifically intended for new cars. ZDDP, along with some other metal additives like manganese, mix with the exhaust and gradually clog up the catalytic converter, significantly shortening its life and reduced its long-term effectiveness. Since the role of ZDDP had been to minimize contact by providing a protective metallic layer or sacrificial surface, especially where the flat tappets rub against the cams, and that isn’t a critical issue in modern engines, it made sense for the EPA to recommend its removal, and gradually, that’s what the API has done.


The problem is that without ZDDP, an older engine will wear out faster, especially in the tappet to cam contact area. Fortunately, ZDDP has not been removed from all oil, just the types of oil specified for newer engines, and not all brands of oil choose to or need to comply with API standards in all of the types of oil they offer. So, all you’ve got to do is make sure you get oil that still has ZDDP in it when you next change the oil on your Healey, and you’ll be fine, right?


Yes, but there is a bigger, and potentially much more expensive problem that you need to be aware of. The problem is when you rebuild that tired old engine as you do your restoration, or have an engine rebuilder do the job for you. You see, what is a small problem, played out over tens of thousands of miles on an older, long-since run-in engine, is a much bigger problem in an engine rebuild. Without the proper additives, some owners have been reporting total cam/tappet failures on engines in their first 500 or thousand miles, necessitating a second tear-down and rebuild, and replacement of some very expensive parts.


Here’s what I learned while reading the research and information being exchanged on the discussion groups. In a “flat-tappet” engine, where the tappets ride directly on the cams, metal wear is a necessary part of the break-in process on a new engine.


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 broken in, those high peaks wear down from rubbing, but at the start, all of the pressure is concentrated on a very small part of the surface area. In addition, if the rebuild process is done properly, are building paste containing high amounts of molybdenum is rubbed on the cam and tappet tips. 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 possible achieve.


And there, literally, is the rub. The process, which takes place primarily during the first few hours of the break-in period, but continues to take place for several thousand miles or hundred hours after that, requires one other ingredient. That ingredient is ZDDP in the oil used during the break-in period which, in combination with the molybdenum-disulfide in the break-in lubricant paste, creates a continuously renewing metal surface in the low spots that helps spread the pressure. No ZDDP in the break-in oil, and/or a careless engine builder who forgets to use lubricant paste, and instead of a smooth surface after break-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 making the lobe surface of the cam shaft ruinously rough.


So here’s the moral of the story:


On older engines, or engines that have been rebuilt and completely and successfully broken in, be careful not to use the lower-viscosity oils intended for modern engines. These oils are typically 5W- or 10W- weights and have the API seal of approval, and are often also marked “energy conserving.”


Instead, for your Healey the specialists and the oil companies recommend use of higher viscosity oils, such as 15W-40 in synthetic oils, such as Mobil1 or Redline 15W-40, or 20W-50 in non-synthetics, such as Castrol GTX or Castrol High-Mileage20W-50, or Valvoline VR-1 20W-50 Racing Oil. (Update: In February 2008, Castrol released a new formula Syntec in 20W50 which is labeled “For Classic Cars” and contains the necessary amount of ZDDP.)

In addition, oils intended for diesel engines (which don’t have catalytic converters, yet and require ZDDP to minimize gudgeon-pin wear) in the same weights, such as Castrol Syntec Blend Truck 15W-40, Mobil Delvac, or Shell Rotella are also recommended. Finally, the boutique oils -- available from independent vendors: just ask on the online lists for sources -- such as Redline, Royal Purple, Penrite, Amsoil, and Torco, in these 15W-40 and 20W-50 weights are also acceptable, since they are intended primarily for older-technology, high-performance engines and don’t choose to meet API standards.


Equally critical, if you are rebuilding your own engine, discuss the recent changes in oil additives with an experienced engine builder, get the best advice you can from the oil companies and the company that grinds your cam, and educate yourself on the break-in process. If you are having your engine rebuilt, ask your rebuilder to tell you what he’s doing to cope with the materials changes, and if he doesn’t know what you’re talking about, consider finding another rebuilder. Then be very careful what you do during the break-in period.

Basic cautions on engine rebuilding for a flat-tappet engine are to use a moly paste on the cam during the assembly, use an oil that you know has ZDDP and is recommended for engine break-in and/or use an oil additive supplied or recommended by the cam manufacturer, change the oil after 500 miles, and seriously consider using a zinc additive in your oil for the first several thousand miles of engine operation.


Thanks to Keith Ansell, Mike Salter, and Roger Moment for their assistance in preparing this article.


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