The MGA With An Attitude

I only write this because some people insist on asking my opinion on the subject, even though I may deem it a non-issue. This should be the only page on my web site to make any mention of ZPPD, and I hope to avoid any other public discussion.

First of all, what is ZDDP? Zinc dialkyldithiophosphates are a family of coordination compounds that feature zinc bound to the anion of dithiophosphoric acid. If you're curious about what all that means, you can look it up yourself. For the purpose of BBS chat, this is commonly referred to as "zinc" in oil. The stuff is an anti-wear additive, especially beneficial at the interface between camshaft lobe and tappets in a flat tappet engine.

The reason for removing ZDDP from engine oil is NOT because the stuff is hazardous to your health (like lead was). It is because some government regulatory agency thinks the the miniscule amount that might sneak past piston rings to invade the combustion chamber may foul a catalytic converter. You may recall that catalytic converters have been in use since 1975, and 3-way catalysts have been used since 1981, so we may well wonder about the validity of that issue. All reasoning (or unreasonableness) aside, the fact is that the amount of ZDDP in engine oil has been reduced rather dramatically in recent years.

Nearly all modern engines use roller tappets which are not much affected by the reduction of ZDDP, so this is a hot issue only for vintage flat tappet engines. For a description of the design and function and wear of flat tappets you may review CM-101 and CM-102. The significant point is that flat tappets are not actually flat but are very slightly crowned on the interface end, and they are mounted slightly off center from the cam lobe. This causes the tappet to rotate in operation to distribute wear more evenly over the entire end surface. As the tappets and cam lobes are run in from new they both wear slightly with initial run in resulting in line contact between these parts. This presents a high pressure contact point that can be a long term wear point. In this case "long term" means that in normal circumstances the cam and tappets should last at least 100,000 miles (and hopefully longer).

Bear in mind that ZDDP is not the only anti-wear additive available, so removal of some of the ZDDP from engine oil does not necessarily make your flat tappet engine into junk. The critical period for cam and tappets is during the initial break in run, and perhaps through the first oil change. This is when it may benefit from elevated levels of ZDDP in the oil. Once the cam has survived the initial run in, and maybe the first 1000 miles, it should do quite well with modern motor oil.

The last new cam and tappets I installed was done in late 2000. This is a Crane Cams fast street cam with a little extra lift and heavy duty valve springs, so the cam and tappets are subject to slightly higher loads than stock issue parts. Initial assembly included grease on the cam lobes and 10W40 oil fill. Run in was 20 minutes at 2500 rpm, after which it was put into normal service. For me normal service includes a moderate amount of city trips, lots of expressway speed cruising, some significant amount of trailer towing, and a few brief bursts of autocross competition (sometimes revving past the red line). I run 20W50 oil in summer, 10W40 oil in winter, and no special additives. This engine has now accumulated 75,000 miles with no oil related problems, and the cam and tappets are still in good condition. As said, standard modern oil and no special treatment.

In recent years there has been a rash of poor quality "off-shore" tappets on the market. I once had a set that was not even ground on the bottom end (sent back of course). There are likely lots of cases where a pro shop might build an engine, but then the car owner may install and drive it without the proper run in procedure. There will be select cases of sub-standard assembly work. I am always suspicious of hardness and quality of any re-ground camshaft (so far have had no reason to use one). In the end I suspect that most of the reported cam failures are not the fault of the oil used or the reduction of ZDDP in the oil.

One should exercise proper care when installing and running in a new cam and tappets, but I don't believe any special additives are required for modern engine oil to serve well in a vintage engine. There is nothing wrong with using zinc additives or oil with higher zinc content if you don't mind the bother and if the "peace of mind" is worth the added expense. For cars that predate catalytic converters there will be no issue with emissions controls. Otherwise I figure that much of the public discussion bordering on religious war is often blown out of perspective.

For a good article on modern issues with flat tappets and modern motor oil, check out the February 2009 issue of Hot Rod magazine.

In this last (off-site) article, notice this comment:
"Motor oil industry sources maintain that even with their significantly reduced antiwear additive content, the new oils still pass standard industry tests that measure valve train wear (including with flat tappets). But cam grinders counter that the type of heat-treat used on the reference test cams was atypical of standard industry practice.Who's right on this one is hard to determine".

The inference I get from this is that "standard industry practice" today in camshaft regrinding and tappet manufacture may be sub-standard. If you can manage to procure good quality parts it should be fine.

Thank you for your comments -- Send e-mail to <Barney Gaylord>
© 2010 Barney Gaylord -- Copyright and reprint information