Thursday, May 10, 2007

Cam Hard = Chunkie Attacks

Failure to properly prep the cam is one of the most common methods of trashing an engine. (Originally posted in 2003 after being rejected by the usual magazines :-)

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Stock VW engine, you’re looking at more than fifty years of continuous development and production.

Not a lot of secrets in a stocker. Which wasn’t always the case.

When it was introduced the Volkswagen engine violated many Conventional Wisdoms associated with automobile engine design. For example, each lobe of the cam actuates two cam followers. Conventional Wisdom insisted the four lobes on a VW wiggle stick would wear twice as fast as the eight lobes on all other four-banger cams... unless you came up with some way to precisely control the hardness of your cams and lifters, which no one had back then. Oh, there was that new gaseous nitriding process but that only worked with steel. VW cams and lifters were cast iron and no one had come up with an accurate hardening process that was economical enough to be used for mass produced cast iron parts. Except an outfit called Krupp.

Ditto for that magnesium alloy crankcase. Never work, not for mass production. Way too expensive. Unless you can come up with a better method of extracting magnesium from sea water. Like that Dowmettal company. Same story for those crazy molded rubber parts in the torsion-bar suspension system. Just won’t work, unless you can come up with a synthetic rubber that’s actually better than the real thing. Maybe some of that Buna stuff would work... (nowadays we call it Neoprene).

Professor Porsche and his gang of engineers didn’t see such things as limitations, they saw them as challenges and came up with an engine that remains in production today. (You can get new replacement engines from the VW plant in Puebla, Mexico. Pretty good little engines.) And if you liked the torsion bar suspension system on the original People’s Car you’ll find it still going strong under our main battle tank.

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One of the tricky bits on a VW cam is getting the hardness just right. Not a big problem nowadays, thanks to Krupp and Adolph Fry. Today you simply look it up on a chart and set the dials to produce whatever hardness and depth is required, duplicating a process that has been in common use now for more than sixty years. It’s no more difficult than, say, programming your VCR. (Yeah, I know... But there it is :-)

For those of you not familiar with surface hardening, take a look at CAMHARD01. Nowadays there are lots of ways to harden the surface of iron, steel or cast iron but one of the handiest heats the metal in an atmosphere of nitrogen gas. The depth of the hardened surface can be controlled by the temperature to which the metal is raised, how long it stays in the oven, the concentration of nitrogen inside the oven, how the part is cooled and so forth.

Hardening a cam is a bit tricker than most other hardening chores because cams have lots of corners. When hardening a part the corners are exposed to the heat & gas on two sides and tend to become harder than other areas of the part. To give you some idea what I’m talking about go see CAMHARD02.

A brittle corner on a cam can be fatal to an engine. The corners approach the hardness of a diamond (seriously! Nitriding can produce a Mohs hardness of better than 9. [Diamond is a 10]).

Talk about the perfect abrasive! Microscopic fragments of diamond-hard material being chipped off and distributed around the inside of an engine... You can bet your bean-bag it caused the VW engineers more than few headaches before they figured it out.

Corners on a cam? (Someone said.) Where the hell are there corners on a cam!


See Figure 3. That’s a picture of a typical after-market cam, fresh back from nitriding. See all those nice sharp edges? That is where the metal turns a corner. Those edges are so brittle that casual handling can cause them to chip like glass. (Look closely. See that tiny notch near the nose?) Even, worse, see the mold-lines on the cam? (Remember, cams are just high-density cast iron.) The blank comes out of the mold with a chilled hardness that is nearly as good as nitriding (although not nearly so deep). Nitride a chilled-cast surface, you end up with hardness well past 9 on the Mohs Scale and a virtual 100% guarantee of chipping those edges unless you do something about it.

Figure 4 shows the pointy end of the lobe on an after-market cam. It also shows all those un-dressed edges. This is normal for after-market parts. It is up to the person assembling the engine to determine which edges need to be chamfered, by how much and the method most suitable for doing so. For comparison, Figure 5 is a stock VW cam. (Notice that the end of the stock cam is not as sharp.) Note that all of the edges on the stock cam are nicely chamfered.

Take another look at CAMHARD02, the drawing showing how the hardness penetrates the metal. The tip of the lobs concentrates the heat during the hardening process in much the same fashion as does a corner. In fact, the tip of the cam’s lobe has to be harder than its slopes or heel if you want the thing to wear at a slow rate. And if the tip is harder than the heel, you can bet your bippie that the edges of the lobe’s nose are even harder still.

You simply can’t allow fragments from those edges to get inside your engine. Even with a full-flow oil filter such debris still gets one shot at your oil pump. And with stuff approaching the hardness of a diamond, one shot is all is takes.

So we don’t let that happen. And neither did Volkswagen. But I don’t have to tell you that because you can see it for yourself. See those nicely chamfered edges in Figure 5? That’s a stock Volkswagen cam. Look at Figure 6; there it is again. That’s a used VW cam, something I pulled out from under the bench. But you can clearly see the chamfering and, if you look real close, the VW logo cast into the metal.

See the lower lobe in Figure 6? You can see that the chamfer is a bit smaller than on the heel of the upper lobe. The chamfer doesn’t have to be very big if all you want to do is get rid of the chunkies. In fact, a chamfer of only thirty-thou or so is enough to make the edges of an after-market cam safe for society. Sure enough, there’s a picture of a lightly chamfered after-market cam lurking in Figure 7. (Wider would be better. I just whizzed this one up for the photo-op :-) If you’ve never clearanced a cam nor chamfered one, one slip of the grinder can screw the pooch in a major way, as in trashing the cam. I suggest you cover the lobes and journals with masking tape before doing any grinding.

Normally, you chamfer an after-market cam when you grind the notches that allow it to work with a stroker crank. That is, you do all your grinding - and clean-up - at one time, usually in a ‘dirty’ area of your shop. (Engines are always assembled in a clean area. It’s not an operating theater but the assembly area should be cleaner than the average kitchen.) No stroker? Then you can chamfer it any time you wish. (It’s called dressing the edges and is a standard pre-assembly procedure with any after-market part.) Just be sure to clean that sucker to within an inch of its life after doing any grinding on the thing. The idea here is to keep abrasive debris out of your engine. Grind on the cam (or anything else) then use the part without a perfect clean-up simply doesn’t make sense.

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Cams are made from cast iron because it is easy to grind to the required curves. After the lobes are ground, the surface of a Volkswagen cam is hardened to a precise degree. The result is a cam with lobes just hard enough so that the rate of wear for one cam-lobe is compatible with that of the distributed wear across the face of two cam-followers (i.e., the lifters rotate to distribute the wear). This results in uniform rate of wear allowing reliable long-term performance.

The process of surface hardening concentrates the harness along edges and thinner sections. By the time you have achieved the desired hardness in the middle of the piece any sharp edges will have been hardened to the point of brittleness.

Cast iron has a granular structure; harden it to the point of brittleness, it will chip like a piece of glass. But only if you let it. Standard automotive engineering practice is to ensure such debris is never allowed inside an engine.

When hardened debris passes through the oil pump, it will create a scratch or score. Once the metal has been scored, it will not heal. The more times such debris is allowed to pass through the pump, the more wear that will accumulate.

When building an engine, any edge capable of spawning debris is chamfered, rounded, stoned or even polished, as the case may be. ANY EDGE. Throughout the engine. The need for such attention to detail is understood by every competent mechanic. The proof of that need and the practices required is clearly evident by simply examining a professionally built engine.

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There are no secrets in a VW engine. Or so I thought :-)

Are you using an after-market cam? Did you clean it up and chamfer the edges? Gap your rings? Stone the edges? Balance everything? That’s your job, you know; attending to all those ‘unimportant’ details the phony experts brush aside. Because when you build an engine, you’re the Mechanic in Charge.

-R.S.Hoover
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