The engine head is a critical piece of the structure and mechanics of your average (and not-so-average) automobile. In addition to performing several critical functions, it can (unfortunately) also be one of those things that Goes Wrong, requiring an injection of ca$h and/or elbow grease to fix.

What is it?

The engine is constructed (usually) out of two or three major bits. The central bit is called the engine block; it's essentially a whacking great piece of various metals and/or alloys. The cylinders are bored through the block, and the pistons travel in the cylinders. This means that the cylinders need something to seal the top end of them off, as well as a surface to put the valves in - and the engine head is our servant. Typically, the engine block contains threaded studs coming out of the top - and onto this, a highly customized plate of metal is bolted. This plate is the head - so called because it sits atop the engine. Note: Transitional Man informs me the proper name is cylinder head!

The head is not a simple bit of metal. First of all, it may or may not be constructed of the same metal as the rest of the engine block (note: an engine block without a head is typically referred to as a short block in 'Murcan auto circles). Second, it contains many critical cuts which allow it to do its job (At least, it may; there are various types of heads with different characteristics. Read the excellent writeup below for more info!) Each engine valve is installed into the head. The valves are each set into a socket on the head that matches their shape; usually, a liner of some softer and/or ablative metal (called a valve seat) is placed into this socket to absorb the impact of the valve closing. This allows wear and tear to occur without damaging the head itself; also, if a valve is ever replaced, this allows the seat to be replaced as well since the new valve will be shaped differently than the old one.

In addition to the valves, there are additional holes in the head. Usually there are cooling passages, which are small tunnels drilled in both the block and the head through which the engine coolant passes before heading for the radiator. This produces a constant flow of heat away from the head and block out to the radiator and hence the air; this in turn allows the engine to run without seizing, deforming or melting. These cooling passages tend to consist of grooves in the head which, when assembled, connect two separate holes in the block. This allows coolant to flow out and around the block passages without requiring piping or other complications.

Finally, there may also be oil galleries in the head. These serve much the same purpose as the coolant passages, but rather than transferring radiator coolant they pass oil through the block to where it is required for lubrication. The oil also serves to pump heat out of the block; high-performance cars, or those driven habitually in hot environments, tend to have separate oil coolers installed. These are nothing more than a radiator for the engine's oil supply; the cooling will stave off the lubricant's tendency to break down at high temperatures and stresses.

So, what goes wrong with it and how do I tell?

There are not that many things that can go wrong with the head, given that it's a fairly basic piece of metal. When something does go wrong, however, it's usually a Very Bad Thing(tm). First, of course, the head can fail structurally - crack, for us peons. This is Bad. A cracked head can and will do one or all of the following:
  • Vent a(some) cylinder(s) to air, destroying its compression and rendering it useless
  • Allow coolant to travel into the cylinders and/or oil galleries, and oil into the coolant passages
  • Fluids (oil, coolant) may leak away even if they do not cause direct problems.
  • Valves can stop working, damage themselves, or even tear loose from the engine.
It's usually very easy to determine that there's something wrong if your head is cracked. Severe loss of power, smoke/steam from the engine, loud unfamiliar noises, burning smells - all will let you know that you have a large crack. But what if it's a smaller one, or even a problem with the ridges on the lower side that doesn't compromise the entire piece? Finally, the head is usually sealed to the block with a piece of softer material called a head gasket - problems with the head gasket can sometimes do just as much damage (although they're usually cheaper to fix!)

Broken head gaskets or coolant passages are perhaps the most common non-catastrophic problems to which the head will contribute. If your car is suffering any of the following:

  • Lack of power
  • Missing or stalling
  • rough, unsteady idle
  • White smoke in the exhaust

The way to confirm this is quite simple. Stop the car and the engine, and have a look into the oil reservoir (either the oil pan or even just under the valve cover where you add oil to the engine). If you have a cracked head (or block) or perhaps head gasket, then oil and coolant will be mixing in the engine. When coolant gets into hot engine oil, it forms a thick light-colored emulsion, somewhat resembling mayonnaise. If you see anything mayo-like inside your radiator OR under your oil cap, then beware! This needs to be fixed ASAP. One unfortunate effect of mixing glycol compounds (as in coolant) with petrol (as in lubricant) is that the two will quickly form highly acidic compounds which do all forms of nasty things to metal parts. See your car care professional.

If you hear clattering, lose power, hear backfiring and the like, then there may be a problem with your valves. Make sure there's oil on 'em, close the valve cover and head for home - there's not much you can do to realign/repair valves on the road.

Gotchas and Tips

Although a blown head gasket sounds (and can be) quite nasty, it is in fact (if caught quickly) a relatively minor problem to resolve. If you have an older car, and are on the road and need to fix this, it's technically possible to do so. Note that newer cars, with more closely machined tolerances and a much more 'NO USER SERVICEABLE PARTS INSIDE' design philosophy, usually will punish you for trying this. The head can be removed with any decent wrench (usually after removing valve lifters and other such stuff). There exists a material called gasket paper which is essentially a roll of flat sealant material; you can place your head on the gasket paper, scribe around it, and 'cut out' a head gasket. This is a risky do, and is recommended only for EXTREME emergencies, especially if you're in an area that offers roadside assistance!

One other notable warning: the head is almost always attached to the block using very specific torque loading on the bolts. See your shop manual for specifics. Tightening the bolts down using loading other than that specified can result in blown gaskets, bolts working loose, or even damage to the head and valves.

There are currently five major varieties of engines in use and their heads vary quite markedly.

Two-stroke engines are the simplest. They have no valves in the head or cylinder, as the gases are controlled by the piston covering and uncovering ports on the sides of the barrel. The head is usually just a flat piece of metal with a hole in it for a spark plug.

Sidevalve engines were most popular in the 1930s, and under the guise of flathead designs, survived into the 1970s especially in large-capacity American designs. They don't have valves in the head, they have them in the inlet and exhaust tracts. This means that the head is easy to machine, but limits the compression ratio that can be achieved, and thus limits the efficiency. They're called flathead engines because the head is flat.

Overhead valve engines are the next most complex; they use an arrangement of tappets and pushrods to allow the crankshaft to more-or-less directly operate the valves in the head of the engine. They are capable of much higher compression than sidevalve engines but the manufacture of the head is much more involved. You can't actually see the head on an overhead valve engine because it's obscured by the rocker cover, which is where all the gubbins for adjusting the valves and their pushrods lives.

Overhead cam engines put a camshaft or pair of camshafts (in dual overhead cam engines) above the head. This greatly reduces moving part count and removes a lot of need for adjustment. The camshafts live underneath the cover on top of the head. Because the valves are operated directly from the cams, there's a lot more room for more valves, bigger valves, and more variation in timing is possible.

Hydraulic valvegear is the Next Big Thing. It promises to bring head complexity back to sidevalve levels while giving all the benefits of overhead cams. We'll see, if and when they actually get it working...

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