A pushrod is a component of the valvetrain of certain piston engines. As its name implies, its function is essentially to push the valve open.
In the traditional layout for an automobile engine -- say, a V8 in this example -- the crankshaft is located at the "bottom" of the engine, with the camshaft above it, in between the two banks of cylinders, being driven at a 1:2 ratio, usually by a timing chain. The lifters (or followers), one for each intake or exhaust valve, sit against the lobes on the camshaft, which push them up at the appropriate time. Each lifter is attached to a pushrod, which transfers the motion upward to a rocker arm on the cylinder head, then downward to the valve itself.
Many newer engines, especially smaller four-cylinder designs, have one or more overhead camshafts with lifters that directly actuate the valves, eliminating the need for pushrods. This has the advantage of reduced mechanical complexity and lower rotating mass, which potentially gives the engine a much higher redline. However, pushrods can still be found (chiefly in larger engines) even in auto racing applications, such as NASCAR, where the league rules mandate certain design parameters for all engines. In these cases, engine builders often try to reduce the length of the valvetrain (and hence, the drag on the engine) by the tactic of keeping the camshaft as close to the valves as possible, making the pushrods very short and light.
Generally, very little goes wrong with a pushrod unless it gets bent, in which case it would fail to open the valve far enough, or at all. This kind of thing is more apt to occur in horizontally-opposed engines, which often place the pushrods below the engine in harm's way to all kinds of road debris. In the best of cases, repairing this problem should be inexpensive enough, but could wind up costing you several hours of labor if your car requires the engine head to be removed, or if more extensive valve train damage has resulted. To illustrate this, we can use the more colorful example of the air-cooled Volkswagen engine, which you can think of as a V4 (half a V8) with the two cylinder banks spread out until they are 180 degrees apart, which gives you a so-called boxer engine. Also, in the VW, the camshaft is on the bottom, driven by a gear instead of a chain. Because of the absence of a water jacket, the pushrods are housed below the cylinders in very thin aluminum pushrod tubes, held in place by the tension of accordion pleats at each end, like a flexible drinking straw.
So here's the thing: Sometimes a rock or some other piece of debris will bash in one of the tubes (especially if you have removed or lost the cooling tin which normally sits below them). This could bend the pushrod, or worse, cause a major oil leak. Many people are tempted to replace the stock aluminum tubes with flashy two-piece chrome-plated ones held on by jam nuts, which JC Whitney and other parts houses enthusiastically sell, but amazingly, this often causes the engine to overheat. Here's why: The original Third Reich-era engine designers, in their endless quest to save materials and complexity, made the tubes so thin because they realized that as the hot oil splashes up into them, hanging out in the cooling air stream, it helps cool the engine! Can you believe those Germans?
The aftermarket two-piece tubes have one important plus, however: they can be installed without removing the head, which means that if one of your V-Dub's pushrods somehow gets knackered, and you have a spare jam-nut tube and some decent tools with you, all you have to do is:
- Remove the valve cover on the affected head.
- Carefully take off the rocker arm assembly.
- Slide the suspect pushrod out through the hole in the head. Roll it on a flat surface to see if it's bent, and replace if necessary.
- Remove the crunched tube (you can even snip it in half with bolt cutters or similar) and put the split tube in its place.
- Put the rod back in, and reinstall the rocker arms, making sure to obey the torque specs (by whatever means available).
Wasn't that fun? They sure don't make them like that anymore.
Sources: John Muir's indispensable How To Keep Your Volkswagen Alive, and a very old LISTSERV posting from VW guru Bob Hoover, reproduced in countless places.