How Do Pistons And Connecting Rods Attach To A Crankshaft?

Kevin Cameron explains how straight-line up-and-down motion in the cylinder bore is transformed into rotary motion at the crankshaft

I want to talk about how the up-and-down motion of the pistons in their cylinder bores is translated into the rotation of the crankshaft. The wrist pin fixes the piston to the connecting rod, which is assembled on the crankshaft in two split-and-bolted halves. The crankshaft main bearings revolve around their own axis. The crankpins, however, are offset. They describe circles of their own.

Before attaching the piston and connecting rod to the crankshaft, I would add some prelube. Everything in a plain-bearing engine has to be prelubed to make up for the several seconds it takes for the oil pump to send fresh oil around every place it’s needed.

The connecting rod goes on to the crankpin, and the connecting-rod cap is slid into place. Connecting-rod cap bolts are then installed. Those bolts are responsible for making sure the piston obeys orders at top dead center and doesn’t keep going and smash all the valves flat. Instead it is obediently dragged by the crankshaft down on either its suction or power stroke. So connecting-rod cap bolts have to be remarkably strong; the metallurgy in them is usually of the best.

Once the bolts are in place and torqued to the correct preload, the piston rises and falls when the crankshaft revolves. The straight-line motion up and down in the cylinder bore is by means of the wrist-pin hinge point and the connecting rod transformed into rotary motion at the crankshaft.

To anyone who has ever worked with gas turbines or jet engines, the idea of pumping air with pistons just seems hopelessly medieval. But gas turbines have been tried in automobiles, buses, and trucks, and where are they now? The advantage of piston engines is their speed and load are easily controlled.

Until the fabled electric revolution is upon us, we’re stuck with back-and-forth motion of pistons translated into rotary motion at the crankshaft by means of the hinged connecting rod.