What is a "crank throw" and how can I determine the number of crank throws on an engine?

Question

This term is confusing me a little and I wanted to make sure I understood it correctly.

Based on my understanding of the Wikipedia article, it looks to me like a crank throw is what allows the piston to translate (or "throw") linear motion to rotation of the crankshaft.

Also, what I infer from this article on firing orders is the number of unique crankpin offsets in a complete cycle will determine the number of crank throws that the engine is characterized by.

So a flat-plane V8 has two crank throws, while a cross-plane V8 has four crank throws. Have I correctly understood it?

Flat-plane V8

Cross-plane V8

Answer 1

There are two basic usages for the term "crank throw". Both have to do with the crankshaft:

• The crank throw is a measure of the distance from the center-line of the main journal to the center-line of the rod journal. This distance could be considered the radius of the crank arm. Twice this measurement should give you the stroke of an engine.

• Due to the basic nature of the distance described above, the crank throw is also considered the rod end of the crank shaft. In most V-8 engines, two connecting rods share a "crank throw" and there are four throws to a crankshaft. In most inline 4-cylinder engines, a single connecting rod is on it's own throw and there are also four throws on a crankshaft.

In your examples of either cross-plane or flat-plane crankshafts, both of them have four throws each.

There is such a thing as a split-throw crankshaft. GM used such a crankshaft in their 90° V6 engine. Here is an image of a split-throw crankshaft:

In the image you can see how the journal is split. While being on the same crank arm, the journal (also called a "pin") is split. They will have the same throw (or radius), but are offset from one another slightly.

Answer 2

Crank Throw

A synonym to a crank throw is a crank pin. Which is a an offset journal on the crank.

If the offset journal shares a connecting rod with another cylinder you would say the crank-pin has one throw for the two cylinders.

• The F1 V6 engines are considered 3 throw engines. The off set journal is shared by two connecting rods.

• IF the described F1 engine had a unique crank-pin (offset journal) for each connecting rod then it would be a 6 throw engine.

• The Yamaha R1 engine from 2009 and the M1 MotoGP ICE are both cross plane crankshafts with 90 degree "throws" or crank-pins. This is a four throw motor.

• The standard 4 cylinder motorcycle has a 180 degree flat plane crank. This is a two throw motor.

TTBOMK crank throw does not include any measurement of distance.

Here is an image of various crank throw arrangements*

image and citation from the US Navy, Sweet Haven Publishing Systems