Cracking The Engine Builder's Code MEASUREMENTS TO KNOW:

1. Deck height: This is the dimension from the crankshaft centerline to the head face of the block.
2.Crank throw: This is the radius the rod journal sweeps out as it rotates around the main journal centerline.
3. Crank stroke: This is twice the crank throw and represents the amount the crank moves the piston up and down the bore.
4. Rod center-to-center length: Usually referred to as the rod length
5. Piston compression height: Sometimes also called the pin height, this dimension refers to the distance between the center of the wrist pin and the top surface of the piston that makes a close approach to the cylinder head face.
6. Swept volume or Cubic Inch Displacement (CID): This refers to the amount of air the cylinder is capable of drawing in as the piston moves from the top of the stroke to the bottom.

Components To Know:

A. Block deck
B. Cylinder bore
C. Piston
D. Wrist pin
E. Ring belt
F. Connecting rod
G. Rod journal
H. Crank counter weights
I. Main journal

The SCAT Q-Light super crank shown here is the kind used in a maximum-effort engine such as an 840hp Nextel Cup unit. Such a crank seeks to satisfy three distinct criteria. Firstly, it must be strong and highly fatigue-resistant to endure the ultra high loads over extended periods. Secondly, it must have the minimum overall mass and the lowest moment of inertia possible to enhance vehicle acceleration. Thirdly, its windage should be as low as possible to cut internal aero drag and viscous losses. We will take a tour of this crank and detail its design philosophy.


1. The heat treat on the 4340 alloy gives it a very hard surface with a tough core. The super-hard surface, in conjunction with a micro-polish finish, ensures very low wear rates on all bearing surfaces.
2. Mass in and around the center of the crank is of no aid toward balancing the reciprocating components. To keep the crank light, the counterbalance mass needs to be concentrated as far from the crank centerline as possible, hence the undercut crank webs to form a pendulum counterweight.
3. The form of the web connecting the rod journal to the main journal is critical for a maximum performance crank. Too thin and the crank breaks, too thick and the counterweight necessary to balance it goes up. Everything in and around the rod journal needs to be light.
4. This is an extension of the last point, as a hollow journal means less mass is necessary on the counterweight to balance it out. Also, contrary to what you may expect, a hollow journal, if correctly done, is actually stronger than a solid one. A key ingredient here is the radius between the hole and the web of the crank.
5. From the front end of this crank it can be seen just how much the counter weights are cut away to reduce overall mass.
6. The star-form flywheel flange is mostly to reduce the crank's overall weight, although it does contribute a small degree toward moment of inertia reduction.
7. The aero leading edges of this crank cut aero drag and viscous losses. This is a big deal for crankcases that run at near atmospheric pressures. For a highly scavenged, low-pressure crankcase, such as in an all-out engine, the gains from aero mods are significantly less because the density of the air within the crankcase is less than half of normal.
8. The fillet radius on a crank has far more to do with its fatigue life than does the journal diameter. This being the case, attention to the fillet radius is important. Not only should it be as large as possible but also very finely polished to avoid stress risers.