Five Ways To Break The Nose of a Crankshaft - Engine Builder Magazine

Five Ways To Break The Nose of a Crankshaft

Scat Crankshafts has determined that 99 percent of the time it is the parts that are being used in conjunction with the crank and the additional machining done to the parts that are being used with the crank that are mostly responsible for crank breakage, NOT the crank.


• Champher machined at wrong angle. See Figure 1.

• Champher machined with too small an angle. See Figure 1.

• Belt drive gears. The seal sleeve bottoms to the face of the main before the inner face of the gear bottoms against the step in the nose of the crank. All of the above prevents the crank gear from bottoming against the step on the nose of the crank. This leaves a gap between the gear and the step, which allows the crank to flex. A fatigue crack starts – SNAP! The crank breaks. See Figure 1.


Fluid, balls, springs, inertia rings with rubber O-Rings, etc, can you balance a wheel on your race car if the tires are flat? How can your rotating assembly be balanced if to quote one manufacturer: "These units (Dampers) should not be on the crank for balancing as the inertia weight may not be centered until the engine starts." Centrifugal force will always take the inertia weight off center no matter what rpm. Your assembly is never balanced. Metal transferred on nose outside diameter and damper internal diameter will fatigue and start to crack, resulting in a broken crank.


Rotating weight multiplies as rpm increases. Engines have heavier or lighter balance weights and larger or smaller noses. RPM above 5,500 is more risky on a Small Block Chevy than a Big Block Chevy. However, as rpms go up, the weight will try to leave the crank due to centrifugal force. Do not be surprised if at some point fatigue sets in and the nose comes off.


Multi-stage oil pumps, blowers, etc all have belt drives that require torque taking off at 90 degress to center line of the crank. More torque is necessary for driving these things and further away from main bearing support all leads to multiple of leverage wiggling the nose. Fatigue sets in, nose breaks, blower stops. The Small Block Chevy has the smallest diameter nose and the weakest of all. Note: Blowers take substantially more 90 degree torque than dry sump pumps, therefore, more likely to break noses. Not recommended for Small Block Chevy. If a blower is being used, use a crank with a Big Block nose.


The counterweights on a crankshaft are designed to work all together as a system within a certain bob weight range. To correct the balance on a crank where the counterweights are too heavy the following should be followed:

Internal Balance: If more than 2 holes are required in each end, the outer diameter of all the counter weights should be turned in a lathe to correct the out of balance condition in all the counterweights. If you try to drill more holes, you will create a secondary wave which will lead to crank flex and eventually a fatigue crank. See Figure 2.

External Balance: The crank is spun with the external balance and flywheel. If it is determined that the assembly is too heavy where the weight is on the damper and flywheel, do not make the correction on the end counterweights of the crank. The out of balance condition is in the damper and flywheel, which is where it should be corrected. It is very simple to alter the bolt on weight of the damper and drill the balance weight on the flywheel. If these components need to be replaced simply bolt on the proper weight to the damper and match balance the flywheel which has to be balanced anyway. If you correct in the end counterweights, you will create a wave in the crank which will wiggle the nose of the crank which well eventually start a fatigue crack which will snap the crank.figure 1Figure 2

– Tech Tip courtesy of Scat

You May Also Like

Shop Solutions February 2023

Check out February’s shop tips and tricks.

Engine and machine shop tips and tricks.

Dowel Removal

This is an easy and clean way to remove hollow dowel pins found in connecting rods or mains. Start tapping the I.D of the dowel with an NPT tap. Once it starts to cut, it will spin the dowel, and with upward pressure, the dowel can be removed and reused.

Honing the LS

Modern honing techniques are as much a performance concern as they are part of engine design, and that’s thanks to the much tighter tolerances and specifications engines need these days. 

LTR Engine Build

This Late Model Engines build is centered around Concept Performance’s new LTR block, which is the first aftermarket as-cast aluminum Gen V LT block. 

LS Intake Manifolds

LS swaps are popular for many reasons, but there are a lot of variations and details to sort through – more of them than you may expect – and many of them are associated with the intake manifold.

LS Cylinder Heads

The LS engine is known for its cylinder heads, and there are tons of options available to upgrade the factory components.

Other Posts

Choosing the Correct Block for Your LS Engine Build

Whether you’re scouring junkyards, ordering cores, investigating factory options, looking at aftermarket cast iron or aluminum blocks, or spending big bucks on billet LS blocks, you’ve probably noticed it’s been harder to find exactly what you want for the foundation of your LS build than it historically has.

Open Loop/Closed Loop and Learning

Closed-loop control can be programmed to either add or subtract up to a certain percentage of fuel in order for the engine to reach the target air/fuel ratio.

Shop Solutions January 2023

Next time you have set of large journal small block Chevy connecting rods to resize, consider honing the big ends of them for a +.002” outside diameter bearing that the LS engines with fracture cap rods use.

Shop Solutions December 2022

Everyone misses occasionally, and this helps avoid dents and damage.