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Crank Journal: Installation Procedures For Placing Heavy Metal Into Crankshaft Counterweights
By Duane Boes
We’re all familiar with the old saying, "We learn from our mistakes." Recently, I was able to learn from someone else’s misfortune. I make plenty of mistakes of my own, so taking a lesson without the expense was a welcome change.
Installation procedures for placing heavy metal into crankshaft counterweights is a simple procedure to explain and implement. If things are done right, there is little to fear from a shaft with metal placed in the counterweights.
However, when a heavy metal slug does begin to move, things can get ugly fast. Consequently, proper installation steps to prevent a slug’s movement are important to know and understand.
Counterweights are subjected to fairly simple forces. Their vibrations, on the other hand, are quite complex. These vibrations can create strange reactions in a slug placed into a counterweight. The photo below is an example of something unpredictable happening.
Notice how the slug has rotated within the counterweight. The pen in the photo points out a portion of the slug’s O.D. that has been exposed through the balance hole. This can only happen if the heavy metal begins to turn.
You would expect a slug to slide one way or the other should it begin to lose its press fit. But to see it rotate is completely unexpected. Interestingly, while this piece of heavy metal rotated roughly .200", it shifted very little. For me to try and explain these vibrations would be a case of trying to explain more than I really understand.
Diameters are "Job #1" when it comes to installing heavy metal. The best insurance against a costly problem starts with the press or interference fit between the heavy metal and the hole it’s placed in. Slugs of 1" diameter require between .002" to .004" interference. To maximize the holding power of this interference fit, both the O.D. of the slug and the I.D. of the hole need to be as round as possible.
Even with every effort possible being made, the slug and the hole will not match perfectly. To overcome this variable I recommend using Loctite 640. I have tested the advantage of using this product several times; the results have been consistent.
Without the use of such a bonding agent a 1" diameter slug installed with a .004" press fit will require nearly seven tons of force to be placed upon it before shifting occurs. With the bonding material, the amount of force needed to shift the slug jumps to roughly 15 tons. Once the slug has been broken free, the force required for continued movement drops to seven tons.
In consideration of the oily environment and operating temperatures, Loctite Corp. recommends its 640 compound. The product can be used very sparingly and still be effective. Anyone who is not comfortable with their press fit, and is staking the O.D. of the slug, should seriously consider using some type of bonding agent.
The problem shown in the photo to the left is the direct result of poorly planned placement and drilling. A slug’s placement is nearly as important as its fit. Figure 1 below is a drawing of recommended minimum section thickness for both the outer and separating walls of a counterweight that is expected to securely hold a piece of metal.
An adequate amount of material must be provided between the slug’s O.D. and the circumference of the counterweight. You can visualize that in the case of a section that is too thin; the counterweight’s material will stretch either during the installation or while in operation due to the centrifugal force. In either event the press fit is reduced, opening the door for unwanted movement.
The same type of deformation can occur at the separating wall. In situations where two slugs are adjacent, the wall is holding one-half of each slug. In this system, the separating wall must withstand a load equal to the total weight of one slug at maximum rpm.
Drilling locations are the next point of concern. The root cause of the problem in the photo stems from a poorly planned drilling into the counterweight. The wall thickness dimensions detailed in Figure 1 may seem a little extreme. Unfortunately, it’s not uncommon to find yourself in a situation where the weight to be removed will fall directly on a series of slugs. In these instances the extra material will be good insurance.
Figure 2 illustrates the effect that drilling has on the counterweight material responsible for holding the heavy metal in place. Attention has to be given to these drill locations. The object being to remove weight without significantly weakening the interference fit between the slug and its surrounding counterweight material. This can be accomplished by limiting drilling to the locations detailed in Figure 2.
In summarizing heavy metal installation, there are four important points to remember. First, make sure your press fit is adequate and that the hole and slug are round as possible. Second, use some type of bonding fluid to fill voids, ensuring a solid footing. Third, make sure the metal is placed into the counterweight in a manner that will maintain a good press fit. Fourth, and last, be careful not to drill out the counterweight material responsible for holding the slug.
A number of engine rebuilders work on modified big block engines. Most of these engines are balanced externally from the factory. In OEM applications external balancing presents few problems. This situation, however, changes as rpm and compression ratios are increased.
In an externally balanced system number 2 and 4 main bearing loads are significantly increased. Externally balanced crankshafts that have broken as a result of a fracture starting at the main bearing side of the overlap are common. Most of these broken cranks also exhibit signs of main bearing wear particularly on numbers 2 and 4. This is the result of higher loads placed on those mains.
The simplest cure for this on higher horsepower engines is an internal balance. In cases where this is not an option, the crankshaft should be heat-treated. By placing a hard wear layer on the journal, bearing life is significantly increased.
The photo below is a close-up of a broken big block Chevy crankshaft. The fracture on this shaft began at the main bearing and progressed toward the rod journal. The photo also shows heat discoloration on the main bearing. The discoloration is uniform around the journal circumference.
This type of heat is consistent with a bearing that is running in distress. Heat generated on a broken shaft is not uniform around the journal; the heat from this type of failure is localized in the areas of interference as the shaft is wedged into the block.
On this crank, the primary failure was the loss of an acceptable bearing surface due to wear. Clearances opened, oil left the journal at a rapid rate taking with it the film barrier, resulting in the generated heat.
A customer’s perception of the problem will usually begin and end with the failed component. For the rebuilder, however, being able to explain why the component failed will always require a much deeper understanding.