“Internal Versus External Balance”

By Ted Eaton

When getting an engine balanced, it’s important to note that there are two different methods in which to have the engine balanced, either internally or externally. As the Ford Y-Block family of engines are all internally balanced as part of the factory design, this is not expected to be an issue but for other engines it’s a subject worth touching base upon here briefly. External balance refers to when the damper, flywheel, and/or other rotating parts outside of the engine block are counterweighted and must be installed as part of the crankshaft assembly to insure the balance of that particular rotating assembly. Whenever possible, the customer should opt for internal balance unless the cost is prohibitive or not effective for the result being desired.

There are two basic reasons an engine becomes externally balanced as part of a factory design. The first is through evolution where an existing engine series was originally designed as internally balanced but has an increase in stroke to the point that the crankshaft counterweights can not be made any larger within the confines of the block in which to compensate for the additional mass requirements. This then requires additional mass to be attached at the flywheel and/or the damper to supplement the existing crankshaft counterweight mass in lieu of a complete new engine crankshaft or block design. The second reason would be in initial engine development and design where the crankshaft can be made lighter which in turn uses less of the costly nodular material typically used to strengthen cast iron. This then allows inexpensive materials to be used at the flywheel and damper to provide for the additional required rotating mass.

There are two inherit flaws in the factory external balance designs. The first would be the stackup of balance variances due to three pieces (flywheel, crankshaft, & damper) that must match up in balance after being independently balanced of each other. Second would be the forced out of balance at high rpm’s this type of balance promotes due to a given amount of non-evenly distributed mass being located in a non-rigid manner outside the confines of the block. This then allows a given amount of flex or twist in the unsupported ends of the crankshaft at that point. The more the counterbalance on the ends, the greater the flex or distortion throughout the crankshaft at a given rpm and the potential for crankshaft deflection or breakage at high rpms.

Although not recommended, there are instances where a balance shop will externally balance an engine that was originally internally balanced. These occurences most often originate from substituting parts that are much heavier than the originals such as heavier connecting rods and/or pistons which causes the crankshaft counterweights to be too light in which to compensate inexpensively. Another instance is where the crankshaft stroke is being increased while the crankshaft counterweights are remaining the same. Although externally balancing as a result of either scenario is performed as a cost savings measure in both time for the shop and expense by the customer, a major problem presented by an externally balanced engine is the inability to change out the modified balancer or flywheel with alternate units at a later date without having the assembly rebalanced. Rather than externally balancing an engine in such a circumstance, there are other options rather than repurchasing lighter components. One such manner is to balance the crankshaft internally through the use of Tungsten, Mallory metal, or other heavy metal in the crankshaft counterweights in order to make them physically heavier. And yet another option depending upon the crankshaft design is to use lightening holes in each of the rod journal throws which will in turn make the crankshaft counterweights artificially heavier without actually having to add weight. Ultimately, cost is typically the deciding factor on what method is used to retain internal balance characteristics in lieu of leaving an engine externally balanced.

Ideally, the engine is best balanced internally which allows replacement dampers and flywheels to be balanced as separate units at a later date to a zero state and placed back on the previously balanced crankshaft with a minimum amount of upset to the crankshafts previous state of balance. The next article in this series will cover in detail how the piston set is match weighed and machined by your local balance shop.

 

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Here’s a Ford Y-Blk damper that’s been externally balanced!!!  This should be avoided as relacement of the damper without rebalancing the crank becomes extremely difficult.

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All these SBF standard transmission flywheels are externally balanced to the 28.4 oz Ford specification.

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All three of these flywheels will fit either a SBF or 300 Ford Six but all three exhibit different states of balance depending upon the engine and its application.

 

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This SBF flywheel has been balanced to the Ford 50 ounce external balance specification.

Originally published in Y-Block Magazine, Apr-May 2004, Vol 11, No. 2, Issue #62