Eaton Balancing

“Piston Match Weighing”

By Ted Eaton

Match weighing the piston set is just one of the steps that’s performed as part of having an engine balanced. When balancing a V style engine, this operation must be performed before the crankshaft can be spin balanced due to the piston weight being required as part of the bobweight calculation. Although piston weight matching appears to be a relatively simple and straight forward operation, the machining method in which it is actually performed can jeopardize piston strength or its integrity if not done correctly.

In order to weight match a piston set, essentially the lightest piston in a set is identified and the remaining pistons are lightened through various machining operations so that all pistons end up weighing the same. Piston pins can be either weighed with the piston that it’s going to be used with or all balanced independently so that the pins themselves all weigh the same. Whether the pins are weighed with the pistons or done separately is typically determined by the shops preference. Regardless, either method will not affect the final state of balance if performed with tight tolerances that keep weight variation and any subsequent stackups to a minimum. The typical tolerance for this operation is ½ gram for the whole piston set with the pins in their respective pistons but the closer to zero, the better. Merely lightening the heavier pistons so that they match the weight of the lightest is not the only machining that can be performed at this point. Depending upon the piston design, the potential for additional piston weight reduction can be of benefit in that a lighter piston will subject the connecting rod, connecting rod bolts, and the bearings to less stress as well as reduce the total amount of reciprocating mass. This is conducive to a rotating assembly that can accelerate or increase in rpm’s at a quicker rate due to a subsequently lighter crankshaft which ends up having less stored or kinetic energy in which to release. Additional piston lightening at this point could also make the difference in removing weight at the crankshaft counterweights as opposed to actually having to add additional mass to the counterweights if only match weighing the piston set. Still, the emphasis here is to only lighten the pistons to the point where overall strength is not jeopardized.

Where to take weight out of the pistons depends particularly upon its design. If taking mass out of the piston’s inside deck portion, then rule of thumb dictates leaving a minimum of 0.200″ thickness. While some pistons have as much as 0.600″ deck thickness and give adequate material to work with, other piston designs are already at the 0.200″ thickness value leaving no room in this area for material removal. Blown applications will typically require much more than 0.200″ material in the deck surfaces so this is yet another consideration. Special care must also be taken in the deck areas under the valve reliefs to insure that sufficient material is remaining under the reliefs after machining. Other areas inside the piston in which to work at for weight removal is in the pin boss, both above and below the pin as well as inside the skirt area or behind the piston ring lands. In extreme cases, the whole of the vertical sides of the pin boss can be machined. All these different options are dependant upon the piston design and exactly how much material is available to work with. Some piston designs create quite a challenge depending upon how much weight must be removed in order to merely weight match. To know how much material is available for removal, a gauge or measuring device is necessary in which to know in advance what the thickness is in the area being contemplated for weight removal. The apparatus in Fig. A shows a homemade fixture that holds a dial caliper in which to perform this operation. A dial indicator gauge can also be retrofitted into a similar fixture to do the same measuring operation.

The tools or equipment in which to actually remove material from the pistons for weight reduction purposes can be quite varied. A piston vise or other fixture that will hold the piston in an inverted position while removing weight from it would be a prerequisite at this point so that machining operations can be duplicated within the piston set. The preferred piece of equipment for actual piston material removal would be a milling machine with a moderately sized cutter. The larger the cutter, then the greater the amount of surface that can be removed with a minimum amount of depth. This gives a very good ratio of minimum depth to maximum weight being removed. The use of drill bits for weight removal is discouraged both from the standpoint that not much overall weight is being removed simply by the diameter of the drill bit being used, but also that the drill point hole that is left behind leaves a potential stress riser in the piston for piston failure to originate from. There are also those instances where a lathe can be used for piston lightening depending upon the material available to work from within the piston.

After all the pistons have been weight matched, the final weight is then recorded on a balance card or work sheet for future reference. The piston set can then be cleaned of machining debris at this point and reboxed until actually engine assembly takes place. The next article in this series will cover connecting rod balancing. Until then, happy motoring.

Originally published in Y-Block Magazine, Sep-Oct 2004, Vol 11, No. 5,  Issue #64

“Connecting Rod Balancing”

By Ted Eaton

An engines connecting rods exhibits traits of both rotating and reciprocating mass and hence, must be match weighed end for end to insure these two masses are kept independent of each other. As a point of clarification, the reciprocating end is the small end of the rod or the portion of the rod that is representative of up and down motion in the cylinder while the rotating end of the rod is the bearing end which rotates with the movement of the crankshaft. Your balancing shop will have a rod weighing fixture that’s designed for separating these two masses and then being able to have all the rod small and big ends match in weights throughout the particular set of rods being balanced.

Simply finding the lightest rod in a set for total overall weight and then reducing the weight of all the other rods without any regard to which part of the rod the weight is being removed from to match the lightest does not make for a balanced set. This is because the weight being removed is most likely being taken from the wrong spot on the rod and thus actually making the rods even more out of balance than before attempting to weight match them. This method fails to take into account whether the mass being removed is reciprocating or rotating mass which is a major consideration in a dynamically balanced engine.

Connecting rod balancing requires a fixture that allows each end to be weighed independently. There are several different fixture designs available on the market but all utilize the same concept; each end of the connecting rod is isolated from the other for weighing purposes.

Similar in concept to the match weighing of the pistons, the ends of the rods must be weighed with the lightest small and big end of each rod within a set being found and isolated. Very rarely will the same connecting rod from a factory installed set have both the lightest small end and lightest big end on it. After finding the lightest ends, it is then just a matter of taking the remaining heavier rods and making the ends match the previously found lighter end weights.

Your balancing shop can employ one of several different methods in which to reduce the connecting rod end weights. Typical tools for this operation can vary from using a grinder, belt sander, or a milling machine. The design of the connecting rod in itself can dictate what machining or weight removal operation will be used. Most stock style connecting rods have a balancing pad on each end which is a convenient spot from which to remove material for balancing purposes. Many of the newer aftermarket rods and especially the H-Beam style do not have these balance pads on the ends and do require some forethought before attempting to remove any material from them. For many of these newer designed rods, material from the big end is removed at the rod bolt edge instead of the very bottom. The small ends for rods without balance pads are usually best done on a belt sander using a nice rounding motion in which to remove material evenly from around the pin end. Regardless of the method used for weight removal, it’s important that the metal not be unduly overheated. This may require repeated quenching if excessive grinding must be performed in which to remove the required amount of material. Excessive overheating of the big end can cause out-of-roundness to the big end bore which can prove disastrous to bearing clearances besides affecting the structural integrity of the metal itself on either end.

After all the connecting rods have been weight matched, the reciprocating and rotating end After all the connecting rods have been weight matched, the reciprocating and rotating weights are then recorded on a balance card or work sheet for the upcoming bobweight calculation.  All that remains at this point is to clean the connecting rods of any debris or grinding/sanding residues caused by this particular balancing step and rebox them until engine assembly takes place.

The next article in this series will cover the nuances involved within the bobweight calculation in preparation for spin balancing the crankshaft. Until then, Happy Motoring.

Originally published in Y-Block Magazine, Nov-Dec 2004, Vol 11, No. 6,  Issue #65

“Bobweight Calculation” By Ted Eaton

The previous articles in this series have expounded upon match weighing the pistons as well as the connecting rod small and big ends. Now it’s just time to start thinking about the crankshaft bobweight calculation. The bobweight will be a specifically weighted fixture that attaches to each of the connecting rod journals for electronic spin balancing purposes and will in turn simulate the rod and piston assembly weights for those mass characteristics necessary for a perfectly balanced engine. Like most V8 engines, the venerable Y-Block will require four of these bobweights, one on each rod journal. Each bobweight will take care of the rotating and reciprocating mass requirements for two connecting rod and piston assemblies along with their respective rod bearing and piston ring packages.

With the weights of the pistons and each end of the connecting rods already recorded on the balance job worksheet, there are still some miscellaneous weights required before calculating what the total weight requirement will be for the bobweights. At this time, the weights of the piston rings and connecting rod bearings for one cylinder are needed. This is a simple matter of weighing these pieces on a gram scale and recording their values on the same work sheet or balance card. Piston pin locks are also weighed and recorded if being required on the engine being balanced.

All parts continue to be weighed in grams due to the increased resolution garnered by this measurement system versus that of using ounces. As a for instance, there are 28.35 grams in an ounce and for a point of reference, a typical dollar bill weighs a gram. Saying a dollar bill weighs a gram is much simpler than saying it weighs 3½% of an ounce or 3/85th’s of an ounce. Thus it is grams as they can then be further broken down as fractions or tenths for additional detail or resolution.

The final value required for the bobweight calculation will be a nominal value in grams for the estimated amount of residual oil that resides at any given time within the crankshaft and on any given pair of piston and rod assemblies. Although industry standard for this oil is 2-4 grams, different shops will add an additional amount based on their experience or preference. Some engine designs will even mandate a much higher value due to its engineering attributes that has the crankshaft or its attached components holding more oil than the standard amount within them. An example would be hollow crankshaft rod journals that hold additional oil either by function or machining ease during the crankshafts manufacture. The Flathead Ford V8 crank would be a good example for simplifying the manufacturing process by using oil reservoirs in the crank pins while the 427 Ford steel crank would have even larger crank pin oil reservoirs designed specifically for stored oil in the event of momentary oil pump starvation. The Ford Y-Block crankshaft design is such that the industry standard could be used but an increase in the oil value may be required to simulate some of the other weight variables that can work their way into the mix.

Adding a specific amount of weight for a given bobweight in excess of what is initially called for would be referred to as heavy balancing or being over-balanced. This is done in instances where anticipated weights or forces will be changing either during the course of an engines life or if the rotating and reciprocating mass characteristics are expected to change at a given rpm range or condition.

If a carbon build-up on the piston top was anticipated over the long haul, then this could be also added to the oil value at this point. If you have a preference for a different oil value to be used on your rotating assembly upon getting it balanced, then talk this over with your shop and get their input on this. Most shops will be agreeable to sutle changes in the bobweight values if you have specific preferences.

There are a variety of other conditions which would require “overbalancing” as part of the balancing process. A change in rod lengths or crankshaft stroke can benefit from a given amount of overbalance depending upon the amount of change in rod/stroke ratio. The use of nitrous oxide, superchargers, or turbo chargers typically also requires a certain amount of overbalance. Using nitro methane in conjunction with a blower is likely the worse case scenario as cylinder pressures are extremely high under detonation which artificially increases the piston weight by a more than a normal amount. Any form of blown engine will benefit from a given amount of overbalance simply due to the weight of the piston averaging artificially heavier not only from the increase in cylinder pressure at ignition, but the increase in cylinder pressure taking place while the cylinder is also filling during the intake stroke. In this instance, the piston is averaging an overall heavier weight when running at speed. A normally aspirated engine has a given amount of pressure counterbalance in that the piston is subjected to negative pressure when the cylinder is filling but is under increased pressure during compression and ignition. If an aspirated engine is working with an extremely well designed induction system and is benefiting from a ramming effect to fill the cylinders at the upper rpm ranges, then overbalancing also helps here. And then there’s the rpm factor. Balancing is linear up to a point throughout the rpm range but depending upon the masses at work within your particular assembly, there is a point in which the crankshaft rpm starts to out run the dynamics of the existing state of balance. Overbalance allows these dynamics to stay in tune or “caught up” to the rpm’s of the crankshaft. There are proprietary formulas that calculate these amounts of overbalance for all the different variables and will vary somewhat from shop to shop. Again, talk with your balance shop regarding overbalancing and determine if this would be best applied to your application.

Now that all the rotating assembly’s component pieces have been weighed, it’s time to calculate the amount each bobweight will weigh before building them and attaching them to the crankshaft. To repeat what was stated in an earlier article, a 90° V8 engine will normally require a bobweight that simulates 100% of the rotating mass and 50% of the reciprocating mass. Because a single bobweight is being used for each V8 journal and represents a pair of connecting rod and piston assemblies, the weight of one piston with its pin, ring set, and a single rod small end will be added to the weight of two connecting rod big end weights along with the weight of two complete rod bearings. This in effect will give the required 50% reciprocating (that which goes up and down) and 100% of the rotating mass. The appropriate amount of oil and desired overbalancing is also added at this point.

With the bobweight calculation now being complete, it’s then just a matter of assembling the bobweights on a grams scale to replicate the calculated weights and then attaching these bobweights to the crankshaft in preparation for spin balancing. The next article in this series will cover exactly this. Special thanks goes to Ernie “Bounty Hunter” Phillips in allowing the use of his balance card for his racing Y as an example. Until then, happy motoring.

Originally published in Y-Block Magazine, Jan-Feb 2005, Vol 12, No. 1,  Issue #66