“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