Balancing pistons? How much is too much?
 

To make a real long story short, here is what I have. Last winter, I built a fresh 565. Due to fuel availablilty issues this Summer or lack there of, I am forced to lower the compression this winter. Knowing this was coming, over the Summer I ordered a custom set of pistons from Wiseco. They where supposed to copy my existing pistons exactly, expect for a different dome. I ordered them with same ring package, pin location and weight. I figured the first set was Wiseco, so they should be able to make me a matching set. And thats what they told me when I ordered the new pistons. " No problem, we can match them up exactly"
I just tore down my engine for what was supposed to be a simple piston swap. Not trusting anyone, I weighed both sets of pistons. To my surprise, the new set weighs 7 grams more than my old set. I dont want to pull the crank and add heavy metal to compensate for the additional weight of the new pistons. The idea was to have 2 sets of pistons set up, with different compression ratios, for the same crank. While i have assembled many engines, I have never balanced a rotating assembly. I dont know what is an exceptable tolerance for difference between the crank counter weight and piston/rod assemblys. I beileve that a 7 gram difference is too high for safe running. I looked into lighter wrist pins, but it seems the next lighter pins are 13 grams lighter. Too light for me. That would put me 6 grams too lite. Wiseco told me they cant help me. That 7 grams is no big deal, just run it. I asked them if they could mill off at least 5 grams, they said no. The pistons should not be milled at all. According to the piston bulld sheet, the decks are .280 " thick. Is that enough to mill out a few grams?
Any ideas for me???

even our stock rebuilds get balanced to within 1-1.5 grams on both the big and small end of the rod. all our pistons are done to a .5 gram of each other.

7 grams is no big deal. Any engine builder that tells you 7 grams will cause problems is simply bowing smoke.
Balancing an engine is not like balancing a tire. The bob weight of a 90* V8 is based off of a theoretical equation. It is a comprimise. Overbalance works better on high RPM engines, underbalance works well on low RPM oem stuff.
I've seen an engine lose a 50g slug of tungsten during a race. It looked like someone shot a 50 cal shot from the inside out. The only reason we knew it happened was because there was a puddle of oil under the car after a pit stop. The hole was patched during a pit stop (luckily it was a dry sump). After 200-300 miles at 8800rpm the engine finished the race with no problems. The bearings looked beautiful, no signs damage. The bottom line is: If balance is close, it will be fine. :)

Did you weigh the pistons and the pins together? They may have got you to the right weight using a different pin to compensate for the difference..

Thanks for the input. I weighed the pistons and pins seperately hoping I could mix and match to get closer to my goals. Unfortunately, the pins were all dead on 157g.

I think you need to keep in mind that a +7 gram across the board addition to the pistons isn't a 7 grams of additional out of balance weight. You're looking for piston to piston difference in weight, not an increase or decrease among the entire set.

Yes, I agree. I understand all the pistons are +7 grams. It would definately be worse if only 1 or 2 were +7. My concern again is that the crank was balanced for a set of pistons that are 7 grams lighter.

Hey Vinney, see if Wiseco can up with a 150 gram pin. A long shot might be to wiegh the pins from your other set of pistons. If your pistons are solid domes, you might be able to get a few grams from the pin side.

Other than a custom set of pins at $75 per pin, nobody makes a 150 gram pin that fits my application. I also thought of weighing my original pins, 157 g as well.

Vinny,

What rpm are you turning the engine ? Although I personally shoot for everything to be within 1/2 gram when I am balancing a rotating assembly, if you were to weigh the parts out of a stock 502 Mercruiser engine, you would wonder why we bothered :( and they generally do not have many bearing issues.

I certainly do not want to downplay the importance of balancing, but to be honest with you I doubt you will ever notice the difference of 7 grams on the pistons, either in the way the engine runs or in its lifespan.

As far as removing weight from the pistons, 7 grams is a LOT of aluminum :( You might be able to get a couple of grams out from the wrist pin bosses, but if Wiseco said not to mill them, I would be inclined to leave them alone.

Like Jeremy said, although we think of balancing as a science, it is somewhat of a guessing game. A perfect example is the amount of oil that is sticking to everything as it rotates, anyone care to take a guess as to how many grams of oil is on each piston ? Is there more on the #1 piston than on the #8 ? etc.

Bill Koustenis
Advanced Automotive Machine
Waldorf Md