You can do it!!
Is balancing your engine THAT important?? YES it is!! If you were to just put parts together, the engine would most likely run but it would be very rough. It would idle poorly and would vibrate badly at high speed. I once bought a cheapo rebuilt engine made from a host of probably out of tolerance, but well cleaned parts that vibrated so badly at idle that my bumper shook loose; NO JOKE!!! If you're from CA's Central Valley, e-mail me and I'll let you know who the crappiest engine builder I've ever come across is in private.
It's very important to balance all the rotating parts of an engine so it will run smoothly. A balanced engine runs smoother; balancing reduces wear on engine parts; less wear gives you better oil pressure and a tighter engine; tighter engines have more power and last longer; etc, etc, etc. . . Face it, for long & happy engine life, you NEED to at balance your engine.
NOTE: I will ONLY be covering STATIC BALANCING since that is really all the home builder can do with basic tools. Static balancing is better than nothing at all, but I'll admit, it's only a start. To get the most out of your engine, ALL the rotating parts (i.e. - crankshaft, rods, pistons, flywheel, clutch, pressure plate and gland nut w/ washer) should be sent to a machine shop capable of doing DYNAMIC BALANCING. This is a process where all the parts are balanced end-to-end as a complete unit kind of like a spin balance on your tires and wheels at your local tire shop. Dynamic balancing is truly the way to go, but with a price of between $150 - $250+, it's often more than a lot of folks can reasonably afford (like ME). A good static balance will really be quite adequate for most.
First you need to find either a scale or a balance beam. If you don't have a scale, you can make a balance beam. Since I have access to a really nice scale, that's what I will be using.
Balancing the Rods
AFTER your rods have been rebuilt and are assembled with their caps and nuts, weigh all your rods individually. Find out which one is the LIGHTEST of the four. It's also best to weigh each end individually in order to get a better balanced rod. Your goal will now be to get the remaining three rods to weigh the same as that rod within 1/2 gram or less (stock is within 5 grams). I know that getting it 4 1/2 grams closer is a pain and might take a little longer, but of you have a good scale why not?? Just think SMOOOOOTH. . .
Above are a few pictures of where you can remove material in order to balance everything. No more than 8 grams should be removed from any one rod in a set.
Above is one of my rods after it was balanced. Only a slight bit was necessary on this rod. Since the scale I used could measure to within 0.1 of a gram, I got them all EXACTLY the same according to that scale. I expect this motor to be the smoothest engine I've built to date!!
Before you do anything to your pistons, give them a good look over. Once you start removing material, most likely you will not be able to take them back if they are found to be defective. This is a photo of one of the two (YES I SAID 2!!!!) cracked pistons that came in my "NEW" set!! An Eddy Current test revealed that the cracks were all over .040"+ deep!!
The parts inspection process for "obvious defects" should be done at the time of sale while you are at the parts counter and NOT JUST during the assembly process. These cracks were found under the wrist pin bores on BOTH sides of two different pistons in the same set!!! This is especially embarrassing for me since I'm a full-time NDT (Non Destructive Tester) by trade and I missed this at the parts counter. These were difficult to see with the wrist pins installed, but sheesh,. . . I'm an NDT!! I should have smelled these :-P Oh well, at least I caught it before it was too late!!
Balancing the Pistons
Just like the rods, the pistons need to be balanced too. This should be done with the wristpins AND clips installed as pictured above.
Proceed just like the rods only remove material from heavier ones at the above indicated boss. NO OTHER AREA of the piston should be touched by the home balancer during the balancing process. It is possible to thin the piston skirts on a mill at a machine shop, but unless you have a mill in your garage or access to one at work, DON'T try and free hand it!! It's best to save that for the pros.
NOTE: If you weigh the wrist pins individually and find the exact weights, you may be able to mix and match the lightest wrist pin with the heaviest piston and minimize the amount of balancing you need to do. This little trick could come in handy since you are so limited in where you can take away material. Remember, aluminum is light and you're limited to that one little boss.
Balance the piston assemblies to within 1/2 gram if possible (Stock is within 5 grams), by removing **small** amounts of material at a time and re-weighing. I've found that how close your pistons are in weight, prior to balancing, is a direct reflection of the quality of the set. Hopefully you won't have to remove much.
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Revised: August 29, 2003 .