If I work on my rods with a grinder to smooth the rough edges, ridges, etc but remove the same amount of material from each rod (so that each rod's weight is decreased by the same amount) is it necessary to have the rotating assembly rebalanced? It's a stock rotating assembly that came with the engine.

Also, would it somehow be possible to remove a corresponding amount of weight from each crankshaft counterweight by radiusing them? (Say X grams from each connecting rod and then radiusing each of the crank counterweights as equally as possible.) I'm thinking that since these things weren't precisely balanced from the factory, I shouldn't require a rebalance if I remove a similar amount of material from each rod and crank counterweight.

Any thoughts?

 

Yes. Without a proper rod balance scale, you don't know just how much your taking off each end. The rods are also only a part of the balance weight. The rods, pistons and pins, rod bearings, rod bolts, rings are all totaled up, then the crank is balanced to match that weight.

Having all the pistons the same weight and rods the same weight is only part of balancing. There'a no way you're going to know where to add or remove weight from the crankshaft without a balance machine. The weight could be adjusted anywhere on the crank.

If you keep the engine below 5000 rpm like a typical "stock" engine, you should be able to get away without a balance but you'll never know how bad it is unless you get it done.

 

Considering that the stock pistons, rods, pins, etc were only "balanced" to within generic tolerances, would it be beneficial to match all the piston/rod assemblies to the same weight (leaving the crank alone)?

 

That still doesn't balance a rotating assembly. It would be like using a 305 crank in a 350. They're identical except for the bob weights. Trying to use 350 pistons and rods which will be a different weight than a 305's will cause an imbalance.

A factory balance is close but not 100% accurate. A factory engine also isn't designed to operate above 5000 rpm for any length of time. Most factory engines operate at less than 3000 rpm where an imbalance won't be noticed as much.

When I threw a rod a month ago, my engine still ran with one less con rod and piston. That's a lot of weight removed from the rotating assembly and the imbalance was very noticable. A small imbalance won't be noticed and could last a long time before doing any sort of damage. Since I take my engine to 7000+ rpm, I want the balance to be perfect.

Like I said, you could probably get away without a proper balancing but if you're spending all the time trying to modify the internals for more power or strength, why skip an important step?
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Also, getting the rods the same weight means weighing them on a rod scale. Both ends of the rod are weighed and weight is removed from the end bosses to match the rod with the lightest weight. You can't just weigh the entire rod and try to match that weight. If you don't take the weight off the proper spot, the rod will be imbalanced.

 

I don't intend to be revving the engine into the high RPM so a balance really isn't necessary for me, but I am curious as to whether balancing out the weight of the stock pistons, rods, etc would actually have a beneficial effect on the motor.

I have them on my bench, I also have access to the equipment needed to weigh them properly ...it wouldn't be a big job to match them to the same weight ...but would it be of any benefit?

 

if nothing else even though this might not lead to massive failure it will lead to a smoother running engine meaning less vibration to the cockpit, long bearing lift, less things to go wrong, and possibly a little more power.

also if the weight does go off and it causes some nice imbalance that creates a lot of harmonics your really not going to like it

 

For a 'street' engine that doesnt see 5000RPM or more EVER, it shouldnt hurt it with a little bit of imbalance.

But getting the balance perfect would give you a bit more peace of mind when you hold it at higher RPMs and will get you a few more miles out of the bearings.

And like Stephen was saying, you cant just weigh the entire part and call it good (unless you dont really care if your doing any good...), the part itself has to be balanced along with the entire assembly!

 

Back before I even knew what it meant to "balance" a rotating assembly I happily changed stuff in the bottom end without ever thinking about weight. I did this for many motors- changing rod bolts, grinding down stuff off the rod beams, swapping stock pistons for ones of totally unknown weight, etc. I never had a problem and I never knew that I could even possibly be causing one.

For a street motor that's going to see very limited use in the upper RPMs you have to be pretty far off. If you're talking about a difference of 10-20 grams that's nothing, especially if you're making the rod/piston assembly lighter. If you're changing it by, say, 50 grams, then you are definitely going to need a re-balance.

If you're going to be "off" it's always preferable to be a little too light on the rod/piston assembly than to be a little too heavy vs. perfect balance. Ask the pros who do balancing and they'll confirm this.