Crank balancing question
 

So I've already had my crank balanced back to factory specs, and now the rods and pistons I have bought may be lighter then the stock rods and pistons that were in there before. my question is, do I have to get my crank rebalanced because i now have a lighter rotating assembly?

im pretty sure as long as all the rods are the same weight you should be fine but i would find out for sure from sombody else

The rotating assembly should be balanced as a whole.. Anything that spins with the crank should be included.. Even the minute details can make a big difference, espically with the balance..

Gotcha Josh, Ok I'll have the guy rebalance everything then, I had a feeling, but I wasen't sure.

just curious how do you balance pistons and rods with the crank?

its about the bob weight. i am sure josh can explain

In short, the masses (not the weight) of everything that spins and the counter weights on the crank need to be in correct proportion to each other.. Not always the same but that would be getting into over and under balancing and I am not the best at explaining things in depth like a textbook.

We had a member (johnjzjz) that was a much better teacher but some peoples ignorance caused him to no longer share his knowledge here..

really?........ here?...... noooo! i don't belive that. LOL

i just dont understand how you would go about measuring a counterbalance

When calculating theweight that is to be attached to the crank, the rod bearings and the bigend of the rod are considered rotating weight. The small end of the rod,piston, wristpin, rings, and pin locks are all considered reciprocatingweight (instead of spinning around, it moves up and down).

Typically, 100 percent of the rotating weight and 50 percent of thereciprocating weight are added together to form the total for the bobweight. Some engine builders include four or five grams in the total tosimulate the weight of oil clinging to the various parts. The bobweights are actual weights that are attached to the crank's rod journalsto simulate the weight of the rod/piston assembly.

how is that possible if theres oil gallys running from the crank main journals to the rod journals? also does the weight of the piston and rod get calculated into the counterbalance on the crank?

don't forget to use a neutral balanced hub and flexplate/flywheel since you are going to be internally balanced.

The short answer is Yes, to answer your original question, Yes, if you changes the weight of your rods, you are basically taking the crank out of balance.

The way I understand the process ( and I'll agree with Josh that JohnJZ would know tons more then I and could probably explain it far better ).....

You aren't actually balancing the crank, but more so balancing the crank TO THE reciprocal weight.

Cranks are basically built with an assumed weight of the rotating mass, but it still is never exact. On a standard V8, the crank is balances so that the counter weight matches the weight at 4 - 90 degree points so that the mass is balanced as it spins these points are the counter weights and material is removed or added at these points to balance the rotating assembly.

The overall balance is done by calculating mass in two sections: the piston, rings, wristpin, top of rod and 1/2 of bearing (remember the rod journal is a pivot because there is mass on the other side). So rod cap 1/2 bearing, bolt heads ect are weighted separately.

From the weight the machinist will determine the weights (bobweights) to attached to the crank ( with what I'll call ) given inner and outer or upper/lower weights. That is the weight always above the throw of the crank and weight always below it. Remember two rods/pistons to each throw/journal, so it's a combined weight of the two, attached to a journal. (why it's also very important to put rods+pistons back in the holes they can out of). Typically you can machine some of the weight out of the rods so that the 8 rods/pistons are near the same static weight, but most often they won't be exact, but will be paired to better balance then once the engine is assembled.

This is one of those areas where more expensive cranks/rods are closer/better balanced during manufacturing/machining. Balancing a rotating assembly can be expensive and the more weight needed to be remove or more importantly added (in the form of Mallory) can drive up the cost.

The machinist gathers the info of the weight of the mass, if you are externally balanced then you must install the flywheel/flexpalte and the harmonic balancer. Attach the bobweights on the journals and spins the crank to determine where weight needs to be added or removed from the counter balances. The bob weights only represent the weight being exerted on the crank, they don't need to free spin like a rod would, the entire unit, crank, bobweights, etc are spun as a whole. The machine wil indicate at what location of the counter weights, that the weight would need to be removed or added. I would assume much as a tire balancer would. you could add weight at one point, or remove it from the throw that is across ( 180 degress ) from that point.

Little side note, with my engine and the concern of weight of the street twin clutch setup the machinist balanced the flywheel to a tighter tolerance then McLeod, and then and balanced the crank with my flywheel attached to ensure it was correct.

if you look at the pic, the counter weight on far right of the crank has a hole drilled where weight was removed.