i just bought a eagle crankshaft its balanced. this one kid keeps telling me i absolutely have to get the crank balanced to my pistons rods wrist pin exe.

i dont understand this cause if all the pistons weigh the same, and the crank itself is balanced then why would i have to get the pistons balanced to the crank. this guy says the bobweight couterbalanced the piston rods exe.

on the other hand my father is telling me itl be alright. he has built engines in the past and his take on things is what i said if all the pistons weigh the same and the crank is balanced it will run right.

i could understand this in pure drag racing with blueprinting your engine but this is a street car occassional drag racing.

so my question is is this abosolutely necessery.

 

it should make as much as of an effect unless you are spinning 6000+ RPM, but.

i am also in question to this in some degree.

subscribed.

 

listen to your dad . you should be fine . if still in doubt send eng. to me .
i will then put it in my 91r/s and run it as a daily driver . in 2 or 3 years i should be able give you a good honest result . such a deal , how could you refuse ??? oh well , just a small joke . you will be o.k. , good luck .

 

Every time you mix and match rotating assembly parts, it should be balanced.

Your crankshaft isn't balanced. You either have an internally or externally balanced crankshaft. That just determines where the balance weights are located.

The weight of the rotating assembly determines where weight is added or removed from the counterweights to balance the engine. The weight of the rod, rod bolts/nuts, piston, wrist pin, rings, rod bearing are all taken into affect.

The weight of each item is not the same. You may buy new pistons and they'll all be the same weight or very close to it but the wrist pins also need to be the same weight. If they're not, it's easier to adjust the weight on the pistons to match a piston to a wrist pin. The rods will have different big and small end weights. These are adjusted to be all the same.

Once all the components are adjusted to weigh the same, the total weight is then added to bob weights on the crankshaft and the crankshaft balance is adjusted for that weight. It doesn't matter if it's an internally or externally balanced crank, using non factory parts means the rotating assembly should be balanced.

You can throw it together like it is but you're investing a lot of money to take a chance there won't be a vibration. A few grams imbalance will translate to pounds at higher rpms.

 

you are right if all the pistons, rods, and pins weight the same you should be ok. The problem is their is only two ways to know for sure. One have it balanced. Two put it together and try. Now having said that, I have ballanced several engines and I have seen pistons that were very close, rods very close, all not needing any material removed to balance. I have seen almost as many pistons that the weight was all over the place requiring weight removal. The crank is another story. When Eagle are any one else sells you a crank they can't balance it properly because they have no way of knowing what pistons, rods, or engine size the crank is being used.

 

 

Those cranks come pre-balanced to something like 1800 grams of bobweight, but if your rods and pistons work out to be like a 1900 or 1700 gram bobweight, and you don't re-balance, you'll destroy the engine very quickly.

 

Wow. I wish my BBC bob weight was that light. I use lightweight pistons but they still have a huge dome and I use H beam rods. I dug out my balance sheet. My bob weight is 2333.4 grams.

The pistons with wrist pins are 805.6 each. Times 2 per throw and and that's 1611.2 just for the pistons and wrist pins. The rest of the weight is from the rods, rings etc.

My balance sheet shows the crank was spun up 10 times before it was perfectly balanced. The 9th time, it was still 3 gm out on the right plane. On the first spin, it was out 89 gm on the left plane and 42.8 on the right. I know it had some heavy mallory slugs added to help correct the balance to offset for the lightweight pistons.

 

I'm not sure that 1800 is the right number for the OP's crank, but it served to illustrate the point.