balancing crank - turning counterweights

R.Brown · Post by **R.Brown** » Tue Dec 23, 2008 12:41 pm

Hey gang,

Got a billet crower crank to balance for a pontiac v8, seems they targeted this for a pretty hefty bobweight as I have to take 550 grams from the center of the front and 410 from the center of the back counterweight.

I have done this on other cranks before, but it was not that much to come off... In the past I guessed at how much a sq inch of metal weighs and figured out the cubic inch area of the amount to machine off the crank and did it... got to a spot where I could drill the rest... and all worked out fine. BUT... we can't fit this crank in the lathe so it has to go out to be turned.. I'd like to get it close the first time.

Any advise from those in the know regarding a calculation technique?

Also, should I keep the counterweights all the same (or close to) diameter?? I think this would be the best for taking the minimum amount off...

Please opine,

rb

ProPower engines · Post by **ProPower engines** » Tue Dec 23, 2008 12:52 pm

I would call Crower and tell them the crank # and tell them what your requirements are.
They should be able to give you a better idea on how much weight to take off.
I went through this same deal recently with a Scat crank and I had to make to trips to the outside shop to get it down to where it had to be.

In my case it was .350" off the end c/weights and still had to put a couple of small divots to be right on the money. The amount of weight removal was about the same.

andyf · Post by **andyf** » Tue Dec 23, 2008 1:02 pm

If you know how much weight needs to come off then you can calculate how many cubic inches of material to remove. If it has to come off the center of the counterweight then removing it equally around the counterweight should accomplish close to the same trick. If it was me I'd just be conservative with the material removed on the lathe. If you need to take 550 grams off total then maybe calculate it to take 450 off on the lathe and leave 100 on the balance machine.

R.Brown · Post by **R.Brown** » Tue Dec 23, 2008 1:19 pm

ProPower - yeah I'd call but they are in their yearly holiday mode until jan 4th.

AndyF - that has served me well in the past, just wondered if someone had a different way of doing it than what I tried to explain in post #1... Like some kind of factor for the closer you get to the center of the crank you multiply the amount by 1.1 or something... That probably does not make much sense the way I typed it.

Strange Magic · Post by **Strange Magic** » Tue Dec 23, 2008 1:50 pm

If you pull off about .185 on the first two counter weights and .185 on the back two counterweights you should be able to dial it right in on the balance machine.

Wolfplace · Post by **Wolfplace** » Tue Dec 23, 2008 2:00 pm

Do this all the time & I have never found a way to be real accurate except for making a few trips to the lathe
Even if I had one I would not trust it,, I hate it when I have to add Tungsten because of my bonehead move,,,,

Another very important point here
If you machine all the counterweights,,
Keep an eye on what removing counterweight in the lathe does to the point of imbalance
In other words do not shoot yourself in the foot
You do not want to rotate the position of needed removal off the counterweights & machining the counterweights will usually rotate it

I also have been known to stick it up in the mill & remove weight on the ends of the counterweights to get it closer
Usually after cutting the weights in the lathe & finding I need more than I really want to drill at the edge or to bring it back up on the counterweight

Nothing wrong with drilling it except my opinion is it looks like crap with more than a hole or two to finish

speedmachine · Post by **speedmachine** » Tue Dec 23, 2008 3:41 pm

I have not found an acurate way of figuring the amount of metal removal either. The trick is trying to get it close ,about 75grams, so that you dont have to drill more than 2 holes. Not only does alot of holes look like crap, but it could induce an unwanted secondary wave that could cause crank flexing and possibly breakage.

R.Brown · Post by **R.Brown** » Tue Dec 23, 2008 7:32 pm

thanks for the info fellas, I appreciate it.

To those who are doing the math, what are you using for the gram weight of 1 cubic inch of steel / 4340? I don't have my 'lil book with me right now, but from memory I think it was 128 grams?

tx,
rb

Post by **Dave Koehler** » Tue Dec 23, 2008 8:41 pm

127.35 to be exact.
If it helps: 1" long by 1.2 wide by .100 thick will be 19.10 grams.

strokersix · Post by **strokersix** » Wed Dec 24, 2008 8:11 am

On my sixes I've set them up on the mill to roll about two crankpins, then cut the counterweights on a radius about their respective crankpins. I put a bar on the crank and pulled it through the cutter by hand. Works great on an inline six because you have always have two crankpins in line to support the crank. V8 you only have one. But if you can figure out how to accomplish this in a similar manner then you can do a real nice job of trimming the weights where it counts. I did this work for wristpin (piston bottom) clearance but there is no reason you couldn't do the same thing for balance purpose.

Just set up on a lathe between centers limits you to cutting them on a radius about the main centerline which is less effective as already noted.

Racerrick · Post by **Racerrick** » Wed Dec 24, 2008 2:30 pm

I remove counterweights in my mill using a fixture like this. http://www.abs-products.com/specialty-t ... ocks.shtml

Post by **Dave Koehler** » Wed Dec 24, 2008 2:55 pm

That throwhead thing looks simple enough but way dangerous to me. How do you make it work without running the risk of grabbing the crank and impaling yourself?

strokersix · Post by **strokersix** » Thu Dec 25, 2008 10:37 am

Yup, that's how I did it, only not quite as fancy. If you pay attention to cutter direction (don't try to climb mill!) it works really well.

Post by **SchmidtMotorWorks** » Thu Dec 25, 2008 11:14 am

If you want to do the calculation you have to determine the center of mass for the volume you will remove.

Generally I would rather reduce the fan of the first and last CW rather than turning because the first and last CW are overweight in 99% of cranks.

Depending on the design of vise and jaws you have, you might be able put the vise sideways on the table, and then clamp it on the counterweight that you are milling. I have balanced hundreds of billet cranks this way and never drilled a single hole.

Since the CW is probably not symmetrical across the first pin, be aware that the amount you remove form each end (leading / trailing) will be different.

Wolfplace · Post by **Wolfplace** » Thu Dec 25, 2008 2:14 pm

Jon
Define "fan"?
Thanks