Mikey 97Z M6

I need to calculate my bob weight, but I don't have a way to get the weight of the rods at both ends. Is there a way to get a "rough" estimate without actually weighing the rods at both ends? I'm sure there are some machinists on here that probably have a rough guess on weight at each end.

Here is what I know, or at least a rough estimate:
635g Eagle 6" H beam rod w/L19 bolts
515g JE Blower piston -22cc
120g Pison Pin
60g JE Rings (unsure of actual weight)
70g Rod Bearing

So let's say the crank has a targeted bob weight of 1800g. As long as I'm below that number, I shouldn't need to add any mallory metal to the crank, just remove material, correct? Any help is appreciated..

Mike

contactpatch

Is there a reason, why, the rod vendor won't
tell you the 'big end, small end' weights,
of their product?

OldSStroker

What contactpatch said above.

With any reasonable big end-small end weight percentage, with the numbers you quoted I get over 1800 gm bobweights, so you'll probably nead heavy metal added for an internal balance. (I used 67%/33% and 75%/25% rod weight distribution)

Ask the guy who's going to do the balancing to weigh the parts.

Denny McLain

Guess I’m a little confused as surely you’re having someone balance your rotating assembly who has a scale, bob weights and a balancer. Not exactly something where you spit in you hand and pound it to see which direction it goes.

Weather or not you need heavy metal is also determined on how the crank is setup and if you are going to externally or internally balance the assembly.

Guess I’m just a whooose, but I’m so **** that I have someone balance it and pay another person to check the work the first person did. I’ve had problems in the past with people not getting the balance right.

OldSStroker

Does it ever agree EXACTLY between two different balancing machines?

So, how many grams do you allow for oil? I like approximately 7.63.

Denny McLain

5 grams for oil.

How about an ounce and a quarter difference? Enough that I had to pull the engine and do a rebalance. Not sure if I would consider that inside standard deviation. Ever since that happened, I have it double-checked expecting few grams, but not a few ounces.

So what does the Performance Trends software say it should be?

OldSStroker

It's probably not a very important factor in determining hp/torque in a simulation program. As in everything else in simulation, GIGO applies.

FWIW, isn't balance (or out-of-balance) expressed in oz-in or gm-cm or lb-ft(if it's really out!)?

Mikey 97Z M6

Not usually on a weekend.

To clarify this situation, I had an oportunity to buy a "used" crank that had already been balanced with a bob weight of 1790g. I really just wanted to know if I was going to need a bunch of mallory to re-balance it, which would offset the "good deal" part of this oportunity. Make sense now?

Thanks,

Mike

SStrokerAce

Mike,

I would guess a crank balanced at 1790-1800 is way light for what you have.... You are easily at 1900+ with what you have there. No matter what that's going to need heavy metal on a balance on most SBC cranks.

From what I have seen in terms of balancing, have a guy who does it right and someone you trust is worth everything. Even better is that you understand how the deal works and stand there and watch him spin it up the last time.... I've seen 40 year old machines get a balance dead nuts.... The biggie in how I see it is that guys who are going to check a balance need to balance the crank with the same bobweight, if you don't it's a pointless exersize. Sometimes being **** about some little things is worthless because you can chase something with money but in the end it doesn't net you anything in the power department.

OTOH in terms of software.... it's only as good as the info you give it. You can get anything close if you put good info in and say xxx combination with xx parts is going to do this.... sometimes there are other variables too, or a combination gets changed.

Bret

racer7088

Mike, I am home but I do many of those rods a week. They are "around" 455g big end and 180g small end and I will see exactly tomorrow for you. Also there are no 70g bearings but rather around 45g a set of two shells (22.5g a shell on CB663H) per rod in the heavier steel backed H bearings and also of course even lighter for others in the 2.100 journal chevy sizes.

Your total reciprocating weight is "ONE of the two or anotherwords 50%" which is one piston + one pin + one set of rings + one set of locks + one small end of rod so we have 515 + 120 + 60 + 4 + 180 = 879g

Your rotating weight is "TWO of the two or anotherwords 100%" which is both or two rod big ends + both or two sets of rod bearings so we have 455 + 455+ 45 + 45 = 1000g

Add these two up and then add 5g for oil and you have a 1884g bobweight so you will need a bit of heavy metal in both ends of the crank most probably to even make a 150% street bobweight. You'd need even more heavy metal for any overbalance.

The Eagle cranks are usually good to around 1875G plus or minus 20 grams or so so it would be very close. The pistons are a little heavy so if you needed to you could sell those and get some lighter ones and drill a little or also you may be able to weld up the prebalancing holes as well is they rough balanced it already but 100g is a lot. Obviously if they are blower pistons they may NEED to weigh 515 grams too. Good Luck!

SStrokerAce

Good point Eric I didn't catch that the bearing mass was that far off. Either way the pre-balanced crank you have is going to need some heavy metal to get it up to snuff.

bret

racer7088

Bret,

Yes, I didn't know if he meant one set or if he had like King aluminum style bearings that were lighter and was using both pairs for that weight?

Denny McLain

I was just spoofing and having a little fun on the EA simulation. As mentioned….. really don’t fully trust the program and yes balance equipment is set up in the metric system. It was 30 some (don’t remember exactly) grams off on one end.

However, dead serious about have my rotating assemblies double checked. Did exactly what Bret recommended and watched the person balance my current assembly. Didn’t do my usual of having another shop double check it, and there is some vibration in my current motor.

The prior 398ci rotating assembly was balanced dead nuts on by one shop, double checked by another as within 2-3 grams and you could put a cup of coffee on the dash @ 6500 rpm an see no ripples.

Lessons learned through the school of hard knocks as scales and equipment can get out of calibration.

Having one balanced on the money really makes the car much more enjoyable to drive. Any more power?? I seriously doubt it.

Mikey 97Z M6

Thanks for the help guys.

Mike