Weighing and balancing of connecting rod, pistons, rings, wrist pin

[old31]

I have watched many videos of the weighing and balancing of connecting rod, pistons, rings, wrist pins.

Why do all the videos show weighing each part individually vs weighing the entire finished unit as one piece?

Now that I have my pistons out I thought I would for the heck of it balance what I could. For an ice cream getter wouldn't taking the entire rod, rings, piston and wrist pin and weighing it against the others be ok.

What am I missing on the procedure?

[Richard Knight]

One thing you are missing is the wrist pin end is reciprocating and the big end is rotating which makes a different when the Bob weight is calculated for spinning up the crank. Google crank balancing you will learn some more. Modern pistons are within 5 grams or better. The rods will be all over the place weight wise. What you have seen is the way it is done.

[Dan McEachern]

A Model A is a flat plane crank- no bob weights are required for dynamically balancing the crankshaft. Balance the pistons and pins only for equal weight. Rods need to be balanced in two steps- 1: Big end weight with the rod hanging from the wrist pin and 2: total weight . The big ends and overall weight should be within at least 2 grams of each other.

[BTERRY29]

Originally Posted by Dan McEachern

A Model A is a flat plane crank- no bob weights are required for dynamically balancing the crankshaft. Balance the pistons and pins only for equal weight. Rods need to be balanced in two steps- 1: Big end weight with the rod hanging from the wrist pin and 2: total weight . The big ends and overall weight should be within at least 2 grams of each other.

When removing weight from a rod, what is the best place and technique for doing so?

[Kurt in NJ]

The areas highlighted with black marker is where the factory machined to adjust weight,
the factory machine weighed the chips to know when the target weight was reached
perhaps the easiest is a fixture on lathe faceplate, or custom ground cutter on boring bar on milling machine

if you look at each rod you will see that area is different on each rod depending on the forging weight

[Richard Knight]

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Over the years I have used several balancing shops. It seems every balancer has his own way of doing things. Although the A and B cranks are flat plane I have had at least one balancer who did balance the crank with bob weights. He also wanted to spin the crank where it ran rpm wise. He just felt he could do a better job. I think most shops skip the Bob weights and spin at a percentage of the. Rpm ? I am not completely sure on that though. I have no doubt Terry is correct on the factory removal of weight but I can tell you some rods will not fit the babbitt molds due to the removal of weight. I can assure you the balancers find places to remove weight on the rods.

[Jim Brierley]

The reason pistons are not balanced with rings is that rings are replaced at times, and rings are light nd precision items so each set weighs almost exactly the same per piston. Pins can be switched to help balancing the pistons.

[Pete]

Originally Posted by Richard Knight

Over the years I have used several balancing shops. It seems every balancer has his own way of doing things. Although the A and B cranks are flat plane I have had at least one balancer who did balance the crank with bob weights. He also wanted to spin the crank where it ran rpm wise. He just felt he could do a better job. I think most shops skip the Bob weights and spin at a percentage of the. Rpm ? I am not completely sure on that though. I have no doubt Terry is correct on the factory removal of weight but I can tell you some rods will not fit the babbitt molds due to the removal of weight. I can assure you the balancers find places to remove weight on the rods.

We have "overbalanced" certain RACE engines for many years. Both V8's and 4's. That is, used bob weights to balance the crank. The amount depends on the engine and the usage. It helps in the operating range but the unbalance is NOT read in that range, it is taken at resonance.

[old31]

Pete what do you mean by" it is taken at resonance?

[Pete]

Originally Posted by old31

Pete what do you mean by" it is taken at resonance?

That is the rpm which the maximum unbalance occurs. The crank is spun up to an rpm ABOVE that and let come down through it by itself. The unbalance is read on a meter on a Stewart Warner balancer.

[Richard Knight]

Pete, thanks. Of all the engines I have had balanced I have never seen a crank spun up. Interesting about the resonance. Would you happen to have any video of spinning a crank up? Thanks, rick.

[Jim Brierley]

There are 3 points of resonance in our engines. I forget them exactly but from memory they are about 900, 1200 (?) and 2300. They are heard more than felt.

[ModelA29]

Don't confuse flat plane with non counter weighted. Ford recently did a flat plane crank for the GT350 Mustang as you can see it's counterbalanced.
The A crank would have been better balanced if it was cross plane.

[Dan McEachern]

Resonance points are a function of the natural frequency of the rotating mass. You can shift the 1st, 2nd and 3rd harmonic points by changing the mass of the rotating assembly, but you can't eliminate them, just move or shift them away from your operating speed.

[johnneilson]

Many factors contribute to the resonance of the rotating assembly including the torsional stiffness or the crank. The flywheel MOI also influences this greatly.
As for the configuration of the crank, the 4 cyl motor has a imbalance as the piston/rod combination doesn't experience symmetrical acceleration/deceleration at top and bottom of the strokes. The crank layout does not matter, this pic one done for firing order change to utilize the intake ports more efficiently.

[Richard Knight]

John, that is a pretty exotic looking bottom end. WOW. For me this has been a very interesting discussion, illuminating!. I would like to ask the brain trust is it my imagination or is the deceleration vibrations, harmonics?, more noticeable when driving a model A than when accelerating. Thanks
.

[Bob Bidonde]

Three types of vibrations produced by the Model "A's" engine are reciprocating (vertical), rotational and torsional impulse.

[Jim Brierley]

with a 180* crank in a 4 cyl. engine there are 2 choices of firing order, 1-2-4-3 or 1-3-4-2. Changing them makes absolutely no difference on the 'putts' going into the engine. No matter which order you choose, each port draws in the same sequence, 1-2 together and 3-4 together. On 4-ported blocks things get better because the ports are separated a bit more.

[johnneilson]

Originally Posted by Jim Brierley

with a 180* crank in a 4 cyl. engine there are 2 choices of firing order, 1-2-4-3 or 1-3-4-2. Changing them makes absolutely no difference on the 'putts' going into the engine. No matter which order you choose, each port draws in the same sequence, 1-2 together and 3-4 together. On 4-ported blocks things get better because the ports are separated a bit more.

Jim,
if you look at that pic closely, this is a 2up/2dn (ala Winfield) crank. The firing order is 1, 3 or 4, 2, 4 or 3. What this does is take the Siamese port and make the crank duration between cyls 360 degrees instead of 180. This really make the Siamese port much more efficient closer to a 4 port design.

This really screwed up the cam grinder, it took longer to explain the cam layout required than it took to make the cam.

John

[johnneilson]

Originally Posted by Richard Knight

John, that is a pretty exotic looking bottom end. WOW. For me this has been a very interesting discussion, illuminating!. I would like to ask the brain trust is it my imagination or is the deceleration vibrations, harmonics?, more noticeable when driving a model A than when accelerating. Thanks
.

Richard,
good question, I have no idea for a stock Mod"A" whether on Accel or Decel, I would imagine it depends on the timing set on accel.

I have noticed on a modified motor, that yes accel does have more felt vibration. This I would attribute to the increased combustion stroke energy generated. At 100 HP, you notice it, at 300 HP that's all you feel as your eyes get pushed back into your head.

John

[Pete]

Originally Posted by Richard Knight

Pete, thanks. Of all the engines I have had balanced I have never seen a crank spun up. Interesting about the resonance. Would you happen to have any video of spinning a crank up? Thanks, rick.

I have seen videos but don't remember where they were.

[Richard Knight]

Pete, I was hoping someone might be willing to put up a video of spinning up a crank or even a show and tell on balancing rods!

[Pete]

Originally Posted by Richard Knight

Pete, thanks. Of all the engines I have had balanced I have never seen a crank spun up. Interesting about the resonance. Would you happen to have any video of spinning a crank up? Thanks, rick.

Here's a video on Youtube that covers the basic process.
Minor details will vary with type of engine and model of balancer.

https://youtu.be/zwDRGNNl5qk

[old31]

Follow up:

I balanced the connecting rods (off by 8 grams) pistons (off by 6 grams) wrist pins ( off by 7 grams) and the connecting rod end caps (off by 12 grams)

That's a total of 33 grams that i zeroed out. l used a scale that claims it was accurate within 1 gram. i used a scale and did not use the balancing tool that weighs the ends of the rods separately.

This may not have been the best way, but it is certainly better than the way it was in the engine before i took it apart.

[Richard Knight]

Thanks Pete, interesting video.

[Dodge]

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John,
I was wondering if anyone was going to do a modern 2up 2down motor.
How did that thing run?

[johnneilson]

Originally Posted by Dodge

John,
I was wondering if anyone was going to do a modern 2up 2down motor.
How did that thing run?

It had moments of greatness, then well, you know. It showed pretty well on the dyno during break in but didn't beat it too much.

We had issues with ignition and injection in the car and that sidelined the project.

The plan is to either run it in a Miller 2 port or another flathead, but that will be determined later.

John

[Richard Knight]

It seems as though what I have felt in decelaration is the engine rpm passing through a resonant frequency? So if the rotating mass is lightened does this change the resonant frequencies to slightly higher rpms. Has anyone spun up a burtz rotating assembly ? And determined where the resonant frequencies are????