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12-23-2022, 01:08 PM | #1 |
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Weighing and balancing of connecting rod, pistons, rings, wrist pin
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? |
12-23-2022, 02:11 PM | #2 |
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Re: Weighing and balancing of connecting rod, pistons, rings, wrist pin
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.
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12-24-2022, 03:48 PM | #3 |
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Re: Weighing and balancing of connecting rod, pistons, rings, wrist pin
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.
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12-25-2022, 08:10 AM | #4 | |
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Re: Weighing and balancing of connecting rod, pistons, rings, wrist pin
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12-25-2022, 09:38 AM | #5 |
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Re: Weighing and balancing of connecting rod, pistons, rings, wrist pin
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 |
12-25-2022, 05:34 PM | #6 |
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Re: Weighing and balancing of connecting rod, pistons, rings, wrist pin
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Last edited by Richard Knight; 12-25-2022 at 05:43 PM. |
12-26-2022, 12:59 PM | #7 |
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Re: Weighing and balancing of connecting rod, pistons, rings, wrist pin
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.
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12-26-2022, 05:03 PM | #8 | |
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Re: Weighing and balancing of connecting rod, pistons, rings, wrist pin
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12-26-2022, 06:05 PM | #9 |
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Re: Weighing and balancing of connecting rod, pistons, rings, wrist pin
Pete what do you mean by" it is taken at resonance?
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12-26-2022, 07:16 PM | #10 |
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Re: Weighing and balancing of connecting rod, pistons, rings, wrist pin
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12-27-2022, 11:26 AM | #11 |
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Re: Weighing and balancing of connecting rod, pistons, rings, wrist pin
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.
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12-27-2022, 12:49 PM | #12 |
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Re: Weighing and balancing of connecting rod, pistons, rings, wrist pin
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.
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12-27-2022, 10:28 PM | #13 |
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Re: Weighing and balancing of connecting rod, pistons, rings, wrist pin
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. |
12-28-2022, 12:46 AM | #14 |
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Re: Weighing and balancing of connecting rod, pistons, rings, wrist pin
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.
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12-28-2022, 10:53 AM | #15 |
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Re: Weighing and balancing of connecting rod, pistons, rings, wrist pin
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.
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12-28-2022, 12:06 PM | #16 |
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Re: Weighing and balancing of connecting rod, pistons, rings, wrist pin
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
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12-28-2022, 12:07 PM | #17 |
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Re: Weighing and balancing of connecting rod, pistons, rings, wrist pin
Three types of vibrations produced by the Model "A's" engine are reciprocating (vertical), rotational and torsional impulse.
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12-28-2022, 12:41 PM | #18 |
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Re: Weighing and balancing of connecting rod, pistons, rings, wrist pin
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.
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12-28-2022, 01:39 PM | #19 | |
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Re: Weighing and balancing of connecting rod, pistons, rings, wrist pin
Quote:
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
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12-28-2022, 01:45 PM | #20 | |
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Re: Weighing and balancing of connecting rod, pistons, rings, wrist pin
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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
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