One rod 10 grams heavier on big end

[smokie]

Someone I know is building a 383. While assembling he managed screwing up the threads on one rod cap screw hole. The rods he's using are scat h beam 6" rods. He found another rod that is one gram lighter overall, but it's 10 grams heavier on the big end. His engine machinist is telling him it won't hurt anything, but to me that's almost a half ounce heavier on the big reciprocating end and at 6,500 rpm it seems like it's going to be a bad idea.

Rod does weigh within 1 gram overall, but is 10 grams heavy on the big end. Is this is a bad thing waiting to happen, or will it be ok?

[KnightEngines]

Unless he's gonna turn 7000+rpm don't sweat it, it'll be fine.

[modok]

We can say..... it's within factory tolerances.
Run it!

[BillK]

Did he try calling Scat to see if they can find one that matches the weight ? They have always been able to get pretty close when I have had to ask.

That being said I have seen a whole lot worse and it didnt seem to cause any type of problem

[jsgarage]

There's probably 5-10 grams of oil temporarily adhering to each rod during operation. This is a non-problem if the engine is balanced.

[smokie]

Thanks everyone, I told him to run it.

[Tuner]

Balancing engines is a wast of time. People who expect to get paid for it are just ripoff artists, race engine scammers, frauds, so why bother.

At 6000 RPM the G force at 5 inches from the axis of rotation is near 5000 G, so 10 gm. is an imbalance force of only 50,000 grams, or 110 lbs, so that won't shake much.

You might carry some extra rear view mirror adhesive with you, just in case.

https://druckerdiagnostics.com/wp-conte ... ulator.pdf

[Baprace]

Balancing engines is a wast of time. People who expect to get paid for it are just ripoff artists, race engine scammers, frauds, so why bother.

At 6000 RPM the G force at 5 inches from the axis of rotation is near 5000 G, so 10 gm. is an imbalance force of only 50,000 grams, or 110 lbs, so that won't shake much.

You might carry some extra rear view mirror adhesive with you, just in case.

https://druckerdiagnostics.com/wp-conte ... ulator.pdf

Only 110 lbs ? Wow that is a staggering number !

[Rick!]

Install the "heavy big end" rod in cyl 4 or 5 to minimize the secondary moment it is going to create and reduce bending stress on the fillets of the number 3 main bearing journal.
I don't agree the above statement regarding "only 50,000 grams" of force being negligible. A 100Hz vibration is easily felt by a human most times through the three touch points; hands, feet, or butt.

[Tuner]

Install the "heavy big end" rod in cyl 4 or 5 to minimize the secondary moment it is going to create and reduce bending stress on the fillets of the number 3 main bearing journal.
I don't agree the above statement regarding "only 50,000 grams" of force being negligible. A 100Hz vibration is easily felt by a human most times through the three touch points; hands, feet, or butt.

Turn up your sarcasm detector, "carry extra rear view mirror adhesive" a clue perhaps?

Don't bother to click the link where there is a handy Nomograph, doo the maffphs


How to Calculate G-Force:

The Force exerted on a sample in a centrifuge is a function of the rotation speed of the centrifuge (RPM) and the radius of the rotor.

If you know the RPM, the equations to calculate G-Force are:

In Centimeters: RCF or G-Force = 0.00001118 x Rotor Radius x (RPM)²

OR

In Inches: RCF or G-Force = 0.0000284 x Rotor Radius x (RPM)²

[modok]

Balancing engines is a wast of time. People who expect to get paid for it are just ripoff artists, race engine scammers, frauds, so why bother.

At 6000 RPM the G force at 5 inches from the axis of rotation is near 5000 G, so 10 gm. is an imbalance force of only 50,000 grams, or 110 lbs, so that won't shake much.

You might carry some extra rear view mirror adhesive with you, just in case.

https://druckerdiagnostics.com/wp-conte ... ulator.pdf

I love the sarcasm but your math is way off by around 4x
It is 5 grams light not ten, and the stroke is not ten inches.

[Tuner]

Balancing engines is a wast of time. People who expect to get paid for it are just ripoff artists, race engine scammers, frauds, so why bother.

At 6000 RPM the G force at 5 inches from the axis of rotation is near 5000 G, so 10 gm. is an imbalance force of only 50,000 grams, or 110 lbs, so that won't shake much.

You might carry some extra rear view mirror adhesive with you, just in case.

https://druckerdiagnostics.com/wp-conte ... ulator.pdf

I love the sarcasm but your math is way off by around 4x
It is 5 grams light not ten, and the stroke is not ten inches.

I admit to WAG' the radius of the rod cap from the center of the crank at the bottom of the stroke, but did you read the OP? Use 4 inches and 6500 RPM and it still comes to 4800 G x 10 gm. = 105.82189 Lbs. of imbalance. If you use 3 inches and 6500 RPM it is 80 lbs of gyrating force. ... so there is no sense in balancing an engine, just let it shake, the girls will like it, like riding a Harley. [/sarc]

His engine machinist is telling him it won't hurt anything, but to me that's almost a half ounce heavier on the big reciprocating end and at 6,500 rpm it seems like it's going to be a bad idea.
Rod does weigh within 1 gram overall, but is 10 grams heavy on the big end. Is this is a bad thing waiting to happen, or will it be ok?

[modok]

I'm highly confident about what I said
lets let somebody else recheck the stroke, rpm, and bob weight calculation

[Tuner]

I'm highly confident about what I said
lets let somebody else recheck the stroke, rpm, and bob weight calculation

Please do, everybody check it.

In Inches: G-Force = 0.0000284 x Rotor Radius x (RPM)²

10 gm. + 0.0220462 lbs.

6500 (RPM)² = 42,250,000 x 4" = 169,000,000 x .0000284 = 4,799.6 G x 10 gm. = 47,996 gm.

47,996 gm. = 105.813067 .bs.

Whatever number within reason you choose to use for radius, 10 gm. on the big end of one rod is a huge out-of-balance problem at high RPM because the imbalance force increases to the square of the RPM.

[modok]

stroke is 8 inches now. that's a big engine
Whatever, I love you buddy. your kinda my hero
it's a shame we don't have better things to discuss at the moment.
You know one thing I never figured out is.....in a wet flow intake manifold it seems like with a sharp enough turn it bounces the fuel back into suspension, but a more gradual turn throws the fuel at the outside wall, and a very gradual turn it stays in suspension.
easy right?
But then there is accelerating and decelerating flow, surface to volume ratio, coarseness of atomization, temperature, speed difference between the wet flow and dry, and renolds number, and all that. easy to say, but exactly how sharp is sharp, in what circumstances, i still have never figured out.