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Should I balance the crank, or

General engine tech -- Drag Racing to Circle Track

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Caprimaniac
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Should I balance the crank, or

Post by Caprimaniac » Thu Mar 26, 2020 5:31 pm

just run it?

So ready to assemble this, and not sure if I'm up to spend time, trouble and $ to get it done.

Internally balanced forged SBF 302 stroker crank. Bobweight on crank 1750 g, actual measured parts give 1660g bobweight.

From factory the throws have some material removed (drilled holes), so assuming reasonably well balanced front to back throws.

Something tells me I won't loose much by running it as is....
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Re: Should I balance the crank, or

Post by Little Mouse » Thu Mar 26, 2020 5:46 pm

Being a hair over balanced good. Under balanced bad' run it dont worry about it.

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Re: Should I balance the crank, or

Post by 1972ho » Thu Mar 26, 2020 6:31 pm

So how expensive is a balance job if they don’t have to do anything other than weigh everything and spin it.

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Re: Should I balance the crank, or

Post by SchmidtMotorWorks » Thu Mar 26, 2020 9:40 pm

Caprimaniac wrote:
Thu Mar 26, 2020 5:31 pm
just run it?

So ready to assemble this, and not sure if I'm up to spend time, trouble and $ to get it done.

Internally balanced forged SBF 302 stroker crank. Bobweight on crank 1750 g, actual measured parts give 1660g bobweight.

From factory the throws have some material removed (drilled holes), so assuming reasonably well balanced front to back throws.

Something tells me I won't loose much by running it as is....
That depends mostly on how much you care about vibration.

90g is a lot.
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Re: Should I balance the crank, or

Post by Little Mouse » Thu Mar 26, 2020 9:44 pm

I know this does not relate completely to a V8. But on the 360 firing twins they were over balanced in the two percent to six percent range and there not the only motorcycles like that. They did that to help keep down there known vibe problems. Also in say 10'000 plus rpm 180 degree honda twins everyone in the book put lighter then stock pistons in them, same with all the 4 cylinder bikes. If you want real vibe problems on anything it's the exact opesit to what you have when the reciprocating weight is heavier then the counter weights then you would start turning it into a paint shaker. Even in the olden days it was well known you cant take a factor iron crank that was weighted for lighter cast pistons and stick forged pistons on it. If you had a forged crank and for some silly reason wanted to use cast pistons that was no problem and far as I know if you decide to turn real high rpms they will advise you to over balance it in the counter weights. As long as you feel your all right piston rod combos are all very close then you dont have a problem.

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Re: Should I balance the crank, or

Post by SchmidtMotorWorks » Thu Mar 26, 2020 11:28 pm

Little Mouse wrote:
Thu Mar 26, 2020 9:44 pm
I know this does not relate completely to a V8. But on the 360 firing twins they were over balanced in the two percent to six percent range and there not the only motorcycles like that. They did that to help keep down there known vibe problems. Also in say 10'000 plus rpm 180 degree honda twins everyone in the book put lighter then stock pistons in them, same with all the 4 cylinder bikes. If you want real vibe problems on anything it's the exact opesit to what you have when the reciprocating weight is heavier then the counter weights then you would start turning it into a paint shaker. Even in the olden days it was well known you cant take a factor iron crank that was weighted for lighter cast pistons and stick forged pistons on it. If you had a forged crank and for some silly reason wanted to use cast pistons that was no problem and far as I know if you decide to turn real high rpms they will advise you to over balance it in the counter weights. As long as you feel your all right piston rod combos are all very close then you dont have a problem.
A parallel twin has zero relevance to a V engine.

A 90 degree V engine will have a noticeable vibration at 100 grams out.
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Re: Should I balance the crank, or

Post by Mark O'Neal » Thu Mar 26, 2020 11:42 pm

This is no time for levity. Have you heard what happened to Machine Gun Kelly?

2% of 1750 grams is 35 grams

Balance it.

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Re: Should I balance the crank, or

Post by Caprimaniac » Fri Mar 27, 2020 4:16 am

Have some Corona. spare time and will contact my guy to see if he can do it in a week. My hands are shaking on getting Things done..... SO, can I wait?

Where I live "Best country to live in the world" according to UN or whoever, salary make it a 400$ job.

Whatever.... No, vibration & shaking is not wanted, But at 8000rpm; will I notice any difference...

Later
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Re: Should I balance the crank, or

Post by Baprace » Fri Mar 27, 2020 5:03 am

Caprimaniac wrote:
Thu Mar 26, 2020 5:31 pm
just run it?

So ready to assemble this, and not sure if I'm up to spend time, trouble and $ to get it done.

Internally balanced forged SBF 302 stroker crank. Bobweight on crank 1750 g, actual measured parts give 1660g bobweight.

From factory the throws have some material removed (drilled holes), so assuming reasonably well balanced front to back throws.

Something tells me I won't loose much by running it as is....
Caprimaniac , you have been on this website for a long time with some great input and I think you are a decent engine builder, I think you know what the answer is, Balance the assembly and make it perfect. JMO

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Re: Should I balance the crank, or

Post by PackardV8 » Fri Mar 27, 2020 12:18 pm

On this kind of work where we want quick turnaround and or lower cost, to make it easier on the balance shop, we make the rods and pistons all the same and just send him one of each to confirm the bobweight. With the tedious stuff done, spinning up the crank is quicker and easier for him to turn around.

Has anyone else noticed today's aftermarket pistons and rods come out of the box balanced, so often nothing beyond weighing for confirmation is required, but the cost of the balance job has gone up?
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Re: Should I balance the crank, or

Post by MotionMachine » Fri Mar 27, 2020 12:30 pm

Is that 1750 number the published "this crank will internal at maximum 1750" figure from the manufacturer? Or was it balanced professionally to that bobweight? New out of the box balance is not remotely close to anything, especially the offshore SBF cranks I've done.

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Re: Should I balance the crank, or

Post by rebelyell » Fri Mar 27, 2020 1:31 pm

Have your assembly balanced.

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Re: Should I balance the crank, or

Post by Dave Koehler » Fri Mar 27, 2020 10:25 pm

PackardV8 wrote:
Fri Mar 27, 2020 12:18 pm


Has anyone else noticed today's aftermarket pistons and rods come out of the box balanced, so often nothing beyond weighing for confirmation is required, but the cost of the balance job has gone up?
I respectfully disagree on that.
It is still a crap shoot.
If you have a scale that does not read in tenths or accept a looser tolerance then yes they are all perfect.
Once in a great while I will actually get a piston set like that but it is definitely not the norm.
Rods are the same way. The only time I get one of those perfect sets is with a set of billet aluminum rods or the high end rods.
Never with low bidder rods.

Capri. Quit screwing around. 90 grams (2.74 ounces) per throw difference? Come on. Do it right.
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Re: Should I balance the crank, or

Post by superpursuit » Sat Mar 28, 2020 12:43 am

So you want to pull 8000 rpm and want to leave your engine way, way out of balance?
Good luck with that.
Let us know how long it lasts please.

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Re: Should I balance the crank, or

Post by Mark O'Neal » Sat Mar 28, 2020 3:12 am

Dave Koehler wrote:
Fri Mar 27, 2020 10:25 pm
PackardV8 wrote:
Fri Mar 27, 2020 12:18 pm


Has anyone else noticed today's aftermarket pistons and rods come out of the box balanced, so often nothing beyond weighing for confirmation is required, but the cost of the balance job has gone up?
I respectfully disagree on that.
It is still a crap shoot.
If you have a scale that does not read in tenths or accept a looser tolerance then yes they are all perfect.
Once in a great while I will actually get a piston set like that but it is definitely not the norm.
Rods are the same way. The only time I get one of those perfect sets is with a set of billet aluminum rods or the high end rods.
Never with low bidder rods.

Capri. Quit screwing around. 90 grams (2.74 ounces) per throw difference? Come on. Do it right.
Or standard was 2.5 grams up or down, but we tried to keep them within 1 gram. A production run varied about 12 grams from lightest to heaviest. I had the luxury of productions runs, it's a lot harder it you're making 8.

But, you've got to admit, they are a lot closer than they were back when it was all manual machinery.

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