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another balance question

Posted: Wed Nov 06, 2019 8:44 pm
by donclark
New pistons came in 17 gr heavier @411 gr. with rings. The old pins weighed 106 gr @ 0.787 x 2.250 x 0.200, H13. There is an H13 DLC 0.180 wall tapered pin available that weighs 91 gr. Problem solved. My questions are: a) is this enough pin and b) is 17 grams even enough to be concerned about. Total bob weight is 1356 gr, max engine speed will be 9400.

Re: another balance question

Posted: Thu Nov 07, 2019 11:45 am
by dwilliams
"Don't worry, be happy." The balance machine can pick up 17g, but it won't make any difference when running.

If you're concerned about the pins call the manufacturer and ask their tech guys.

Re: another balance question

Posted: Thu Nov 07, 2019 12:19 pm
by mag2555
A lot of engineers state that even 25 grams of unbalance is not a issue, but this it at 7000 or so rpm, at 9400 I would want far better tolerance then 17!

Re: another balance question

Posted: Thu Nov 07, 2019 12:57 pm
by BillK
What engine is it ?

Re: another balance question

Posted: Thu Nov 07, 2019 2:17 pm
by donclark
SBF, 3.25 stroke

Re: another balance question

Posted: Thu Nov 07, 2019 3:19 pm
by steve316
When Bill Jenkins was running Pro Stock with a SBC he would over balance @ 52%. I have over and under balanced 35g and more. It didn't run any better or any worse, also the bearings looked like new on tear down.

Re: another balance question

Posted: Fri Nov 08, 2019 1:12 am
by Roundybout
My biggest concern would be going to a heavier piston and a thinner wall pin. The weights involved in how it will balance out is secondary. I’m assuming a drag race application and 9400 RPM is at the shift/stripe. I’d talk to the piston/pin manufacturer for some possible guidance. If the current piston and pin combo looks good it may be marginal with the new stuff especially with the forces Involved at that RPM. My hunch is you’ll be ok but skimping on the pin to save weight is my last resort unless there’s a comfortable safety margin there.

Re: another balance question

Posted: Fri Nov 08, 2019 1:28 am
by KnightEngines
I put pistons that were 45g heavier in a motor without a rebalance.
It turns 8200rpm & looks mint on pull down.

Just run it, you'll never know the difference.

Re: another balance question

Posted: Fri Nov 08, 2019 7:05 am
by mag2555
My main concern would also be for the unneeded weight placing a greater load on the Rods.

Re: another balance question

Posted: Fri Nov 08, 2019 9:59 am
by donclark
KnightEngines wrote:
Fri Nov 08, 2019 1:28 am
I put pistons that were 45g heavier in a motor without a rebalance.
It turns 8200rpm & looks mint on pull down.

Just run it, you'll never know the difference.
This was also the opinion of the man that did the original balance. His shop serviced engines for several ARCA teams back in the day. If you compare with the total bobweight, it amounts to 1.2%

Re: another balance question

Posted: Fri Nov 08, 2019 10:12 am
by Old School
mag2555 wrote:
Fri Nov 08, 2019 7:05 am
My main concern would also be for the unneeded weight placing a greater load on the Rods.
Would you define the " unneeded weight" in this application?

Re: another balance question

Posted: Fri Nov 08, 2019 12:07 pm
by mag2555
Just talking in general since we strive for light weight to have a more rev happy motor.