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Old 01-12-2010, 08:53 PM   #1
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changing pistons without rebalancing?

in my garage i have a professionally built 355 using forged probe pistons that weigh 541 grams. the pistons are high compression and im converting the motor to be used with a blower, the motor is fully balanced.

if i find another forged piston with a similar weight, can i install them without rebalancing? what differance in weight would be considered "acceptable"?
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Old 01-12-2010, 08:56 PM   #2
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I wouldn't chance it, no way.
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Old 01-12-2010, 08:56 PM   #3
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Quote:
Originally Posted by sn8ke eatr View Post
in my garage i have a professionally built 355 using forged probe pistons that weigh 541 grams. the pistons are high compression and im converting the motor to be used with a blower, the motor is fully balanced.

if i find another forged piston with a similar weight, can i install them without rebalancing? what differance in weight would be considered "acceptable"?
Using a new piston that is lighter is no problem, assuming it was balanced to 50 or 51% in the first place. Substituting a heavier piston can be a problem if it's more than 10 grams or so heavier.

Rich
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Old 01-12-2010, 09:05 PM   #4
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thank you guys for the fast responses,

so the keith black pistons i was *hoping* to use weigh 514 grams apiece, as opposed to the current pistons weight of 541 grams.

so is that 27 gram differeance too great to chance? what would you fellas do? this is for a mild motor that will top out at about 6400prm
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Old 01-12-2010, 09:14 PM   #5
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Originally Posted by sn8ke eatr View Post
thank you guys for the fast responses,

so the keith black pistons i was *hoping* to use weigh 514 grams apiece, as opposed to the current pistons weight of 541 grams.

so is that 27 gram differeance too great to chance? what would you fellas do? this is for a mild motor that will top out at about 6400prm
I would use them w/o rebalancing. With one iteration of my bracket motor (~900hp @ 7,500rpm) I used a piston that was at least that much lighter (don't remember the exact numbers) with some trepidation, even though a couple of people told me it would be fine. It was and that is where I learned about "overbalancing". Some engine builders routinely overbalance and believe it makes a smoother, more reliable engine. Overbalance means extra weight on the crank counterweights, equivalent to switching to a lighter piston. This is assuming the rotating assembly was not overbalanced in the first place. You should have the numbers from the person who balanced the crank. If not, assuming it was a routine balance job where overbalance was not requested, it is almost certainly balanced to 50% and in that case, the lighter pistons will be fine.

I think I once made a long post about this, you may want to do a search.

Rich
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Old 01-12-2010, 09:25 PM   #6
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im almost positive i have a "standard" balance, your post helps me out very much, thanks rich.
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Old 01-13-2010, 08:17 AM   #7
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Very cool to know.

I switched from a -5cc SRP to a -24cc SRP piston and had no problems. Using the same crank/rods and going to a lighter piston I might just skip the balance this time around again.
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Old 01-13-2010, 12:54 PM   #8
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Very cool to know.

I switched from a -5cc SRP to a -24cc SRP piston and had no problems. Using the same crank/rods and going to a lighter piston I might just skip the balance this time around again.
Some good reading. I've had old school guys say if you've balanced the rotating assembly once, and you rebuild and marked every thing, no need to do it twice on the same parts. The again I've had some machine shops say you absolutely need it done even with same parts. Although I'm sure it's a CYA issue for them.

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Old 01-13-2010, 12:56 PM   #9
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My new pistons arrived yesterday and I was thinking just the same as the OP.

My old SRP 4.030 piston weighs 400 gr. + 131 gr. the pin.
New Ross 4.040 piston weighs 376 gr. + 120 gr. the pin. Even though the piston is bigger, all in all it weighs 35 gr. less.
So I won’t be balancing either.

Thanks for the info
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Old 01-13-2010, 02:59 PM   #10
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using a new piston that is lighter is no problem, assuming it was balanced to 50 or 51% in the first place. Substituting a heavier piston can be a problem if it's more than 10 grams or so heavier.

Rich


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Old 01-13-2010, 08:34 PM   #11
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You're fine with that much over-balance - run it.
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Old 01-14-2010, 11:59 AM   #12
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For $200 why would you not rebalance it?
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Old 01-14-2010, 12:30 PM   #13
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Where I come from it's more like $400!!!
Just honing my block from 4.030 to 4.040 was like $540 (friendship price)
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Old 01-14-2010, 12:52 PM   #14
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You can do it as long as its close in weight. What you could do too is if you get a piston that is slightly heavier for instance within 10 grams you could always take off weight off the piston to match up to what you already have.
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Old 01-14-2010, 01:18 PM   #15
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Is there a piston/ rod combo that we could use in a stock motor(stock crank) without re-balancing? Looking for a low compression setup.
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Old 01-14-2010, 01:18 PM
 
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