changing pistons without rebalancing? - CamaroZ28.Com Message Board


Reply
 
Thread Tools Search this Thread
Old 01-12-2010, 08:53 PM   #1
Registered User
Thread Starter
 
Join Date: Feb 2007
Location: smithtown, NY
Posts: 1,099
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"?
sn8ke eatr is offline   Reply With Quote
Old 01-12-2010, 08:56 PM   #2
Registered User
 
RamAir95TA's Avatar
 
Join Date: Sep 2001
Location: Woodstown, NJ
Posts: 4,160
I wouldn't chance it, no way.
RamAir95TA is offline   Reply With Quote
Old 01-12-2010, 08:56 PM   #3
Moderator
 
rskrause's Avatar
 
Join Date: Dec 1969
Location: Buffalo, New York
Posts: 10,756
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
rskrause is offline   Reply With Quote
Old 01-12-2010, 09:05 PM   #4
Registered User
Thread Starter
 
Join Date: Feb 2007
Location: smithtown, NY
Posts: 1,099
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
sn8ke eatr is offline   Reply With Quote
Old 01-12-2010, 09:14 PM   #5
Moderator
 
rskrause's Avatar
 
Join Date: Dec 1969
Location: Buffalo, New York
Posts: 10,756
Quote:
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
rskrause is offline   Reply With Quote
Old 01-12-2010, 09:25 PM   #6
Registered User
Thread Starter
 
Join Date: Feb 2007
Location: smithtown, NY
Posts: 1,099
im almost positive i have a "standard" balance, your post helps me out very much, thanks rich.
sn8ke eatr is offline   Reply With Quote
Old 01-13-2010, 08:17 AM   #7
Registered User
 
Join Date: Feb 2004
Location: Moore OK
Posts: 2,183
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.
jsetzer is offline   Reply With Quote
Old 01-13-2010, 12:54 PM   #8
Registered User
 
Bersaglieri's Avatar
 
Join Date: Sep 2003
Location: The Wild West
Posts: 5,904
Quote:
Originally Posted by jsetzer View Post
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.

-Dustin-
Bersaglieri is offline   Reply With Quote
Old 01-13-2010, 12:56 PM   #9
F6
Registered User
 
Join Date: Jan 2003
Location: Lengede
Posts: 165
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
F6 is offline   Reply With Quote
Old 01-13-2010, 02:59 PM   #10
Registered User
 
Join Date: Feb 2003
Location: Mooresville, NC
Posts: 2,637
Quote:
Originally Posted by rskrause View Post
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


+1
marshall93z is offline   Reply With Quote
Old 01-13-2010, 08:34 PM   #11
Registered User
 
MachinistOne's Avatar
 
Join Date: May 2004
Location: Bay Area, CA
Posts: 2,002
You're fine with that much over-balance - run it.
MachinistOne is offline   Reply With Quote
Old 01-14-2010, 11:59 AM   #12
Registered User
 
ayers1967's Avatar
 
Join Date: Jan 2007
Posts: 164
For $200 why would you not rebalance it?
ayers1967 is offline   Reply With Quote
Old 01-14-2010, 12:30 PM   #13
F6
Registered User
 
Join Date: Jan 2003
Location: Lengede
Posts: 165
Where I come from it's more like $400!!!
Just honing my block from 4.030 to 4.040 was like $540 (friendship price)
F6 is offline   Reply With Quote
Old 01-14-2010, 12:52 PM   #14
Registered User
 
Join Date: Aug 2005
Location: Tampa, FL
Posts: 415
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.
sam pace is offline   Reply With Quote
Old 01-14-2010, 01:18 PM   #15
Registered User
 
cause4panic's Avatar
 
Join Date: Apr 2005
Location: NYC
Posts: 432
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.
cause4panic is offline   Reply With Quote
Reply

Related Topics
Thread Thread Starter Forum Replies Last Post
Rebuilding the Lt-1 from 0 ... Where do i begin ? Sergio LT1 Based Engine Tech 11 01-27-2016 04:27 PM
94' to keep up with new SS Dmonty LT1 Based Engine Tech 33 06-02-2015 11:19 PM
Mahle 4.030 pistons & rings 1963SS Parts For Sale 1 02-06-2015 10:05 AM
New 383 stroker DART block 1000+ Horsepower D1SC383LT4 Parts For Sale 1 01-26-2015 01:41 PM
Crank Replacement 1996LT1Z28 LT1 Based Engine Tech 8 12-01-2014 01:01 AM


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 05:40 AM.


Copyright © 1996-2017 CamaroZ28.com. All rights reserved.