Based on the previous posts I've read about engine balancing, I have the feeling this will be a controversial question, but here goes ...

I'm buttoning up a rebuild on a 1993 302 for an F150. I ended up replacing the pistons with hypers (Summit kit) and a new set of bushed con-rods from Eagle (basically I just didn't like the pressed-pin stock rods). Here's how the new parts compare to the old (all weights in grams):

<TABLE style="WIDTH: 383pt; BORDER-COLLAPSE: collapse" cellSpacing=0 cellPadding=0 width=510 border=0 x:str><COLGROUP><COL style="WIDTH: 128pt; mso-width-source: userset; mso-width-alt: 6253" width=171><COL style="WIDTH: 47pt; mso-width-source: userset; mso-width-alt: 2304" width=63><COL style="WIDTH: 50pt; mso-width-source: userset; mso-width-alt: 2413" width=66><COL style="WIDTH: 67pt; mso-width-source: userset; mso-width-alt: 3254" width=89><COL style="WIDTH: 91pt; mso-width-source: userset; mso-width-alt: 4425" width=121><TBODY><TR style="HEIGHT: 13.5pt" height=18><TD class=xl28 style="BORDER-RIGHT: #d4d0c8; BORDER-TOP: windowtext 1pt solid; BORDER-LEFT: windowtext 1pt solid; WIDTH: 128pt; BORDER-BOTTOM: #d4d0c8; HEIGHT: 13.5pt; BACKGROUND-COLOR: #ffff99" width=171 height=18></TD><TD class=xl24 style="BORDER-RIGHT: #d4d0c8; BORDER-TOP: windowtext 1pt solid; BORDER-LEFT: #d4d0c8; WIDTH: 47pt; BORDER-BOTTOM: #d4d0c8; BACKGROUND-COLOR: #ffff99" width=63>Old</TD><TD class=xl24 style="BORDER-RIGHT: #d4d0c8; BORDER-TOP: windowtext 1pt solid; BORDER-LEFT: #d4d0c8; WIDTH: 50pt; BORDER-BOTTOM: #d4d0c8; BACKGROUND-COLOR: #ffff99" width=66 x:str="New ">New</TD><TD class=xl24 style="BORDER-RIGHT: #d4d0c8; BORDER-TOP: windowtext 1pt solid; BORDER-LEFT: #d4d0c8; WIDTH: 67pt; BORDER-BOTTOM: #d4d0c8; BACKGROUND-COLOR: #ffff99" width=89>Difference</TD><TD class=xl26 style="BORDER-RIGHT: windowtext 1pt solid; BORDER-TOP: windowtext 1pt solid; BORDER-LEFT: #d4d0c8; WIDTH: 91pt; BORDER-BOTTOM: #d4d0c8; BACKGROUND-COLOR: #ffff99" width=121>% Difference</TD></TR><TR style="HEIGHT: 12.75pt" height=17><TD class=xl32 style="BORDER-RIGHT: #d4d0c8; BORDER-TOP: #d4d0c8; BORDER-LEFT: windowtext 1pt solid; BORDER-BOTTOM: #d4d0c8; HEIGHT: 12.75pt; BACKGROUND-COLOR: #ffff99" height=17>Piston + Rings + Wristpin</TD><TD class=xl30 style="BORDER-RIGHT: #d4d0c8; BORDER-TOP: #d4d0c8; BORDER-LEFT: #d4d0c8; BORDER-BOTTOM: #d4d0c8; BACKGROUND-COLOR: #ffff99" x:num="785.14285714285711">785.1</TD><TD class=xl30 style="BORDER-RIGHT: #d4d0c8; BORDER-TOP: #d4d0c8; BORDER-LEFT: #d4d0c8; BORDER-BOTTOM: #d4d0c8; BACKGROUND-COLOR: #ffff99" x:num>818.5</TD><TD class=xl30 style="BORDER-RIGHT: #d4d0c8; BORDER-TOP: #d4d0c8; BORDER-LEFT: #d4d0c8; BORDER-BOTTOM: #d4d0c8; BACKGROUND-COLOR: #ffff99" x:num="33.35714285714289">33.4</TD><TD class=xl33 style="BORDER-RIGHT: windowtext 1pt solid; BORDER-TOP: #d4d0c8; BORDER-LEFT: #d4d0c8; BORDER-BOTTOM: #d4d0c8; BACKGROUND-COLOR: #ffff99" x:num="4.2485443959243127E-2">4.25%</TD></TR><TR style="HEIGHT: 12.75pt" height=17><TD class=xl32 style="BORDER-RIGHT: #d4d0c8; BORDER-TOP: #d4d0c8; BORDER-LEFT: windowtext 1pt solid; BORDER-BOTTOM: #d4d0c8; HEIGHT: 12.75pt; BACKGROUND-COLOR: #ffff99" height=17>Rods</TD><TD class=xl30 style="BORDER-RIGHT: #d4d0c8; BORDER-TOP: #d4d0c8; BORDER-LEFT: #d4d0c8; BORDER-BOTTOM: #d4d0c8; BACKGROUND-COLOR: #ffff99" x:num>560.0</TD><TD class=xl30 style="BORDER-RIGHT: #d4d0c8; BORDER-TOP: #d4d0c8; BORDER-LEFT: #d4d0c8; BORDER-BOTTOM: #d4d0c8; BACKGROUND-COLOR: #ffff99" x:num>499.0</TD><TD class=xl30 style="BORDER-RIGHT: #d4d0c8; BORDER-TOP: #d4d0c8; BORDER-LEFT: #d4d0c8; BORDER-BOTTOM: #d4d0c8; BACKGROUND-COLOR: #ffff99" x:num>-61.0</TD><TD class=xl33 style="BORDER-RIGHT: windowtext 1pt solid; BORDER-TOP: #d4d0c8; BORDER-LEFT: #d4d0c8; BORDER-BOTTOM: #d4d0c8; BACKGROUND-COLOR: #ffff99" x:num="-0.10892857142857143">-10.89%</TD></TR><TR style="HEIGHT: 13.5pt" height=18><TD class=xl34 style="BORDER-RIGHT: #d4d0c8; BORDER-TOP: #d4d0c8; BORDER-LEFT: windowtext 1pt solid; BORDER-BOTTOM: windowtext 1pt solid; HEIGHT: 13.5pt; BACKGROUND-COLOR: #ffff99" height=18>Total Assembly</TD><TD class=xl25 style="BORDER-RIGHT: #d4d0c8; BORDER-TOP: #d4d0c8; BORDER-LEFT: #d4d0c8; BORDER-BOTTOM: windowtext 1pt solid; BACKGROUND-COLOR: #ffff99" x:num="1345.1428571428571">1345.1</TD><TD class=xl25 style="BORDER-RIGHT: #d4d0c8; BORDER-TOP: #d4d0c8; BORDER-LEFT: #d4d0c8; BORDER-BOTTOM: windowtext 1pt solid; BACKGROUND-COLOR: #ffff99" x:num>1318.5</TD><TD class=xl25 style="BORDER-RIGHT: #d4d0c8; BORDER-TOP: #d4d0c8; BORDER-LEFT: #d4d0c8; BORDER-BOTTOM: windowtext 1pt solid; BACKGROUND-COLOR: #ffff99" x:num="-26.64285714285711">-26.6</TD><TD class=xl27 style="BORDER-RIGHT: windowtext 1pt solid; BORDER-TOP: #d4d0c8; BORDER-LEFT: #d4d0c8; BORDER-BOTTOM: windowtext 1pt solid; BACKGROUND-COLOR: #ffff99" x:num="-1.9806711979609152E-2">-1.98%</TD></TR></TBODY></TABLE>


I've read a fair amount about reciprocating vs rotating mass in the engine balance calculations. I didn't measure the recip-vs-rotating breakdown on the rods (not sure I actually could do this accurately) but since they are bronze-bushed, and the factory ones weren't, I assume a good bit of the weight difference is on the reciprocating end (bronze vs steel), and thus countered by the heavier pistons. So it seems to me these overall weight differences are minor ..

Questions:

... If this were your project, would you get the crank re-balanced? Needless to say I'm trying to save money here.

... if I were to get the crank re-balanced I assume I need to take a full piston set (with rods, pin and rings) to the machinist along with the harmonic balancer and the flexplate. Since I already have the heads back on, can I remove a piston assembly from the bottom, and then re-insert it from the bottom with my ring compressor?

Final comment:

I've read a lot about the 28 oz flexplate vs the 50 oz. The steel counterweight on the flexplate is about 10 cm long by 2 cm wide by 1 cm thick, so 20 cc * 8 g/cc ~ 160 g = about 6 oz (28 g/oz). So where do the 50 or 28 oz come from? If these numbers actually refer to the moment of inertia (weight times radius of rotation) this might make more sense since this counterweight is located about 6 inches from the center of rotation. The reason I really want to know is I'm trying to "calibrate" my worst-case weight differences (assuming all of the differences in the rod weights are rotating and not reciprocating mass) with the other reader's experiences in this forum: i.e. that using the wrong flexplate definitely leads to engine vibration problems!

For what it's worth, I'd prefer to hear only from knowledgeable readers who have relevant experience in this area. I'm an engineer (PhD actually, but only a poor ChemE) and I can speculate with the best of them! But I know enough to know that experience is #1 in technical areas like this.

Final FINAL comment:

I ride a big V-twin motorcycle (VTX1800) and I PAID for some "good" vibration. Honestly, how important is the issue of V8 balancing anyway? Does anyone know a better reference site than this one ?

http://www.babcox.com/editorial/ar/eb10330.htm

At face value this article reads as much or more "how to convince your customer that they need a balance job" than "why the balance job is really needed"!!

 

There's no way to get a piston out through the bottom (at least not intentionally) in one piece. Even though your overall weights are not that much different, there are usually small variations in the individual weights of each component. Theoretically, there is the possibility that during assembly, you may place, say, all the "heavier" parts at one end of the crankshaft. That would definitely cause a "wobble". The assembly has to be balanced as a whole, and the components installed in the same position as it was balanced. Make sense? I would definitely have it balanced. It's worth the $150 or so. JMO.-Pete

 

With all those changes, get it balanced. Pistons, you could get away with , but changing rods? No way I'd try it. The better the dynamic balance is, the longer the motor's going to last. Vibrations are hard on internal parts. Mainly the bearings (they get hammered) and the block(good chance it's going to get stress cracks down the road from the shakes). Question: How many hundreds of thousands of miles do those V-twin engines last ?

 

flexplate will cause SERIOUS vibration. get the crank cut and polished, then balanced with oversized bearings.

 

questioning the bob weights....

stock is ususlly about 1750-1835
the long rod moter we balanced came in at 1630.
1348 sounds awful light. considering the difference of weights on the rods, i'd definitely rebalance. You have picked up almost 35 grams on the large (heavy ) end of the reciprocating weight. how did you weigh the rods, total, or big end vs small end...just another variation to consider. All things considered.....a balance charge at the machine shop would be worth it, IMHO