<VV> rod balancing - Oops, don't get me started

[BBRT]

Great info!

Chuck S
----- Original Message ----- > Don't concentrate on the total rod weight.
> As each end of the con rod moves differently, the big end is considered 
> rotating weight, and the pin end is considered reciprocating weight, like 
> the piston, pin and rings.  Thus all the rotating ends should be equal, 
> and all the pin ends should be equal.  A rod could be 8 grams light on the 
> pin end, and 8 grams heavy on the small end, and checking the total weight 
> would miss it completely.
>
> I believe you will find it difficult to get all the ends within a few 
> grams, even with a factory original set.  Selectively grinding I-beams, 
> and maybe even picking candidates from multiple sets will be required to 
> make a "balanceable" set..
>
> To make a nice set of rods follow this sequence.
> - weigh as many good used rods as you can find end-for-end to get a 
> balanceable set of 8. Number the rods and caps carefully so they will 
> remain partners.  DO NOT file notches in the I-beam ! Do not cut, stamp or 
> file any part of the rod corresponding to where a crack developed on this 
> rod
> http://myfiles.stonybrook.edu/~nmachtay/sae/engine/images/IMG_0174.JPG
>
> Only put ID marks as shown here. 
> http://www.autozone.com/images/cds/gif/small/0900823d80050daf.gif
>
> - On each rod, with bearings out , Tighten the nut, then loosen the nuts, 
> and try peek thru the parting face against a brightly lit background in 2 
> directions.  You're trying to find gaps, which indicate rods with bolt 
> holes machined out of perpendicular to the faces or not parallel with the 
> side faces. A parting face gap of more than about 0.002 inch moves the rod 
> to 2nd string, or the trash.
> - magnaflux the rods to check for fatigue cracks or other flaws. No sense 
> reworking a cracked part. Small problems can be ground out.  A key area to 
> inspect is the broached notch in the rod for the rod bolt's head. Removing 
> the bolts is necessary to get a good look there.  REmoving the bolts 
> guarantees the big ends will need to be rebuilt, but they probably did 
> anyway.
> - Check the rod relative lengths and pick the 6 closest ones.  Discard any 
> varyijng by more than 0.010 inch.  Dial verniers with one jaw in the pin 
> hole and one jaw in the big end bore are close enough. Use the thin angle 
> section at the tip of the jaws.  Measure the pin bores to check for proper 
> interference ( 0.0008 - 0.0012 inch smaller than wrist pin diameter). This 
> needs a bore gage or inside mike
> - Check the squareness of the parting faces to the side clearance faces 
> with a combination square. Like the "gap" test above, More than 0.002 can 
> create problem with bolt bending and odd side clearance geometry after 
> rebuilding.
> - Smooth the bolt head notch and deburr the rods, always grinding 
> lengthwise. Install the old bolts snug, and Pre balance the 8 rods end for 
> end within a few grams.  If the rods end-for-end weights are close enough 
> that all the I-beams can be ground to remove the forging flash, do it, 
> only grinding lengthwise
>
> - Magnaflux again to see if grinding the I-beams uncovered flaws
> - Have the rods shot peened with cast steel 230 shot to the almen 
> intensity in the old Chevy power book  Maybe Almen 15 A (I forget) .The 
> important area is from the bolt seat up to the wrist pin bore.
> - Have the rods rebuilt using new US made bolts. Explain you do not want 
> the parting faces angle cut (that pooches in the sides of the bore and 
> makes the big end clean up 100% pretty , but puts the bolts in bending) 
> and the roundness, taper, bell mouth and barrel shape must be less than 
> 0.0002 inch ( "2 tenths" ! yes, that is hard to do, and some techs in some 
> shops don't even know how to measure like that.  )  Also request that the 
> minimum, and similar amounts be removed from the rod and cap faces,  to 
> maintain center to center length that we already know are pretty equal 
> because we measured them.
>
> An important function of the big end is to maintain a round housing for 
> the rod bearing at all times.  That requires stiffness, so leave the ribs 
> full height.  Rib bending stiffness is a ~cube function, so a rib 2X high 
> is 8X stiffer.  Conversely a rib 1/2 as high is only 1/8 as stiff. 
> Artistically grinding the ribs thinner compromises stiffness less.  The 
> block between the ribs on the cap is fair game, but difficult to remove 
> without a milling machine.  Removing the un-threaded nub on the threaded 
> end of the stock rod bolts is worth a gram or 2, but I'd save that for the 
> finish balancing later
>
> Don't bevel the rod like this since it reduces the "footprint" of the 
> parting face between rod and cap.
> http://www.skidmarkracing.net/photos/Connecting%20Rod.jpg
> See how these 911 rods have a large "footprint" between rod and cap 
> designed in.
> http://franksplace2.com/images/Connecting%20Rods.jpg
> That's important to reduce the bending loads on the bolt when at 6000 rpm 
> plus the crank journal pulls on the big end like this.
> http://www.mayfieldclinic.com/Images/PE-PSR_Figure3.jpg
> or this
> http://newsimg.bbc.co.uk/media/images/42393000/jpg/_42393771_hook.jpg
>
> Big end stiffness is usually not a pass-fail game.  Reduce the stiffness 
> too much, and you won't know it until the day you are busy dealing out 
> vigilante justice to that Camaro or Honda at 6800 rpm rounding dead man's 
> curve, and maybe that rod bearing seizes and spins.  "Gee, it never did 
> THAT before."
>
> After the rods are rebuilt check the length and record for use equalizing 
> deck heights, check for straightness (100 years ago 0.005 Oversized Harley 
> wrist pins made a good slip fit checking pin), straighten and gently 
> finish balance.
>
> Scrub clean, lightly oil and bag for use at trial assembly.
>
>
> Dan Timberlake
> Westford, Massachusetts, USA