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

[djtcz at comcast.net]

snipped and bottom posted 

-------------- Original message -------------- 
I have a set of .40 over cylinders and forged pistons that I've never  used.  
The rods and pistons were balanced.  I have always been a  little unsure 
about the fact that the rod cap casing ridges were ground down to  balance them.  
I've only completely rebuilt one engine before and that was  stock without any 
balancing beyond weighing pistons to get a close weight  match.  And I 
probably only have one more that I'll do in my Corvair life  (unless I get that 
garage of my dreams) and I'd like to use these .40 over jugs  in a new engine 
this 
year.  Anybody with experience building performance  engines have any 
recommendations or guidance as to how much rod caps can be  ground before they 
become 
too weak?

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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