<VV> connecting rod culling / balancing

[djtcz at comcast.net]

snipped and bottom posted 

-------------- Original message -------------- 
Bryan Blackwell  wrote 
Greetings,

Could somebody tell me the approximate weights of a Corvair piston  
and rod?

Reason - I'd like to go through some rods and see if I can make up a  
set closer than stock to use as a base to make a balanced set.  I see  
some inexpensive scales on eBay that might serve the purpose well,  
but I don't know what weight it needs to handle.

Thanks!


========================================================
Hi Bryan,

 The rod end-for end weight variation was always my problem ( I worked in a balance shop for 23 years). I'm not sure total weight, especially of rod and piston assemblies, would make even a  troublesome gross variation evident.

The reason it is a  problem is the factory weight adjustment is for total weight, made by removing steel from the boss at the "top" (side?) of the pin end. No big end boss Probably because lack of clearance to engine innards.  Most con rods are provided with extra metal just for "balancing"at both ends.  Balancing Corvair rods had to be carefully coordinated in with all the other rod work

I'd avoid rods with the balancing boss milled to the max.
Like this Jag rod, except its boss 90 degrees to "ours". 

Some would figure a rod with no boss would likely have more steel in the I-beam, and thus be stronger. Late rods have plenty strong I-beams. A greater danger is fatigue cracks originating from the ugly  notch broached for the rod bolt head.  I'd smooth and deburr that area, and have it shot peened with steel shot to Almen 15 A, or whatever MIL spec 13165 says. That means the bolts have to come out, and that means the rod Big Ends must be rebuilt.

So the sequence is - 
- sort by end-for end weight
- rough balance, maybe within 5 grams end-for-end, while including grinding the I-beams, which is not always possible due to variations within the available sets. Weight removal at big end should not reduc rib height or "foot print" at parting face.  Yes, that is tough to do
- remove bolts, keeping cap and rod together
- magnaflux (wet method, fluoresecent for best sensitivity), paying close attention to the broached notch - remove any indications with small carbide burr.  Dremel OK. DEmagnetize
- use a carbide burr or small stone to smooth and recontour the broached notch and the similar location on the cap
- shot peen, avoiding the parting faces, big end bore, and pin bore
- rebuild big ends with new bolts. Less than 0.0002 inch taper ! That's hard to do. 0.001 inch out of round as long as its big across the parting face (lemon shaped).
- check wrist pin interference and rod bend and twist and strighten

Dan Timberlake 
Westford, Massachusetts, USA