<VV> Re: VirtualVairs Digest, Vol 13, Issue 84

[djtcz at comcast.net]

bottom posted
-------------- Original message -------------- 
From: virtualvairs-request at corvair.org 

bottom poseted
-------------- Original message -------------- 
I'll be putting the '69 Fitch 140 back together soon.
I just need a check of how close the weight of the
piston/rod assys should be. 

I'll lay out what will go back in: * minus the wrist
pin which is about 100 grams. My scale only goes to
1000 g.

#1 903.9 grams*
#2 901.3
______________
     +2.6 

#3 904.0 grams*
#4 901.3
_____________
    + 2.3

#5 904.0 grams*
#6 902.9
______________
    +1.1 


It's interesting that 1,3,5 are heavier than 2,4,6,
but there is little left to lighten on 1,3,5.

For those not too familiar with grams, a large sheet
of Bounty paper towel weights about 2.3 grams.

I think this looks good. Opinions???

MIKE KOVACS
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Dan Timberlake replies 

I don't think total con rod weights mean very much. Especially on Corvair rods. 

Each end of a connecting rod dances differently.  
The big end (BE) describes a circular motion, so is considered part ot the rotating mass, along with the crank, flywheel, etc.
The Corvair wrist pin (WP) or small end describes a straight line side-to-side motion, along the horizon, so is considered reciprocating mass, just like a piston, or the rings, etc.

For most purposes a con rod can be described as 2 masses.  One mass concentrated at the big end centerline, and the other concentrated at the wrist pin centerline.  Added together they are the total rod weight, so knowing any 2, the 3rd can be figger'd with addition or subtraction.
BE + WP = total rod weight/mass

Chevy's balance correction consists of a boss of extra steel on the WP  end.  Most other engines have balance bosses on both end of their con rods, to allow adjustment of the rotating and recip mass.  Some Buicks had a single boss, part way up each I-beam, so as to influence both the BE and WP end proportionally.

If you were to rig up a means to weigh each end separately I believe you will find (1) a 5 to 10 gram difference between BE weights of Corvair rods, even from the same litter, and (2) that total rod weights will offer no clue which big ends are fat.  My hunch is, if right from Chevy 2 total rod weights are the same, but one has a WP boss cut way down, that BE is heavy.

Unfortunately The design of the Corvair rod big end offers very little extra material to remove without reducing big end stiffness.  Reducing rib height or the "footprint" at the parting face are particularly bad, so should be avoided.  That cute little block between the ribs on the cap is probably fair game, but tough to remove without machine tools. As on the I-beam section, crosswise grinding and tool marks are forbidden. 

I worked in a busy East Coast balance shop from 1973 to 1996.   We used to suggest Corvair customers bring in a all their spare con rods so we could get a balanceable set. Even then creative grinding on the I-beam of some rods was required to balance a stock set.  IF the I-beams will be ground for performance preparation then even more careful pre-selection of a set  is required.

As a practical matter, a few grams difference in rotating or reciprocating weight is not a big deal in any engine.  Porsche allowed up to 5 gram difference in piston weights on early 911s and 912s.