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Connecting Rod Balancing Page 2


Looking at both ends of the connecting rods,  one can see the balance pads.  This is the area on the rods which was forged into the part,  specifically to be removed as needed to balance the rods one to another.

The image on the left is obviously the small end (piston end) of the connecting rod,  and the right image is the crank end.   The piston end has been ground down at the factory.  Since all the rods weighed about the same (within 3 grams of one another) and only the piston end was machined,  I assume that this was the factory practice,  to simply weigh the rods and remove weight from one end only.

Following Richard Finch and Bob Sutcliffe's advice,  I balanced each end separately.


Here's the completed stack.  


Rather than just grind more from the top of the pad as the factory did,  I removed most of the mass from the corners,  rounding the pad quite a bit.  This was pretty straight forward.

All in all,  not much material had to be removed from the piston end of the connecting rod.


Notice how the second from the top,  and the bottom rods don't have any work done to the piston end balance pad.  That's because these two were the lightest,  at 113 grams.  The other's were anywhere from 115g to 123g.

On several of the crank ends,  I chose to not use just the balancing pad for weight removal.  In fact if I had,  I would have not only run completely out of material to remove,  but in my opinion,  I would have weakened the cap.  So I opted to remove excess material before starting to grind on the balancing pad.  I did this to every rod,  even if only a gram or two was to be removed.

Also see page 15 of Clark's "Otto" catalog to see where they remove material to lighten rods.

The left image shows a rod which has not been ground.  The right image shows one which only a little bit of weight was removed.  The material removed is part of the excess which Bob removes when he lightens rods,  so I felt it was safe to start here.

This rod needed just a bit more weight removed than the rod in the previous photo...

...but this one needed a bit more.  This is about as far as I would go,  before getting into grinding on the balancing pad on the.

Any and all grinding was followed up with some very fine sanding (polishing) to reduce the stress riser effect of the scratches left by the grinding wheel.

On the left is a stock rod bearing cap.  
  On the right is one which has been ground down by about 1.5 - 2 grams.


This one however has a good 4-5 grams removed.  Notice that I dished out the pad,  rather than just grind it off along with the ribs which make up the "C" section beam.

And this is the finished stack.  The top one has just a little removed,  the next two substantially more,  the next two none, and the bottom one,  a whole bunch.

I started out with the small ends weighing anywhere from 308g to 316g and the big end weighing 113g to 123.

Now the small ends weigh within 1g of 113g,  and the big ends weigh within 1g of 305g. 

All together I took 10g off each rod.