John A. Marker

I promised to post the results of the balancing of my brand new ZZ4 GM short block. (Also posted in C4 Tech) This came assembled, so I had to pull the pistons and press out the pins. I have to assume that since this was assembled by GM, that these weights were within their tolerance for a balanced engine. All of these weights are in grams.

Pistons Rods Pins Pistons & Pins
1 533.9 598.5 159.8 693.7
2 531.4 600.8 160.0 691.4
3 533.8 601.5 159.4 693.2
4 533.6 600.8 159.2 692.8
5 530.8 599.2 159.7 690.5
6 533.3 596.1 160.0 693.3
7 533.5 602.4 159.4 692.9
8 534.8 596.9 159.6 694.4

As you can see, the greatest difference between two pistons is 4.0 grams and 6.3 grams for the rods. There is not too much you can do with the pins to balance them. And the piston and pin combination is the end product that counts. Just think of large rocks at the end of sticks twirling around, you sure would want them to all weigh the same. After balancing the piston/pin combination, they all weigh 690.5 +/- .1 grams. We threw the pistons on the scale and they all weight 690.5 or 690.6 grams. The rods were balanced to 596.1 grams +/- .1 grams.

The balancing was done by Ed's Crankshafts in Hayward California. The shop is run by two brothers, Ed and Carlos. I have know both for years and Ed's wife used to work for me years ago. There are not too many crank grinding shops left in the area. Ed's does most of the cranks for the East and South Bay machine shops.

The GM balance may be okay for those that drive down the freeway or side roads at no more than 2500-3000 rpm. But if you want an engine that will run smoothly and last....BALANCE IT.

Greg G

Where did you remove the weight from the piston/pin combo? Do the rods have to be balanced crank to piston? What do you do place ine end on a pivid and weith the other end?

John A. Marker

It was awhile ago that the balancing was done. The rods were weighed and material was removed around the area where it bolts together around the crank. There are flat spots that can be ground off to remove weight. The rods were also weighed end for end. Material was removed as needed to get them as close to being the same at each end of the rod. When all the grinding and weighing was done, the rods weighed the same end for end and the total weight of the rod was 596.1. This weight varried only .1 from one rod to the next as a maximum weight.

The material was removed from the base of the pistons below where the pin goes through the piston. The pistons were aluminum, so it is easier to drill the piston to remove small amounts of weight. When this was finished the pistons weight was 690.5. Compare one piston to another and the maximum weight difference was .1.

The next item after all the pistons and rods were balanced was the crank. They mounted the crank on a special machine and then added the equivalent weight of the piston/rod/pin combination to each spot where a piston would be located. They then spun the crank. A computer graph then shows the weight difference at each end of the crank and where to remove weight. The crank was forged steel, so carbide drill bits were used to drill the crank to remove weight. Experience is a big factor in balancing everything. You don't want to remove too much at a time, small amounts are better. Remove a little and spin to retest.....remove a little more and spin to test....remove a little more.

If you are building a engine, this is the right thing to do to keep it together and be able to spin into the higher RPM's if you go there.

Balanced 89

Change to ARP rod bolts to complete package.

Greg G

Thanks
Thinking of building an engine and would like to do some of the work myself. Can't do the crank but should be able to do the rods and pistons

Slalom4me

Rods contribute to both the rotating weight and the reciprocating
weight. In simple terms, the big end is rotating weight while the
small end is reciprocating weight.

The customary method of balancing a set of rods entails matching
the big ends of all the rods to each other and the small ends of all
the rods to each other. It is not a case of matching the rods in the
set to all have the same total weight. Nor is it a case of matching
the weight of the big end of a given rod to the weight of the small
end of the same rod.

This weighing process can be performed with a commercial fixture
and scale such as the one sold by ABS Products.
Or it can be done with a home-built fixture and scale like this one
fabricated by Patrick Panzera (based on a drawing by Don McGehee
based on a fixture Mr McGehee saw in a performance shop.)
(The approach above is known as the Center-of-Gravity method.
There is at least one other approach for balancing rods which seeks
to take into account factors relating to rod length and rod/stroke
ratio which are not addressed by C-o-G.)

Edit: The image of the home-built fixture seems not to display
consistently within this post. Try using this link to the image.
Or go to Mr Panzera's page about rod balancing to see several
images and read his remarks.
.

BrianCunningham

Thanks for posting

LD85

Great info!

Crazyhorse