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Balancing connecting rods
Quite a regular
Joined:
2009/8/1 13:01
From Norway
Posts: 40
Do anyone have any information about the maximimum difference in weight between the pistons and connecting rods from the Packard factory in the 1930, or which max weight difference between the connecting rods should be achieved by rebiulding a Packard engine today? On the today modern engine I have been told that the max weight difference between the connecting rods should be 1 to 2grams. But the Packard engine is a low speed and a long stroke engine. Should also balancing the connecting rods by weighing the big and small ends be performed on a engine overhaul today?

Posted on: 7/3 14:00:23
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Re: Balancing connecting rods
Forum Ambassador
Joined:
2007/3/14 16:01
From New Jersey
Posts: 15400
Please don't take my memory as infallible but I seem to recall seeing 1/7 oz. (about 4 grams) mentioned in Packard documentation of the era for piston to piston variation. However, with today's enhanced equipment capability why wouldn't you go to a tighter spec just as a matter of course?

Posted on: 7/3 14:27:33
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Re: Balancing connecting rods
Home away from home
Joined:
2007/10/28 7:49
Posts: 2174
Here is what my reference says about rods:

"All connecting rods should be weighed separately at both ends (using a fixture as shown). Grind material from the balance pad area of each end to achieve equal weight for all rods matching the weight of the lightest rod."

The fixture supports the end being weighed on the scale and supports the other so that the rod is horizontal to the scale table.

Now, a dollar bill is approximately one gram and centripetal force difference depends on the weight, radius of rotation, and speed. A long stroke motor can be, thus, affected as much as a high speed short stroke motor. And the accuracy of the scale should be taken into account as well. If the scale is accurate to 1 percent then for a one pound rod you are looking at an error of .16 ounces per pound or 4 grams leeway per pound.

Crankshaft balancing is a different animal. Bob weights are put on the rod journals using a formula. Well, we don't know what formula Packard used when they built those motors. Some people have said balancing a Packard engine is a waste of money. And after giving it some thought they may be right. I base that on my experience with motors that weren't balanced versus those that were.

I know one thing for sure. I've driven a few that were rebuilt with now unavailable NOS parts and the smoothness was outstanding.

Posted on: 7/3 16:29:32
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