Balancing the pistons of the Model A Ford

[Tom Wesenberg]

Originally Posted by Colonel Biggs

Sir Kenneth --

That's one fancy balancing rig! When you are "adjusting" the weights of the rods, are you using small drill bits -- as in the piston balancing video? If so, can this require several drill spots, or are the rods pretty closely balanced in general?

What is the reasoning for not just balancing the rod as a single entity, and making the drill spot in the center of the rod?

The top end of the rod is reciprocating motion, and the bottom end is rotary motion, and this is the reason to balance each end to match the others.

[Kurt in NJ]

The area highlighted in red is where the factory machines for balance, if you look at the small ends you can see the differences in the amount of metal removed, on original pistons (don't have picture ), there is a boss in the rib connecting the thrust surfaces that is machined to balance, with aftermarket pistons sometimes it is the inside of the bottom of the skirt that is machined

I once found an article in american machinist online that desccribed the balancing, the machine had a scale as a chip pan,

[kenparker]

Very interesting thread and a learning experience for me.. Finding new ways to improve some of my weighing processes. I had two scales (both from Harbor Freight) One weighed in 1 gram increments. The other in .1 increments. Knocked the .1 scale off the bench the other day and it no longer works. Gotta get a new one. The scale you see in my pictures is the 1 gram scale.

Cololenel (sp) Tom W answered the ? about why balance the end separately. Kevin shows where to cut/grind metal off the rods. My pictures below show additional grinding areas. Some of my grinding spots may not take off but 1/4 to 1/2 gram at the time. Some of the grinding only smooth out the rough casting, but sometimes that is enough. The area indicated along the shank is only the rough area of the casting seam. Never take off the sides of the beam/shank.

How much is a gram. The scale and the two cotter pins show one gram, which is not very much metal. Those cotters are 1" x .80 dia. Also one gram is equal to one standard paper clip.

[kenparker]

One more note. I get pistons from Berts or Snyders and they will usually be with in 4-6 gram of one another. The rods come from Pauls Rod Shop (somewhere in Missouri) and they are usually within 4-6 gram of one another. So, Not a whole lot of grinding to be done if you buy good stuff.
"Bout year ago a set of pistons came and one of them was 20 grams heavier than the other 3. Sent them back and new set came and all four were within 4 grams.

Also the number of shim used affect the weight, so weigh theshims with the rods.

[H. L. Chauvin]

Per Reply No. 20 above, (still have the new piston box):

Just in case anyone is interested in trying to buy (4) perfect Model A pistons that all weigh "exactly" the same ............... in lieu of some of the "Good Enough for a Model A Brands" offered ............ the pistons I bought from Mr. Ron Cloat's shop (10) years ago were manufactured in the USA by:

"Silv-O-Lite", United Engine & Machine Company, 4909 Goni Road, Carson City, NV 89706.
Not sure if "Silv-O-Lite" is still in business.

[Mikeinnj]

Sponsored Links (Register now to hide all advertisements)

Silv-o-Lite Pistons for Model A are still being manufactured . Here is the link.

https://www.uempistons.com/index.php...88e34e349e846f

[ursus]

Balancing the rod small end is difficult as the recommended area to grind is a small annulus at each side. Why not balance the small end with the pin installed and take any excess weight off the pin?

[Flathead]

Two things to keep in mind are be sure the scale you are using is accurate enough for this type of work, and since the Model A engine is low RPM a "race" type balance is not needed.

[Kevin in NJ]

Originally Posted by Flathead

Two things to keep in mind are be sure the scale you are using is accurate enough for this type of work, and since the Model A engine is low RPM a "race" type balance is not needed.

So a question, what accuracy is needed?

Ford produced rods to +- 1 gram at each end.

Balanced the crank to 1/4 oz and had the flywheel to less then .001" out of center line.

When you ask a machine shop to do the engine to Ford standards they tell you that you want a racing engine.

So what is the type balance that you suggest?

[H. L. Chauvin]

Per the question in Reply # 29:

"So what is the type balance that you suggest?

In my opinion:

1. The upper limit to Model A mechanical perfection when rebuilding mechanical components for a Model A should be equal to trying to achieve 100% perfection; and,

2. Trying to achieve mechanical perfection the first time takes far less time than the time later spent on repairing less than perfect Model A mechanical assemblies that failed.

[Flathead]

The original Ford specs are fine. My point was that balancing to a tiny fraction of a gram would not yield a noticeable difference in smoothness for a low RPM Model A engine. I am not advocating sloppy work. Sorry for any misunderstanding. YMMV

[Dan McEachern]

Not to rain on any one's parade here, but the correct way to balance connecting rods is to match balance the big end weight and then balance the rod for total weight. The reason for this is so that the rotating assembly "sees" the same rotating mass on each rod jourrnal (big end) and that all the reciprocating mass is the same. Balancing the small end of a connecting rod as shown above rather than the total weight is simply incorrect. Not sure what else to say on the subject. Dan

[hardtimes]

Originally Posted by Dan McEachern

Not to rain on any one's parade here, but the correct way to balance connecting rods is to match balance the big end weight and then balance the rod for total weight. The reason for this is so that the rotating assembly "sees" the same rotating mass on each rod jourrnal (big end) and that all the reciprocating mass is the same. Balancing the small end of a connecting rod as shown above rather than the total weight is simply incorrect. Not sure what else to say on the subject. Dan

Hey Dan,
Thanks for sharing your extensive experience and valued opinion ! Also, for making us such GREAT parts !

[BillLee/Chandler, TX]

Originally Posted by Dan McEachern

Not to rain on any one's parade here, but the correct way to balance connecting rods is to match balance the big end weight and then balance the rod for total weight. The reason for this is so that the rotating assembly "sees" the same rotating mass on each rod jourrnal (big end) and that all the reciprocating mass is the same. Balancing the small end of a connecting rod as shown above rather than the total weight is simply incorrect. Not sure what else to say on the subject. Dan

Dan, please set me straight:

If all of the big ends weigh the same, and all the rod ends weigh the same, how can the total weight of any rod differ from another?

I think I understand that if you first balance the big ends, and then start removing metal to balance the rod end that you can affect the weight of the big end. But wouldn't that imply that you should iterate measuring and balancing the two ends of the rod?

Can you direct me to any papers or website that fully explains the "balancing" of a typical engine? Perhaps some explanation of how "balancing" of the rotating mass and the reciprocating mass is achieved?

Again: not trying to contradict, just trying to understand.

[H. L. Chauvin]

Looks like if two person's heads, necks, shoulders, and arms weigh the same .... and the same two person's feet, ankles, and legs weigh the same, the guy with the 54" pot belly would weigh more than the slim guy with the 30" waist .... but what do I know about dieting and eating too many french fries.

[Kevin in NJ]

For balancing the connecting rods Ford had a fancy machine that would do both ends of the rod. They cut OD on the ends as shown in the pictures. Ford specd certain weight on each end +-1 gram. From a production stand point that was probably the most cost efficient way to have balanced rods.

From the hobby stand point, you are shooting for the big ends to be the same weight. You also need the total weight to be matched so where are you going to make that happen? You have to do something at the little end. This could be in playing with pin mass or piston mass. Your goal, I believe (and may be wrong) is to have the big ends the same on the rotating mass and the total mass be the same on all 4 assemblies. On the rods you really only have 2 places to play with weight and those are the areas the factory used.