Weight Tolerances on Rebuild

Henry's Lady's Man

Henry's Lady's Man

Senior Member
I'm ready to put my engine back together. All pistons, rings and valves, guides & springs are new, all that stuff weighs the same for each cylinder. I am reusing the babbitt rods, since all babbitt looks great, and within specs.The trouble is, the rods are not all the same weight. How far can they vary in weight before it causes more vibration than acceptable? I am including a chart of the before and after weights of parts (somebody threw together some mismatched parts).
Bottom line, since the babbitt is good, I'm trying to save a shipload of money from buying rebabbitted rods that may weigh different as well.

the rods and caps weigh respectively:
1. 734 g
2. 748 g
3. 740 g
4. 718 g

(All other parts are matched weights) -Chuck
 
I'm sure others will "weigh" in on this but I think those rods are too far off to give a smooth running motor. I believe that + or - one gram is closer to what you need. If you try to match the weights yourself be careful where and how you grind.
 
You also need to weigh big end and small ends of the rods and have them weigh the same.
Not just the total weight of the rod.
 
Flathead said:
I'm sure others will "weigh" in on this but I think those rods are too far off to give a smooth running motor. I believe that + or - one gram is closer to what you need. If you try to match the weights yourself be careful where and how you grind.
Click to expand...

Thanx for that. I was thinking the same; or having some material welded somewhere up the sides of the lightest rod. Sheesh; how can rods differ so much? The rods #2 and #3 both have Ford script, and #1 and #4 both have a small triangle on the inner side toward the bottom.
Welding may warp them though. Should I drill some holes through the middle, spacing them?
 
Dodge said:
You also need to weigh big end and small ends of the rods and have them weigh the same.
Not just the total weight of the rod.
Click to expand...

Sounds like I'll need to match center of gravity on the rods then with a balance, by grinding the heavier ones. They REALLY are so far off! This is my first crack at this kinda thing, but I figure, all I need is the right advice.
I could also talk to Bert's and see if Steve has just two to sell me that may be better matches.
 
I'll probably get a shipload of flack for this, but here goes; Why can't you drill a small hole in each of the heaviest rods to equalize the weights? There are 28 grams to the ounce and you're only 16 grams apart, or slightly more than a half an ounce. I don't know how big a hole that it would need to make up this weight, but is this solution feasible?
Terry
 
Trust Herm on this one. #2 at 748 grams and #4 at 718 grams. That a 30 gram difference.You will never be able to grind that much weight off one rod without weakening it. And welding on the shank of a rod is a no-no also. It would well be worth the the expense of the new rods since you already have all the other internal new. Dont take the risk.

Had to replace an engine in a friends car several years ago when a rod broke and threw it through the side of the engine. Guess what. The rods had been welded to balance them. One broke at the weld and one more was cracked.
 
With all that work and expense why cheap out now? Replace with new and do a proper job or you will likely regret your decision at a later date. Wayne
 
From the factory...

The rods were built to a certain weight at each end +-1 gram.

When assembled the piston, pin and rod assembly was weighed and put in one of 4 bins. Any 4 taken from a bin would be within 4 grams total weight.

Crank was done to 1/4 ounce of balance and the flywheel was mounted to less then .001" from the centerline.

You really need the balance to be very tight to get minimal vibrations. Significant out of balance on the crank end will limit you top end and increase the vibrations affecting the life of the babbitt.

You have to build the A engine like it is a race car engine if you want it to be like a new factory car.
 
Kevin in NJ said:
From the factory...

The rods were built to a certain weight at each end +-1 gram.

When assembled the piston, pin and rod assembly was weighed and put in one of 4 bins. Any 4 taken from a bin would be within 4 grams total weight.

Crank was done to 1/4 ounce of balance and the flywheel was mounted to less then .001" from the centerline.

You really need the balance to be very tight to get minimal vibrations. Significant out of balance on the crank end will limit you top end and increase the vibrations affecting the life of the babbitt.

You have to build the A engine like it is a race car engine if you want it to be like a new factory car.
Click to expand...

A wealth of knowledge for sure. Great post. Wayne
 
OOOPZ! My Bad!I see that there are 30 grams difference between the heaviest and the lightest, not 16 as I said. This changes things drastically. Don't know how I missed that!
Terry


kenparker said:
Trust Herm on this one. #2 at 748 grams and #4 at 718 grams. That a 30 gram difference.You will never be able to grind that much weight off one rod without weakening it. And welding on the shank of a rod is a no-no also. It would well be worth the the expense of the new rods since you already have all the other internal new. Dont take the risk.

Had to replace an engine in a friends car several years ago when a rod broke and threw it through the side of the engine. Guess what. The rods had been welded to balance them. One broke at the weld and one more was cracked.
Click to expand...
 
kenparker said:
Trust Herm on this one. #2 at 748 grams and #4 at 718 grams. That a 30 gram difference.You will never be able to grind that much weight off one rod without weakening it. And welding on the shank of a rod is a no-no also. It would well be worth the the expense of the new rods since you already have all the other internal new. Dont take the risk.

Had to replace an engine in a friends car several years ago when a rod broke and threw it through the side of the engine. Guess what. The rods had been welded to balance them. One broke at the weld and one more was cracked.
Click to expand...

This is what I needed to hear. I will go with Herm and Your recommendation. Some kookaburra really threw this engine together. One wrist pin was 124g, three were around 150g! Pistons ranged from 572g to 586g! I'd say that I am surprised it ran so well, but that's the Model A for you... Hard to kill them!
I'll be calling Bert's within minutes for rod exchange.
 
Terry said:
OOOPZ! My Bad!I see that there are 30 grams difference between the heaviest and the lightest, not 16 as I said. This changes things drastically. Don't know how I missed that!
Terry
Click to expand...


Yes Terry,
If they were under 10 grams apart, I would have done some grinding, but ALAS, it is far worse!
Thanks to all for responding. I will post my findings from Bert's this evening.
 
Henry's Lady's Man said:
Yes Terry,
If they were under 10 grams apart, I would have done some grinding, but ALAS, it is far worse!
Thanks to all for responding. I will post my findings from Bert's this evening.
Click to expand...

BE safe, go ahead and drop by and talk to our buddy john cosper. Tell him what you've run into and I bet he'll help you out. Tell him tony white in Lafayette, LA said hi. Have fun modelAtony tony white Lafayette LA
 
We don't have enough information to say those rods can not be balanced. I have balanced rods that were FAR worse than that.
We need the big and small end weights seperately.
Worst case, if all of the unbalance was on the big ends, (not likely) the lightest ones can be brought up in weight easily.

To check them properly, you will need a setup similar to the picture.
NO WELDING or HOLE DRILLING!!!!
 

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modelAtony said:
BE safe, go ahead and drop by and talk to our buddy john cosper. Tell him what you've run into and I bet he'll help you out. Tell him tony white in Lafayette, LA said hi. Have fun modelAtony tony white Lafayette LA
Click to expand...

Tony, I dropped by to talk to our buddy John Cosper in 2012 and found his shop closed and weeds taking over. No answer to phone number I have, so if anyone knows different I'd like to know. The engine he did for me is pretty bullet-proof.
Fearless
 
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