Match-balance specs for B Rods?

JoeWay

JoeWay

Well-known member
I am considering buying a set of B rods. I asked Snyder's if their rods were match-balanced and if so to what spec. I just got this answer from Tom:

"Joe,
B rods are tough to weight match as close as others because we do not
have the supply of rods to sort through.
Typically a set of B rods will be within 10 grams.
Thank you!
Tom J."

10 grams seems to me to be quite a bit. What do the voices of experience say about allowable weight difference on rods?

I suspect Rich Fallukka's inserted rods are much closer, but I'm not really interested in spending $700 for rods for this particular engine as it is not the one I will be installing permanently. This engine currently has 3 rods that are pretty close in weight and one that is more than 20 grams different.

Joe
 
Ford spec on the A rods was a certain weight +-1 gram at each end.

From what I have read, when done, the 4 piston rod combinations in an engine would be within 4 grams of each other.
 
Joe, is this really an issue causing you some indigestion or just a thought-provoking question? Why not weigh what you receive and weight-match to the lightest one??
 
Joe,

You didn't say which end was 20 grams heavy...I would assume it's the
big end. I would get some heaver and lighter rod nuts, then mix and match
too get the weight closer.
 
D. J., I don't recall whether I tried to figure out where the weight is. I suspect I just weighed the whole rod. I agree with you and Kevin that 10 grams is way too much.

Brent, I'm concerned enough about this that I am considering spending $280 for re-babbitted rods if I can't get the ones in the engine to match pretty well. I'm not sure whether there is an extra 20 grams of metal in a rod to get rid of in order to match up what I have. It's been so long since I was into this engine that I can't remember whether the odd one was heavier or lighter.

I guess I need to search Tom Wesenberg's posts for pictures of the beautiful balancing fixture he built.

Joe
 
I have had several sets of "B" rods in the factory box, on the scale the rods in the sets were within 1 gram or less ---total weight, and end for end weight.

Last year at Hershey I bought a set of 21a rods in a ford box (for my 36), they weighed out at 7 the same within 1/2 gram, 1 at 1 gram.

I just bought a "metal thing"(auctioneers description) that I am going to modify into a device to machine the balance pads of rods ---originally I think it was a pipe threading machine made in the teens, as I bought it it is a variable speed welding positioner that was used to weld augers

I have at least 20 original nos pistons, any size not in sets was within 5 grams, the 1 boxed set was within 1 gram

On the model A rod print it said +or - 1 gram for each end and for total weight

I know from my car that 1 rod changed from a 2 ghram difference to a 12 gram difference made for anoticable change in vibration and it lowered top speed from 67 to 64

A weight change of the small end can be made with the wristpin.
 
A weight change of the small end can be made with the wristpin. End Quote

If you did that, you would have to rebalance the set of pistons again. You would have a mess. What ever you would take off the rist pin, would throw the other 3 pistons out that far. Also, how would you know how much weight to take off, and how would you scale the Rod to find out?
 
I thought the same thing myself. Is it possible to match pistons without pins and match rods with the pins? I know this is not how it is done but would the engine still be in balance if done this way?
 
Hello Joe:
We run into this problem with other brand engines when you can't get another rod to get them balanced. I'll weigh the rods and find the heaviest as you mentioned 20 grams. I would add bobweights to the crank to create the unbalance of the rods and then remove the weight from the crankshaft as seen in the picture.
 

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J and M Machine said:
Hello Joe:
We run into this problem with other brand engines when you can't get another rod to get them balanced. I'll weigh the rods and find the heaviest as you mentioned 20 grams. I would add bobweights to the crank to create the unbalance of the rods and then remove the weight from the crankshaft as seen in the picture.
Click to expand...

I got it...tho it is a one shoot deal...you tried this on a V type engine?
 
Hello DJ/Kahuna: These examples are a 1928 Chevy and the rods were worse than 20 grams on the total weights. As shown it was 36 grams off.
off 15 grams on the big end.

I didn't have any extras so what to do?
I balanced the big ends as close as I could and the total weights are demonstrated by the bobweights.
The "Bobweight" part that is clamped to each rod journal represents the total weight of each rod,piston and ring weights added together to duplicate the rotating mass.

If I couldn't make the rods match as what this thread is about I can match the rods to the crankshaft by duplicating the unbalance of the heavy rods and then making corrections to the crank where I have more choices of where to remove the metal from to make it run smoothly.

In regard to the V type engines this is aceppted practice to add bobweights and balance the crank.
 

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"I balanced the big ends as close as I could and the total weights are demonstrated by the bobweights.
The "Bobweight" part that is clamped to each rod journal represents the total weight of each rod,piston and ring weights added together to duplicate the rotating mass."

But the small end of the rod, piston, rings, and wrist pin are all reciprocating mass. How does that play into rotating mass? I don't see how you can add or subtract a rotating mass to correct for a reciprocating mass. :confused:
 
But the small end of the rod, piston, rings, and wrist pin are all reciprocating mass. How does that play into rotating mass? I don't see how you can add or subtract a rotating mass to correct for a reciprocating mass. :confused:

You're right. However the engine runs much better with me duplicating the unbalance of the rods and making the best of an unobtainable situation.

All the pistons were weight matched as well as big ends. only thing off was total weight. nor could I correct it,wasn't enough there to remove.
I took the heaviest placed on number 4 and lightest on number 1 rod journal.
Then the next two which were close enough in total weight to add up to the heaviest on 2 and 3 rod journal. Spun the crank and made the corrections to the crank as if it was the crank that was unbalanced.
Sometimes you have to bend the rules to make the engines run.
Naturally it would of been best to get a set of rods that was within 1/2 gram of each other end to end and total weight but what's one to do when they're not readily available.
 
Just as an example here is a 1936 Seagrave Model G fire engine we received to rebuild. I don't think I could find anyone willing to break up a set of rods to sell me if one was way off.
 

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J and M Machine said:
Hello DJ/Kahuna:

In regard to the V type engines this is aceppted practice to add bobweights and balance the crank.
Click to expand...

I realize that, I worded the question poorly. Lets try this...I was going to
build a "cheater" engine for NASCAR some years back. The front 2 cylinders
would "pump legal", the back 6 holes would be MUCH larger(sleeve the front
2 cylinders too 4.00", the back 6, 4.125"). Can you correct for that on the
bobweights? Internally balanced, 3.48"-3.50" stroke.

Yep...you look like a Hye
 
Dudley: Yes if you can get all of the pistons to match weights then it would work.
The rods and bearings are all the same weight so the main concern would be balancing the pistons.

I have been told from Eagle that these production balance jobs are 4 grams plus or minus. They said it was good enough for street use.? 8 grams total.

However; I have seen production engines,namely Chrysler 440, being out more than 27 grams on rod end weights such as what was mentioned above.

Hye em yes!
 
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