428 balance question

[galaxiex]

I posted this over on the other FE forum but thought I'd ask here.


My 428 has previously been balanced with the (refurbished) C7AE-B factory connecting rods.
Big end resized, pin end re bushed. ARP bolts.
They have been beam polished and are lighter than stock.

I do have the correct 428 flex plate with the weight.
It was used in the balancing of  the rotating assembly.


The engine has been in storage/project stalled for a long time. Never run.
Now I'm back at it, and on the advice of another forum member, he recommended that I upgrade the rods.
I have a custom hyd roller cam and the Trick Flow heads. Should be knocking on 450 - 500 HP?
Max RPM 6000, maybe 6500.

Intended use, well it's going into my 66 Galaxie and mainly street use, with an occasional trip to the drags.

Crank is a factory 428 1U turned 0.020" - 0.020"

I now have a set of Molnar rods for it.
I'm not changing anything else in the rotating assembly, just the rods.

The stock rods total weight now runs from 736g to 740g  Only one rod is 736g, all the others are 739, to 740g.

The new Molnar rods are 3 of them at 751g and the rest at 750g.

I don't have a rod weighing jig so at this time I can't check the big and little end separately.

Should I re-balance? Do I need to?

The only thing I have for the previous balance work, to say that it was properly balanced,
is the good reputation of the shop that balanced the rotating assembly many years ago.
That shop at the time was the "go to" guys for anything FE.
I can only trust that they did it right.
Long since closed up shop, so I can't ask them.
I was in the shop long ago, but did not get to see them doing the actual balance procedure.
I did see the balance machine and setup and as I recall it was professional looking.


Thanks for all reply's.

[galaxiex]

I have a rod balance jig coming today to check the big and little ends and compare to the stock rods.

Depending on results could I simply "match" balance the new rod to the old?

[galaxiex]

Rod weight results... all in grams.

Stock rods

small end            big end
1. 224.8              513
2. 227.5              513.7
3. 228.2              513.1
4. 228                 513.2
5. 227.3              513.6
6. 227.9              513.4
7. 226.7              514.7
8. 228                 514.6

Molnar rods

1. 226.9              524.8
2. 226.2              526.4
3. 227.7              527
4. 226.9              526.3
5. 225.6              526.1
6. 226.4              527.2
7. 226.2              526.7
8. 226.4              526.6

Looks like the small ends are "close".
Big end is where the main difference lies.

Numbering is stock rod cyl location as balanced/assembled.
Molnar rod numbering is arbitrary. I can move them around to better match up.

So is it feasible to just lighten the big end to match the original rods?
Or at least get them close?

Or should I send the entire rotating assembly out for re-balance?

If I can just lighten the Molnar big ends, where do I take material off?
I watched a YT vid where he said to NOT grind on the ribs on the caps, they are there for strength/rigidity.

Thanks!

[427mach1]

I would have the rotating assembly rebalanced and resist the temptation to balance it yourself.  Steel weighs about 7.85 gram per cubic centimeter.  You need to remove almost about 14 grams, or 2 cc from your heaviest rod.  This is about the same volume as two sugar cubes.  That is a lot of material to remove.  You should, instead, have it balanced properly which will likely include the addition of weight (mallory metal) on some of the crankshaft counterweight.  It is not worth the risk to try to try to balance it via Youtube!

[jayb]

I have been told by a very well respected engine builder that a 10 gram weight difference won't be enough to matter.  This was when I was replacing one rod in a reciprocating assembly; he said just throw it in there, that I'd never notice the difference.  He was right, and that engine went to 7500 RPM multiple times at the track.  So, you could probably do the same, and use them as is.  Having said that, if it were me I'd have the crank rebalanced since the whole thing is apart anyway; why take chances?  You don't want to grind on those rods, rebalance the crank.

[galaxiex]

Thanks guys.

I did some bob weight calculations.

For the stock rods, the bob weight came out to 2298.6 grams
For the Molnar rods it came out to 2328 grams, so a difference of 29.4 grams.

Clearly too much to just install the Molnar's and go.

How bad would this shake with almost 30 grams difference in the bob weight? 😁

[My427stang]

I am showing about a 15 gram bpbweight difference plugging it into a recent 428, sort of a little WAG in there because your stock rods are all over the place. 

However, it doesn't mean much because we don't know the original bobweight.  Do you have it?  I supply with every build in the documentation or it could be ground on a counterweight

Because the new parts are a tighter group, and because some guys will slightly overbalance, depending on what the bobweight was, it may be even better than it was.  However, the easiest and safest thing to do is to spin it again

[galaxiex]

I am showing about a 15 gram bpbweight difference plugging it into a recent 428, sort of a little WAG in there because your stock rods are all over the place. 

However, it doesn't mean much because we don't know the original bobweight.  Do you have it?  I supply with every build in the documentation or it could be ground on a counterweight

Because the new parts are a tighter group, and because some guys will slightly overbalance, depending on what the bobweight was, it may be even better than it was.  However, the easiest and safest thing to do is to spin it again

Thanks.

I don't have the original balance info.

I'm gonna get it checked/balanced again and see if it's possible to use a neutral balance flex plate as well.

I have the proper 428 weighted flex plate, but it would be nice to be able to use a 0 balance.