One of the things that troubles me the most, is rotating assembly
balancing. Problem is that in Croatia nobody does it.
You can have crank balanced, but nobody asks for pistons or
rods for this. They just balance crank?!

From what I have ben reading on internet, it's important to know
exact weight of rotating parts to be able to balance the crank.

And also from what I've read, balancing IS VERY IMPORTANT.

And now, it's hollidays, and I have some time, so I took David Vizard's
"How to build max performance Chevy on a budget" book and here is
a C/P of very interesting part of the text:
"... balancing does not mean that there will be more destructive
internal engine loads present. What it does mean is that whatever
forces are generated at one journal may not be fully countered by
forces generated at another. The result is an out-of balance engine.
But the engine is no more likely to fail than if it were perfectly
balanced."

And here is another C/P:
"If the budget exists, balancing the rotating assembly is recommended.
But be aware that your motor will be just as reliable and run just as
well without balancing."

???

 

If you deal with a good shop in the states you can order an assembly that they have balanced for you. Crank, rods, pistons, and sometimes a balancer and flywheel.

If you are going to drive your car like a Vette then I would suggest you get it balanced. If it would rarely break 4,000RPM then you would probably be okay without it. That of course depends on if the parts you have going in it are quality. If it is the stuff that came out of it you should be fine.



Good luck

 

When an engine is balanced, the pistons are weighed and matched to the weight of the lightest. The big end of the rods are similarly weighed and matched as are the small ends of the rods. The rod bearings and the piston rings are weighed, but not altered to equalize their weights, because they are nearly identical as manufactured. Then "bob" weights are made up and attached to the crank throws that are usually equal to the weight of the rotating weight plus 1/2 the reciprocating weight for each throw. If the engine has any external "unbalanced" rotating components, like the flywheel or flex plate of a one piece rear main seal SBC, then it must also be attached to the crank when it is spun up and balanced. A production 400, and a non-internally balanced 383 need to have both the front vibration damper and the flywheel or flex plate attached during the balancing.

A crank shaft could be sent out and balanced without all the rotating components, if you can equalize and supply accurate information for the making up of the bob weights to the one doing the balancing. I don't think it would be wise to split the responsibility of the balance job among or between more than one party.

Does knowing how the engine is balanced help?

RACE ON!!!

 

Yes it helps, and I did read about it before, but it's strange to
read that David Vizard thinks that balancing has almost
nothing to do with engine reliability.

 

Well, only had one balanced and blueprinted.I guess it kinda has to do with application,at least in my opinion, how much ya gonna beat it? As far as reliability I'd have to say it does in the higher RPM range, both my friend and I were told it (ours) were good to 10K, never even came close, but did hit 8K frequently. I can't remember the rods I used, but the machine shop rejected a few before they were satisfied.Just seemed right for me at the time because I was putting alot of $$$ into the engine. Call it added insurance.

 

Balancing an engine isn’t about making more horsepower or turning higher RPM. It is about longevity or giving the engine the best chance of staying together at high RPM. Think of unbalance in the context of the extra forces the balanced engine won’t have at high RPM. Force increases by the square of the speed so a small unbalance force at 3,000 can be a big deal at 6,000.

For example, assume a 1 oz. unbalance at a 6 inch radius. At 1,000 RPM is equal to about 10 lb force, at 3, 000 it is 90 lb, and at 6,000 it is 360 lb. This of course is assuming that all the unbalance is in a single location which it never is.

To those that are inclined to calculate such things and jump, don’t bother to jump at the math. This is just a quick run of the numbers and so I may have errored, but it is going to be close enough to make the point.

I personally balanced several engines years ago just to experience it beyond theory and really learn how, but I used the accepted procedure of matching weights and then used bob weights like CFI-EFI described. Matching piston and overall rod weights were easy, I spent the most time on getting the rod big end to small end right; so with a gram scale you could do that yourself. You would just have to make a jig to hold the big end on a knife edge while you weigh the small end. But even with that part of it done, I’d have to think a while about just spinning the crank and its direct mounted parts and calling it balanced.

I learned from a Stewart Warner book but Schenke and IRD also spec the use of the bob weights. I think I would figure out a way to get it done with bob weights. Have you asked anyone there that does this work if they would do it with bob weights?

 

Or like this. Go ride a bike with one pedal that weighs 1/4lb and one that weighs 3lbs. You wouldn't notice the difference just cruising along, until you were really cranking and then it would throw the bike all over the place.

This is a bit exaggerated but you would be amazed at how heavy a few grams can be at 8,000 RPM and what that weight can do to a bearing.

 

I think I know where David Vizard is coming from:
I bought a set of pistons that were hypereutectic pistons, and when weighed, they were less than 1/10 gram difference between them.

I think that he left out the part that these days, the parts have less human involvement, and when manufactured in today's standards and CnC machinery, the parts are pretty near identical in weight, density, and form.

 

If its a driver & your just changing pistons the factory balance will be ok.

Any other use or component changes get the assembly balanced.

 

My vote is that the quotes are accidently being taken out of context.

In the ©1999 edition of "How To Build Max Performance Chevy Small
Block On A Budget", Mr Vizard's discussion about balancing begins
on pg 57 and consists of just four paragraphs. The opening sentence
reads:
He goes on to give a sketchy (one sentence) overview of how to
calculate the Bob Weight and continues with:
In other words, it is not necessary to 'balance the engine' (ie: have an
outside shop work on the crankshaft) if you:

• self-balance the reciprocating components (rods, rings, pistons,
  pins, wrist pin clips (if applicable) & ect.
• calculate the Bob Weight for the (now) match-balanced
  reciprocating omponents
• know the target Bob Weight that the crank was balanced for
• determine that the Bob Weight of your components falls within a
  +/- range of the target Bob Weight of the crank

One flaw with this reasoning is that it depends on the assumption that
the crank balance remains within tolerance - I can think of more than
one reason as to why it might not.

My vote is that Mr Vizard did not intend to say that engine balance
does not matter. His message is that you can do a lot of the work
to balance the engine yourself and, depending on your findings, there
may be no need to have outside work done to end up with satisfactory
results.

The balancing section in this book is VERY brief. If parts of the rotating
/reciprocating assembly have been changed or modified, my vote is
that engine balance should be verified.

At a minimum, an imbalance will be annoying. In a severe situation,
imbalance may cause parts to wear more rapidly or fail.

.

 

I think that you're right. I think that he wrote this part of the
book very poorly. Acording to him, and according to how little
of text he devoted to balancing, one might come to conclusion
that balancing is not very important thing. And I, and also all
of you, dissagree. Especially since the book has "max performance"
in it's title.

 

Who is the source of the comment?

Perhaps it is true that in Croatia a machinist balances cranks without
the reciprocating components. This isn't to say that the weights of
the reciprocating components are not matched to each other and
used to determine the Bob Weight values for the crank balancing.

Some cultures are more self-reliant and frugal than others.

The crank balancing machine and the expertise to operate it lead a
shop to have to charge higher labour rates. The work to match the
reciprocating components is time consuming and requires patience but
it does not need costly equipment or a journeyman's years of training
and experience.

Perhaps the practice in Croatia is to self-balance the reciprocating
items and then send the crank out along with the Bob Weight that
has been calculated?

.

 

Depends on how fast you spin the motor up....anything that spends time @ or over 6k should be balanced to survive.

 

You could be right.

The source of that comment, actually sources, are a few mechanics
over here. And they are really good mechanics, but they just
don't have any experience with V-8 engines.

I think that the main problem is that Croatia is very small market,
especially for tuning V8 engines.
I would say that at least 90% of tuned engines are 4 cylinder engines.
And balancing straight engines is different than V-engines.

All work that is done here on V-8 engines, is usually just overhauling
with maybe just new pistons (if overbore is required) and that
doesn't demand balancing.

To be sure, I'll contact two companies on Monday (only two that I
know of, that have balancing machines) and see what they'll say.

 

I had the opportunity to travel in the region before it came to be
known as Croatia. Split, Hvar & Dubrovnik stand out in memory.

.

 

Well, if you ever by any chance come over here again, please
contact me, I would be very glad to meet you and to show you around.

If it wasn't Croatia when you were here, then it was probably
20 or so years ago, and a lot have changed since. At least,
roads and highways are much better.

 

One problem solved. I contacted a company (actually it's very small
family business) today about balancing.
One very friendly man anwsered me and first thing that he asked me,
when I told him what I need, was;"do you know exact weight of all
parts, or even better, do you have all of the parts to bring them to me."

This man has worked with balancing for 30 years now, and he also has
experience with v-8 engines. Mostly for marine use, but V-8's.

One thing that he agreed with all of you and me, is that balancing
is a must, even for street driven engines, and of course, even more
for high rpm racing engines.

I don't know what are prices for balancing in US, but it will cost me
arround $150. If it'll be done good, I'll be more than happy with that!

 

Hi!

there´s a shop in germany that balanced my rotating assembly..!

PM me, if you are interested

greetings

 

Thank you!
It looks like I'l be able to do it locally, but if this fails.
I'll contact you.

Thanks again!

 

Does your project consist of verifying the balance of OEM parts
that were originally matched at the factory? Or does it consist
of matching together for the first time a collection of aftermarket
parts gathered from several sources?

Improving the balance of parts that are close to being in tolerance
or restoring the balance of an assembly where OEM pistons have
been replaced to accomodate a cylinder clean-up are both reasonably
straight-forward tasks. It can be more complex to balance a collection
of aftermarket parts.

Visit the shop. Find out if you can get a tour so as to see examples
of the methods employed to add and remove weight for balancing.
Ask questions. Listen to hear what kind of person you are dealing with.
It is likely that with the information you have gathered here and
elsewhere about your engine, you may know more about some aspects
than your prospective machinist. Does he acknowledge that he may
not know everything? Does he ask questions? Does he listen? If you
are uncomfortable - look elsewhere.

A recent thread had a vivid example of what can go wrong when
a shop works on a crank. I was surprised to see that the thread
has been removed from the forum - but this sometimes happens.

The picture is gone now and there is little information left, but
Cuisinartvette had created a link to the thread I am referring to
here:.