Most production rebuild shops don't go to the trouble of balancing every engine they do, as long as you are using the stock components and the pistons are within 3-4g I don't think you will have a problem. Dynamic balance is a nice way to go, I do all My engines but I have a very nice machine at my disposal , it's not always necessary though, if you can make all the pistons weigh the same and get the big ends (rotating) and then the small ends (reciprocating) to be the same you should have a pretty good running engine.

On a V8 engine, the factory balances the crankshafts so as to match the weights of the rest of the rotating assembly.  If the weights are changed on either the rods, pistons, or peripherals, then the crankshaft itself becomes out of balance by lieu of a different amount of mass that is being spun around and the crankshaft counterweights are no longer being correct in which to compensate.  The degree of imbalance simply varies by the amounts any of these components are changed in weight.  As Tim mentions, 3-4 grams is not a significant difference but it’s not unusual to find 30-40 gram differences in replacement components which is definitely significant.  But stockers have been and are still being rebuilt with absolutely no regard towards the balance.  It’s only in those instances where there is a noticable vibration that it’s realized that the weights of the parts being used have deviated too far from what the crankshaft was originally balanced to be compatible with.  But it’s those imbalances that are present but not being felt that are destructive to internal components over the long haul as well as taking away power that would normally be directed towards the flywheel.

But simply match weighing the rods end weights and piston weights will be a step in the right direction but the job is not complete until the remainder of the  parts have been weighed and the crankshaft is dynamically balanced to match the bobweight calculation for those components.  The exception to this is on a stocker where the new pistons are match weighed so that they match the weight of the originals.  In this instance, the rotating assembly is in roughly the same state of balance as delivered from the factory.  I say roughly because even the factory balance jobs varied with some being better than others and with only a very few being ‘on the money’.  Blame it on the day of the week, breaks, the particular guy doing the balancing, or whatever.  For this reason, even rebalancing stockers with all their original components to a much closer degree than was being performed at the factory is of a proven benefit.

    I hope you don't think that I am trying to imply that there is no "benefit" to balancing, there is, all I was telling Paul is that if he is replacing one piston with a used one to try and save a little money and He gets the weight of the used one to match the rest in his engine He will be fine. In the case of a complete fresh, all new parts, rebuild a $150 balance job is money well spent.

I have heard somewhere that factories back in the '40s, '50s and 60's would weigh "representative" parts, balance a crank to those parts, then drill the next batch of cranks exactly the same as the sample.  That means factory balance is iffy from the start.  They also had the luxury of 90 day warranties and most customers only keeping the car a couple of years.  I doubt if the factories individually balanced each engine they manufactured.  Too labor intensive.  Luxury cars of the period may have received better balance jobs.

John in Selma, IN

Tim.  Feel assured that I didn’t take it to mean that you are not in favor of balancing.  I probably came across sounding like it has to be balanced or it will fall apart right away.  In Paul’s case, he is to be commended for at least trying to minimize weight variances in the rods and pistons himself while knowing that he’s not in a position to rebalance the crankshaft at this time.  This is much more than what happens on a majority of rebuilds and at least match weighing what he can will still help in the long run.

Paul.  Don’t be overly concerned about the differences in balance pads as a variance in balance pad heights within a given set is common.  And it’s not unusual for the rod with the largest pad to actually be lighter than the heaviest in a set.  This has to do with how the rod was initially laid out when the machine work was started and the distribution of the weight on the rest of the rod as a result.  Let us know what you actually find on the rod weights in regards to pad height versus the weight and how much variance in grams you actually find from the lightest to the heaviest.

      I know you've covered this before, but explain to them why you have to balance dynamically and why the factors change for different end uses. (Randy's rail vs. a truck engine.) 

Ted,

In the years I've been balancing I too have seen very few Factory balance jobs that were OK and none that are spot ON, But I wonder if you would agree that they did a much better job in the late '50s and through the '60s than you see in the late '70s and '80s. I would say that a 1963 327 scrub is much closer to right on than a 1982 305 or 350.  Also I would like to add, while on the scrub subject, that 305s (5.0) and 350s (5.7) use the same casting # crank and I have seen guys unknowingly swap the two may times, they seem to run somewhat OK when a 350 is in a 305 but the other way around and LOOK OUT.

Disclaimer: I use the scrub for example only because, I'm sorry to say, I see a lot more of them.

Of course you see more chevvies.  They break more.  When I was in the auto repair business in the late '60s and early '70s, I often remarked how much I liked chevvies, if I relied on Fords for business I'd starve to death.

John

        Put the rods in a box and send them to Ted, or Tim. It will probably cost you less and you'll be pretty sure it's done right. Either one of these guys will treat you right.

                                   Frank

If you do as Frank says and send Me your rods and return shipping I have a matched set that I'll trade you. They are not resized or have new bushings, but I will make sure they are good to use and weigh the same. I could use your rods as a broken set for when I need one to replace a bent or burnt one in another set.

The only SBC, other than the 400, that is externaly balanced would be the 5.0 and 5.7 with the one piece rear seal made '86 and later. They have a weight on the flywheel/flexplate only and because of a differant size flange you can't swap the flywheel/flexplate with the earlyer model SBC.

Tim.  That’s exactly my experience also.  The earlier engines as a whole do exhibit less variance in both the static weights and crankshaft imbalances versus those that were made in the Seventies and Eighties.  This just points to quality or tighter controls being more in the forefront on the earlier engines than the later.  The engines I find that exhibit the most imbalance as a whole are the Mopars.  Tolerances or allowances at that camp appear to be much wider than at the other two camps.

To add to what Tim mentioned, here’s a balancing article that goes into more detail on some of the nuances that exist with external balancing.

http://www.eatonbalancing.com/blog/2007/11/20/engine-balancing-part-2/ 

The short version for ‘factor changes’ is that engine balancing on V engines is not linear in regards to the rpm.  Rotational (dynamic) forces change within the engine as the rpm goes up which in turn dictates a change in balancing parameters on the crankshaft itself when balancing for specific rpm ranges.   And then special conditions come into the fray such as compression ratios, boost pressures, rod lengths, etc. which in turn must be compensated for in an application specific balance job.  Although the percentage of reciprocating mass is the value actually being altered in these types of circumstances, the compensating value for these is typically within the oil number on the balance card.  And due to the proprietary nature of some of the formulas dealing with ‘overbalance’, don’t be surprised if a balance card lacking the finished bobweight value is handed back to the customer if a card is handed back at all.

Dynamic balancing is performed when a rotating mass (ie. crankshaft) can have the potential for a number of rotational imbalances in varying degrees at different planes throughout its length while in motion.  On the short end of the scale is a flywheel which will exhibit mostly static imbalance and very little in the way of dynamic balance characteristics.  A crankshaft is on the longer end of the scale and can exhibit both static and dynamic imbalances.  A static imbalance can typically be felt outside the confines of the engine while dynamic imbalance can feel to be vibration free but instead is being couterproductive in power output to the flywheel and being destructive to the engine internals.

Here’s a couple of pictures which may help to clarify the visual difference between dynamic and static imbalance.

And here’s the link to the balancing article which goes into much more detail.

http://www.eatonbalancing.com/blog/2007/11/21/engine-balancing-part-1/

       If you guys read what he said here and in the link, you'll know more about the subject than 98% of your bar buddies. You'll also never again question the value of balancing dynamically. The balanced motors last longer, make more power, get better mileage and are better dancers!

Thanks for the explanation and the picture, you are much more eloquent than am I, now when Mr. Burns reads this he will know why his "race truck" crank was balanced with a 52% reciprocating factor instead of the normal 50% and also why I like to balance the flywheel sperate from the crankshaft, except in the case of an externally balanced engine.

Just to add; the internal/external point on Teds Blog brings to mind my dislike for the 454 scrub. I see a lot of these with broken crankshafts, which I have attributed to the external balance, but a bigger problem I think is that GM had an engine that was not built strong enough for the power that they want it to put out, compounded with the fact that so many motor homes are built on chassis that have this engine in them. Our shop is close to I-5 and I see a large number of burnt up and over worked 454's that give up out on this main north/south thoroughfare.

I have your rods and will ship the balanced set on Monday. Thanks for the extra, It is greatly appreciated.