Data Point on Factory Balance

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nm9stheham

nm9stheham

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For anyone interested, as far as a data point for the even-ness of factory balance work, all 8 pistons and rods, etc., from a '68 273 were disassembled, and all the individual parts weighed to compute the corresponding bobweights, including big and small end rod weights. There are the original stock factory parts, taken out and marked as to their position.

For the individual piston/rod assemblies, the corresponding bobweights varied over a range of 19 grams. (This would be as if identical piston-rod assemblies were installed in pairs.) The bobweight ranged from 2108 to 2127 grams.

When bobweights are computed with the assembles paired like they were actually installed in the engine, the variation goes down in this particular engine, as the heaviest assembly was paired with an average weight assembly. The bobweight variations were 11 grams between the 4 crankpins (journals); it ranged from 2109 to 2120 grams for this engine.

I'd suspect that the worst case factory variation could easily be 15 grams, and might be as much as 20 grams across all production.

If anyone is interested in the detailed numbers, let me know.
 
That is interesting. I can remember vaguely reading about balance in some Hot Rod or similar article in the seventies, and I called Traco who were up around LA I think. They claimed they balanced 302/ 350 Chev combos to something like 1/4 or 1/2 a gram. Seems to me the article was claiming "not much higher" variation for a factory Chev 302 Obviously that was a long time ago. I don't remember specific numbers, only that the variation was MUCH less than I had assumed. Either that...........or someone was lying LOL
 
My machinist is a Mopar guy and does balancing and I remember talking to him yrs. ago and he concurred with your exact findings nm9. We also briefly talked about the accuracy of balancing on GM's and Fords and he said Fords were usually the closest with GM's being next. We didn't talk specific numbers on anything other than Mopars which he said were usually around 15 grams (avg) difference
 
So what I am hearing is that the factory got away with "unbalanced" engines when they left the factory? Or is it that there was so much weight difference they balanced thereself out? Lol
 
dartfreak75 said:
So what I am hearing is that the factory got away with "unbalanced" engines when they left the factory? Or is it that there was so much weight difference they balanced thereself out? Lol
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There were quite a few stockers I've driven that would do the wow...wow...wow over 3200 rpm.
Driveline or engine/convertor balance.
 
dartfreak75 said:
So what I am hearing is that the factory got away with "unbalanced" engines when they left the factory? Or is it that there was so much weight difference they balanced thereself out? Lol
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They can't/don't "balance themselves out". Mopar's were balanced close enough to not shake themselves to pieces but that's about it
 
fishy68 said:
They can't/don't "balance themselves out". Mopar's were balanced close enough to not shake themselves to pieces but that's about it
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Yea that's what I figured. That is pretty crazy. Wonder if that is why newer cars seem to last longer miles? They are better balanced than The older engines
 
as was relayed to us, by machinist, the amount of gram weight can be under 30 grams and will not affect things, over 30 grams need to rebalance. factory stuff is usually under the 30 max. just food for thought.
 
perfacar said:
as was relayed to us, by machinist, the amount of gram weight can be under 30 grams and will not affect things, over 30 grams need to rebalance. factory stuff is usually under the 30 max. just food for thought.
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So when replacing pistons as long as the total weight of each rod piston ring combo is within 30 Grams of the original weight it will be ok?
 
Newer parts are made to tighter tolerances so they balance better/closer. PM rods are an example.

I used OEM PM rods in my kid's race motor... they were from 598 grams to 605. Pretty close. The pistons we used were within 2 grams out of the box. My engine shop only balanced the crank.

Factory production speeds are fast... don't have time to f**k around getting everything as perfect as we'd like.
 
fishy68 said:
They can't/don't "balance themselves out". Mopar's were balanced close enough to not shake themselves to pieces but that's about it
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LOL. My old V code RR which had the heavier 6-pak rods rattled out a rod bearing at somewhere around 40K hard miles. An "engine shop" (San Diego this was around 71) talked me into reworking that rod instead of replacing it, and hard chroming the crank. It came apart AGAIN early one morning on I-5 S of Stockton, headed for Sac on a long weekend. The no6 came apart, swung around broke one or two more rods, and something broke the cam into "a few" pieces. The stopped valves then attacked various pistons, and at some point somebody went through the block AND the pan. I am lucky the oil and coolant didn't cause excitement on the 70mph, 0100 in the morning highway.

Spent the rest of the "weekend off" towing the thing back to NAS Miramar. Removed the intake, distributor, valve covers, and threw the rest in the **** can

We hitched a ride with the wrecker crew--who were drunk---into Stockton. Asked a cop "how do we get out of town?"

"Have ya tried hitchhiking?"
"Oh, no officer, we have money. We are looking for a plane, train, boat, ?"
"Have ya tried hitchhiking?"
So we got a hortel room, ended up a bus to 'Frisco, taxi to the Oakland airport, PSA flight to San Diego. Taxi to my (RIP) buddies house, took his truck out to Miramar, and retrieved my 64 "440" (426) Dodge. Borrowed a clamp on tow bar, and drove all the frickin way up to "S of Stockton" and towed the '70 all the way back to the base

My leading chief:

(Knits eyebrows Columbo style) "I do have one question. Are you supposed to be able to look THROUGH the engine and see the GROUND?"
 
67Dart273 said:
That is interesting. I can remember vaguely reading about balance in some Hot Rod or similar article in the seventies, and I called Traco who were up around LA I think. They claimed they balanced 302/ 350 Chev combos to something like 1/4 or 1/2 a gram. Seems to me the article was claiming "not much higher" variation for a factory Chev 302 Obviously that was a long time ago. I don't remember specific numbers, only that the variation was MUCH less than I had assumed. Either that...........or someone was lying LOL
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I think I can answer that for you.... they may be balancing to a unit of inch-ounces, which is what I now understand balance machines work with. Here is a good article that discusses that. and there are 2 very good companion articles to this one.
Finding Balance (Part 2): Examining the Shortcomings of Typical Engine Balancing Techniques - OnAllCylinders
 
dartfreak75 said:
So when replacing pistons as long as the total weight of each rod piston ring combo is within 30 Grams of the original weight it will be ok?
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Sorta.... it is the bobweight of the assemblies being within a certain range.

The upper end parts have 1/2 the effect on bobweight versus the lower end of the rod for a V8. Example: If you changed a single piston by 30 grams and nothing else, then the bobweight on that crankpin would change by only 15 grams. That is why you can get away with one oversize piston or swapping out one piston-rod assembly in a daily driver car.

In general, for high revving you want the bobweights a lot closer than what we are talking about here for just older production numbers and how bad/good it was out of the factory for stock engines. But, as a guess, if the bobweights for an engine like you are planning (that won't rev much past 5500 RPM) are within 10 grams of crank's original target bobweight, then it will be acceptable. That would allow piston+pin+ring variations of +/-10 grams from the original nominal weights and it might be able to stand a bit more.
 
Food for thought. A 5 cent nickel weighs 5 grams. A copper paper clip weighs 1 gram. Just so you get a idea. Now go find 6 nickels and tell me what you think. :)
 
That is some informative numbers:)

A good friend of mine that is much sharper than i am once said regarding balancing down to 0.5g or so"once you realise how much oil is being dragged and thrown around by the crank and rods you will stop caring about that 0.5gram its totaly irrelevant"

Of course you should balance your complete rotating assembly and while someone is there doing the job there is no reason to not do it with precission just pointless to loose any sleep over that last 0.5g
 
One thing to think about oil flying around..... it will get on all rods and pistons pretty much equally, I would think. So the bobweights on all crankpins ought to increase about the same due to oiling. So in a way, it is a side issue, at most.
 
So, I need to ask a question after taking this all in. Balancing at a machine shop is quite expensive. I can buy a rotating assembly pre-balanced from scat. They give you all the measured weights etc and guarantee it is +/- 2 grams. They charge under $200 extra for the balanced vs non-balanced kit. Everyone says have the machine shop balance it. If it comes from scat at +/-2 grams, that is significantly better than what it is now...why pay the machine shop to balance to a 1/2 gram?
 
a good superstock engine builder told me that on a 340 that pistons within 10% weight of previous balance was ok. i agree
 
stumblinhorse said:
So, I need to ask a question after taking this all in. Balancing at a machine shop is quite expensive. I can buy a rotating assembly pre-balanced from scat. They give you all the measured weights etc and guarantee it is +/- 2 grams. They charge under $200 extra for the balanced vs non-balanced kit. Everyone says have the machine shop balance it. If it comes from scat at +/-2 grams, that is significantly better than what it is now...why pay the machine shop to balance to a 1/2 gram?
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IMHO, the 'let the machine shop do it' statements may have to do with 'don't try this at home' (LOL) or 'make sure it gets done'.

  • Things that can make things less accurate than they seem:
    • Even if the piston-rod assembly's bobweight equivalents are all within 1/2 gram, then the subsequent balancing of the crank to the resulting bobweight has a tolerance that can make the overall result effectively better or worse than that 1/2 gram accuracy.
    • If the shop is doing 1/2 gram accuracy on the rods and pistons individually, then the tolerances will stack up for a larger overall piston-rod tolerance than 1/2 gram.
  • I don't know what SCAT means exactly when they refer to the weights that are being balanced to +/- 2 grams. If the weights you get from SCAT for each piston-rod assembly are 'bobweight equivalents", then pairing heavy and light assemblies on the same crankpin will help overall balance every time. If they are just total piston-rod assembly weights then this will make overall balance better most of the time.
  • If your assembly is of the internal balance flavor, then the dampers and flexplates/TC's enter into it.... they are balanced within a tolerance so unless those are done with similar precision, then that will make super-accurate balance work less effective.
  • We have no way to know of SCAT's machinery and daily balancing work is better or worse than your local shop. Who knows?

If you are getting the whole rotating shebang from SCAT, and can install it unmodified, then, I agree, $200 is a good price. And, if you are getting a 4" cast stroker crank, from talking with SCAT, the crank is going to need heavy metal added, and if you can get that included for under $200, then that is even a better bargain.

I expect the work is gonna be quite good enough for most of us in the moderate HP range, horsing around on the street. It'll sure be tons better than the factory work LOL
 
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