Rotating assembly question

[AlanCasida]

On rotating assemblies do you prefer to get them already balanced or have your machine shop balance them? Looking to (finally) get back in the FE game.

[SSdynosaur]

Personal experience. I ordered a Winberg billet replacement crank for a stock stroke 440 cid FE engine that, previously, had been equipped with a very modified FT crank that was found to have multiple cracks during a tear down inspection. Winberg asked for the rod/piston balance weights stating that it would be a simple matter to balance the crank during machining. Inspection, upon receipt, revealed a shaft without a single indication that it had been balanced; no obvious contouring of counter-weights, no drilling and no added heavy metal slugs. Curious about whether it was actually balanced or not I gave the same balance info to my local race shop that I had furnished to Winberg and paid them to check and advise. The shop reported that the crank came out as close to perfectly balanced as if they had rebalanced one of their own jobs. If you are in a situation where every HP/Torq figure is critical I feel that elimination of the common aftermarket balance machining and the subsequent crankcase windage reduction would be valuable.

[pbf777]

       I would have say that your experience with Winberg may not prove as with many others.  So it's a judgement call in the case of each vendor and their actual definition or acceptable tolerance in the balancing effort.

       On a different forum (not "FE"), a guy stated that he had been responsible for the crankshaft balancing for one of the bigger crank-kit suppliers and that their "acceptable" tolerance in the balancing effort was "+/- 25gr. on the crankshaft!      Do you think that's good?   

       Scott.

[CaptCobrajet]

Personal experience. I ordered a Winberg billet replacement crank for a stock stroke 440 cid FE engine that, previously, had been equipped with a very modified FT crank that was found to have multiple cracks during a tear down inspection. Winberg asked for the rod/piston balance weights stating that it would be a simple matter to balance the crank during machining. Inspection, upon receipt, revealed a shaft without a single indication that it had been balanced; no obvious contouring of counter-weights, no drilling and no added heavy metal slugs. Curious about whether it was actually balanced or not I gave the same balance info to my local race shop that I had furnished to Winberg and paid them to check and advise. The shop reported that the crank came out as close to perfectly balanced as if they had rebalanced one of their own jobs. If you are in a situation where every HP/Torq figure is critical I feel that elimination of the common aftermarket balance machining and the subsequent crankcase windage reduction would be valuable.

A properly executed “no drill” balance is the most desirable.  CAD programs used by folks like Winberg and Crower, among others, allow them to strategically design the counterweights so the heavy metal is not needed, and drilled holes are not needed.  There are people around who will “no drill” balance less expensive crankshafts, but that service costs more.  If you are looking for every bit of power, this should be on your list.

Generally speaking, the lower budget rotating assemblies are useable, but not as “spot on” as the old guy in a one man shop who still cares.

[blykins]

Exactly, that’s why it’s best to have your machinist or builder balance the assembly rather than buy a balanced package from one of the lower priced crank companies.

[cleandan]

What I get from this is buy your engine parts and pay your engine building machinist to balance those parts during the build.

[blykins]

What I get from this is buy your engine parts and pay your engine building machinist to balance those parts during the build.

Bingo.  As Blair stated, the guys whose names are on the engine or standing behind the work will take more care than the guys who are trying to crank through high quantities of crank balances per day and don't see the end result.

Many years ago, I started weight matching components a different way.  Instead of weighing every piston, every rod, etc., then grinding on rods or drilling pistons to match everything to the same weight, I will do some calculations then pair components up based on their individual weights:  for example marrying a lighter rod little end with a heavier piston, etc.  It takes longer, but to me it's cleaner and looks more professional. 

Don't have any proof for this, but if I had to guess, the manufacturers who are sending rotating assemblies out already balanced are not putting any care into it and are just reading the general weight number off the rod box, piston box, etc.  In reality, each individual rod, each individual piston, etc., will weigh differently.

[AlanCasida]

Thanks guys, that's kind of what I was thinking. I've never bought a complete assembly before, I've always gotten my rotating assemblies in pieces. I know my machinist always likes to check all the components anyway.

[Barry_R]

I will agree with the other guys for the most part.
But I also want to play "devil's advocate" for a moment as well.

When doing balance calculations - what do folks doing the work add to the equation for "oil"?
I have seen values anywhere from "nothing" to "10 grams".
I can pretty much guarantee that the value chosen is - in the vast majority of cases - based on nothing beyond legacy, opinion, or wild guess.
The guys at Ford or NASCAR probably know exact values - the rest of bus take the best guess we can with available information.

And many balance folks will promote the accuracy of their balance to 1/4 of a gram.
I had the pleasure of digging into the software of a reasonably new, reasonably nice, and reasonably expensive balancer.
We discovered that it had a +/-3 gram fudge factor built in that would happily print out "zero" on the display.

I am NOT telling anyone that a poor job is OK, nor that anybody is misleading them.
I am telling folks not to get too freaked out if balance stuff is not perfect if/when they check it themselves.
The process catches outliers and gross errors/issues.  It gets thing close.
The minutia can remain minutia without ill effects.

[mbrunson427]

Alan, just curious, what's your project?

We have balanced 3-4 Scat cranks for different engines, they never have enough counterweight. Always have to add metal to those.

If you're buying something fancy like a Winberg I would think that they would be very close.

[AlanCasida]

Alan, just curious, what's your project?

We have balanced 3-4 Scat cranks for different engines, they never have enough counterweight. Always have to add metal to those.

If you're buying something fancy like a Winberg I would think that they would be very close.

If I buy a rotating assembly it will most likely be a Scat 4.25" 482. I have a 452 rotating assembly now that was damaged. I have bought a new crank for it but still need a connecting rod so I may use it but a 482 would be nice. 

[My427stang]

When doing balance calculations - what do folks doing the work add to the equation for "oil"?
I have seen values anywhere from "nothing" to "10 grams".
I can pretty much guarantee that the value chosen is - in the vast majority of cases - based on nothing beyond legacy, opinion, or wild guess.
The guys at Ford or NASCAR probably know exact values - the rest of bus take the best guess we can with available information.

And many balance folks will promote the accuracy of their balance to 1/4 of a gram.
I had the pleasure of digging into the software of a reasonably new, reasonably nice, and reasonably expensive balancer.
We discovered that it had a +/-3 gram fudge factor built in that would happily print out "zero" on the display.

I am NOT telling anyone that a poor job is OK, nor that anybody is misleading them.
I am telling folks not to get too freaked out if balance stuff is not perfect if/when they check it themselves.
The process catches outliers and gross errors/issues.  It gets thing close.
The minutia can remain minutia without ill effects.

These are good comments Barry and I agree, although we do every one and check any one that comes in "balanced", and I think I always will

- No two balancers or scales read exactly the same either, every tool has a variance. Might as well add the scale and rod fixture setup variance too.
- We aren't spinning a round top or a tire, we are balancing a crank with varying load/unload rates from different directions with pressures and vacuums above it during operation, so as you said, gross errors (or mismatch) in the casting or forging of the counterweights are the greatest factor for balancing issues, more than the more minor issues of lesser parts.
- I'll add that I have no idea sometimes what craziness the manufacturer is really using for a target bobweight assumption.
- On an FE with a bob of about 2000, oil is well under 1% of the bob, so you can likely use nothing to 10 grams.  We generally use 5 in the formula from habit, the balancer software calculates 1-2 g higher if I plug it in, but I always do both to keep us honest.
- I'd argue the oil is never the exactly the same value in operation too, but as above, close enough.  All parts are coated in oil in operation, you have bearing surface oil, bleed and what's in the passage.  It's dynamic like everything else.  As I think about this, I think more than 5 g may be wise, but at way under 1% of bob, I think it's measuring with a micrometer cutting with an axe

All that being said, call me old fashioned and/or wasting time, I do the measuring and math on every one, and then verify balancer input to check twice.