FE Power Forums
FE Power Forums => FE Technical Forum => Topic started by: hbstang on May 23, 2026, 12:53:36 PM
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bought a new scat forged 4.25 crank for a build,and builder cant balance crank,its asking to much weight on sides of front and rear counterweights.maybe they are not clocked right?anyone see this?
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Almost every time I had a very odd number, it was because I did a bobweight screw up, machine setup, or rarely a calculation wrong (I keep it in an excel file and plug new numbers in, then let the machine calculate and recheck). Not questioning him, but I would make 100% sure he rechecked his bob weights, setup and his calculations. Been there.
That being said, what would drive you to need that much weight would be a heavy piston and rod combo, or as you pointed out a light counterweight. Last time I talked to them about balance, I think they said you shouldn't need to add material on a forged crank until the bob is over 2000 grams. I have seen FE bobweights climb pretty high especially with H-beams and a short piston. I have been countering that with a 6.800 rod when I can to lighten the piston.
Do you have the component weight? Happy to calculate the bob and see if we came up with the same, but odds are against the math being wrong.
All that being said, I have had a lot of SCAT forged and cast cranks, and the forged have varied the most. No two combos have the same bobweight, but it does seem that the forged stuff varies more than the cast.
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thank you for your reply.i will ask builder on monday for the weights and also call scat to ask about crank.the x he marked is where the machine wants the weight and its not enough room to put that much in the front.the rods are oliver 6.700 and piston is custom race tech dome for 489 sohc.1.345 comp height.i dont think its all that heavy,but i could be wrong.
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Racetec tends to go heavy on pin, both length and thickness and the big bore dome could be heavy. I have never used an Oliver rod but good thing is no doubt they are stout. If an L19 bolt I’d toss the bolts in the trash though.
I’ll cue Brent to talk about bolts but they are dangerous
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I'd think real hard and long about using the bolts that are in those Oliver rods, unless they're CA625 bolts.
If they're not CA625, they're L19. Lots of people use them without issue, but I personally know of two instances where an L19 bolt failed: one was my engine, idling on the dyno, the other was an engine builder buddy of mine who had a bolt head pop off while the engine was on the stand.
The L19 bolt is more subject to hydrogen embrittlement.
You don't hear of this happening with 8740, 2000, or CA625 bolts.
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. . . . . . builder cant balance crank,its asking to much weight on sides of front and rear counterweights.maybe they are not clocked right?anyone see this?
Yep! Many times, and then the finger pointing begins! ::)
Today's pistons, if anything generally run on the light-side and even if you don't have the lightest, they wouldn't insight that sort of needed correction. Now, most modern aftermarket rods do often, in comparison to the O.E.M.'s standard products, add some mass to the calculation, but pretty much the norm for either of these is relatively consistent and understood; so therefore in my opinion ultimately it is the crankshaft manufactures fault that you find yourself in this predicament! >:(
The problem is that the crankshaft forging core options are limited, and it seems that upon initial design of some, the counter-weight values and positioning was based on a shorter stroke version. Then, someone just decided "all-ya-gatta-do" to make a "stroker" is provide enough additional material in the crank-throw location in the forging to allow variable placements in the pin/journal finishing. And this sounds fine until you consider the issue of mass effect in distance, or as in the balancing process is often presented "inch-ounce" of imbalance; aka., the same rod & piston (bob-weight) mounted further from the rotational center requires grater sums in bob-weight effect to offset! ;)
Then, the outer radius in the counter-weight is limited due to its' relationship to the reciprocating piston, take a look at the shape of the counter-weights here, then go look an O.E.M. example. In doing so, what one should notice in a comparison, is that the O.E. piece's counter-weight outline, that in its' relationship to the main journals and path of rotation, is often of an "eccentric" profile, whereas this SCAT example has them of a "concentric" outline; meaning that the latter doesn't present the possibility of being of equivalent mass, particularly out at the outer reaches, but it was easier/cheaper to produce! :o
If being forced to deal with this scenario as presented here, the operator in the balancing effort is going to need to be "creative"; it will be necessary to "coerce" the point of correction to a better position, one that will permit addressing. And this isn't always easy or quickly done, so be prepared to pay additionally for this more advanced service effort! :(
B.T.W., I've "never" had an instance where I "couldn't" mange to balance a crankshaft; though there have been many instances where I advised the customer that it probably wasn't going to be a wise choice on their part.
On one occasion, this back before the aftermarket was filled with the options we have available today, a customer brought in a Ford 460 crankshaft that had been offset ground to 2.100" journal size, and then with the counter-weights cut-down for the stroke increase and which also consisted in the use of a shorter than the 385 series connecting rod. When I first looked at it, I said: this isn't going to be easy, and I suggested to the customer that, well . . . . . . "he was barking up the wrong tree"! He responded with: "I know you can do it". ::)
On the first "spin" (on the balancer) I called him up and said forget it! It's going to take way too much metal to bring it around; or whether even if there's enough counter weight area present to act upon! His response: "what ever it takes"! Well, in the end, I got it balanced; twenty-three pieces of heavy-metal later! :o
Scott.
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I'd hit the brakes until we see some weights
I took a Diamond shelf piston for a SOHC with a 4.25 crank and 6.700 rods, 4.28 bore from the catalog and they are 640 grams, Racetec usually comes in heavier in both pin and piston than Diamond,
Then looked at even the lightest Oliver at 825 grams, and they go up from there, and my make-believe combo comes in at a bob of 2365 grams.
I can't remember if the target for a SCAT was 2000 or 2100, but I believe 2000, so you need to offset 365 grams. Add maybe some more pork from Racetec and if he is running Oliver Max series or a big bore, and it could be real.
I have never had to, but you can weld to extend the counterweight, or even add to the counterweight with a machined piece. I have seen SBC guys do all kinds of crazy stuff.
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... I have seen SBC guys do all kinds of crazy stuff.
It's really not all that crazy. Reciprocating aircraft engines use bolt on counter balance weights to account for the variations of mass throughout engine series. Rather than using dedicated cranks, they just change the counter weight to match the new components. It's worth considering that as a technically acceptable solution.
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This is probably more anecdotal than helpful, but I bought the Scat stroker kit from Barry as a balanced assembly. Mahle pistons, H beam rods. I was surprised to see that some Mallory had been added to the crank.
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I'd hit the brakes until we see some weights
I took a Diamond shelf piston for a SOHC with a 4.25 crank and 6.700 rods, 4.28 bore from the catalog and they are 640 grams, Racetec usually comes in heavier in both pin and piston than Diamond,
Then looked at even the lightest Oliver at 825 grams, and they go up from there, and my make-believe combo comes in at a bob of 2365 grams.
I can't remember if the target for a SCAT was 2000 or 2100, but I believe 2000, so you need to offset 365 grams. Add maybe some more pork from Racetec and if he is running Oliver Max series or a big bore, and it could be real.
I have never had to, but you can weld to extend the counterweight, or even add to the counterweight with a machined piece. I have seen SBC guys do all kinds of crazy stuff.
thats what i am going to do.also if a lighter pin,and rod would help it, but i dont know those weights yet.
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Thats what i am going to do. Also if a lighter pin, and rod would help it, but i don't know those weights yet.
Yes, a lighter bob-weight would "probably" make the job easier! And then although there's nothing wrong with the owner being "involved" in the process, but it 'is' the guy whom has been 'contracted' for the balancing service that 'should' be approaching 'you' concerning what would work to make the job simpler, or in some instances even just feasible. ;)
Seriously, this issue isn't that uncommon, but I don't have your setup in front of me, and I'm not privy to the conversation you might have had with the shop, but when one experiences one of these, although, if one has any sum of experience, you might grimace with disgust saying "rats, another one!", it just kinda goes with the territory! >:(
Of course, the shop might want to contact the customer to advise them that although "we can" balance it, it's moving into the territory of "time & materials" to do so! Either that, or the customer starts spending additional money buying "other" componentry in the effort to reduce the costing in the balancing effort; which sorta just doesn't make a lot of sense, unless otherwise it has been concluded that it isn't feasible otherwise. Which I really doubt in this case. :)
But, it sure would be nice, if the crankshaft manufacture's would "engineer" and "execute" the counter-weights 'properly'! ::)
Scott.
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A question for the old-timers like Randy etc.- and before I get the irate bum-rush, I have not weighed for bobweight yet. I have 454 combo with Crower rods and factory lightened Arias domed pistons, and the option of a factory 428 crank or a welded and offset ground to 3.98 391 FT crank. Love to use the 391 deal for strength, but it is one heavy sumbitch. I understand that back in the day it was popular to cut down the counterweights on the steel cranks when possible. I have a big South Bend so that part is feasible. The local speed shop guy who lives across the street, but is seriously Chebbie cross-threaded between the ears, is willing if I get the part matching and bobweight stuff ready. I swapped into the rods n pistons from a gent who was quitting racing, for a set of Eagle rods and dished pistons, so hopefully the rod and piston matching stuff is well done. Looking for actual been there done that experience from some old buggars lol- not a dozen "opinions". I know the 428 cranks are pretty tough and have been used many times, but it would be nice to be able to lean on this thing, I have some decent heads, cams and intakes. The 391 cranks are seriously heavy bastages, but strong as hell. If this one wasn't already car-modified, I wouldn't even think about it
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Looking for actual been there done that experience from some old buggars lol- not a dozen "opinions".
But honestly "opinions" all your going to "actually" get, as no one here actually has any of your stuff "in-hand"! ;)
I understand that back in the day it was popular to cut down the counterweights on the steel cranks when possible. I have a big South Bend so that part is feasible.
Again, I don't have "your" crankshaft in-hand, but in my experience, even though you might find the "FT" crankshaft to be heavy, it's not carrying that greater mass in the counter-weights! This as these examples were outfitted with "4-ring" pistons which were longer in distance below the wrist-pin than the typical "FE" light vehicle pistons and therefore the counter-weights were already cut-down for these and necessitated external counter-weighting in order to achieve balance. :(
Also, just note that if determined to cut something off the counter-weights, understand that they should be trimmed "eccentrically" in relation to the center-line of the rotation, rather than as is the pictured SCAT example, so this requires off-setting chucking jaws on your lathe, doing the math, and then each counter-weight would be set-up differently in the clocking for the off-setting (watch out for the throws! ::)). And this greater effort as required for doing the job correctly, is why the cranks are so often done incorrectly . . . . . . cost. ;)
I know the 428 cranks are pretty tough and have been used many times, but it would be nice to be able to lean on this thing.
Well, "back-in-the-day", on the first tear-down and rebuild attempt of the 428's, this coming out of some "sedan" with say a hundred, hundred and ten or so thousand miles on them, I'd say something nearing twenty percent wouldn't pass magnaflux inspection; and as for the sportier Mustangs and the like, failure was far greater. But yeah, a lot of guy's "got away" with running 428 cranks in the 427's, . . . . . . for a little while anyway! :o
Scott.
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Just need to get the bobweight stuff done and take both cranks down there and see what's what
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My builder at the time said pretty much the same thing on my 4.25" Scat. After a few calls he did figure it out and it came out fine though.
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so it goes bad to ugly on this crank balance.i bought a new set of molnar h-beams that are almost 70 grams lighter,and a set of pins that are 20 grams lighter,so bob weight went down to 2225 ,sounds good right? well now it moved the front weight completely off the counterweight into space,to the right ,and the rear is to close to edge to put a weight in it.my guess is this is a junk crank from scat!counterweights must be mislocated.i will have to source another crank,as i wont changed pistons/rods etc.so i will contact scat on monday.
any of the engine builders on here can tell me if you had a scat forging that was defective ?counter weights clocked wrong or bad machining?would removing weight from other end of counterweight bring it in?
any one please help.
maybe scat will help.
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This seems a little too hard, any chance his machine is off?
I suppose a counterweight could be off, but it sounds unlikely
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. . . . . . so bob weight went down to 2225 ,sounds good right? well now it moved the front weight completely off the counterweight into space, . . . . . .
Yep, that's how it works! And before you ran out and spent that money (I think I might have previously commented on this) the "balance guy" 'should' have been able to tell you that! ;)
My guess is this is a junk crank from scat! Counterweights must be mislocated. I will have to source another crank, . . . . . . .
No arguments from me on your assessment of the crankshaft execution; but just as in your endeavor sourcing a different set of rods and pistons, I'm afraid the same outcome will be had with another crankshaft! :o Because as I also previously stated:
Seriously, this issue isn't that uncommon, . . . . . . it just kinda goes with the territory! >:(
And even tough previously I might have stated that: "there's nothing wrong with the owner being involved", but here it sorta seems like the "balancing guy" is making it your job to solve the issue of transforming an "unappreciated job" into "easy pickings" for him? ::)
Scott.
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This seems a little too hard, any chance his machine is off?
I suppose a counterweight could be off, but it sounds unlikely
brian has balanced hundreds of cranks and built more engines and is very good at what he does.i am sure he double checked everything.
what i dont understand is how thee location can move to the right with a lower bob weight? but i dont balance/build engines ever!but i like to understand whats happening
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What was your initial bobweight? sorry if I missed it
FWIW, last 4 forged SCATs I did, 2130, 2101, 2230, another 2101 and a 2050, all came in easy.
If he is willing to play a little, SCAT said the target is the high 1900s, maybe hang some bobs around and see if the crank does what it's supposed to with a lower weight.
I never heard of a SCAT being incompatible with a cammer, but I haven't built one either
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the new bob weight is 2225.old one was 2368.do you have pics of the one you did at 2230?where and how much weight were the slugs?was it a scat forged 4.250 fe crank?
did scat change there 4.250 crank forgings at any time?
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What I don't understand is how the location can move to the right with a lower bob weight? But I'd like to understand whats happening.
Again, because that's how it works! i.e., You add or subtract a "little" weight, the imbalance as registered rotors one way or the other, progressively based on the sum and resultant influence on the assembly that that adjustment might produce; add or subtract enough and it'll just flip to, or opposite to, the point of change. :)
And it is this phenomenon that the "guy" must harness, but it often makes for significantly more time invested, in order to achieve a balanced result. ;)
Scott.
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SCAT said the target is the high 1900's
But then, how often does one end-up with a bobweight in 1900's (with a margin for balancing) with an FE? ::)
Or is it that, rather than SCAT having actually "targeted" this, it's just the sum that was left for, in as the counterweights had been executed, sufficient in sum or placement or not? :o
Oh, and that's really not a question! ::)
Scott.
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SCAT said the target is the high 1900's
But then, how often does one end-up with a bobweight in 1900's (with a margin for balancing) with an FE? ::)
Or is it that, rather than SCAT having actually "targeted" this, it's just the sum that was left for, in as the counterweights had been executed, sufficient in sum or placement or not? :o
Oh, and that's really not a question! ::)
Scott.
I agree with your not question....
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What was your initial bobweight? sorry if I missed it
FWIW, last 4 forged SCATs I did, 2130, 2101, 2230, another 2101 and a 2050, all came in easy.
If he is willing to play a little, SCAT said the target is the high 1900s, maybe hang some bobs around and see if the crank does what it's supposed to with a lower weight.
I never heard of a SCAT being incompatible with a cammer, but I haven't built one either
when you did the scat forged crank with 2230,can you tell me where and how much weight you had to put in it?any pictures by chance?
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I have two, o don’t know weights but can show pics of the cranks
Unfortunately can’t be until Friday
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Though pictures of someone else's stuff might be interesting, but it isn't conclusively valuable in providing any sort of solution to your predicament as each assembly sum is unique.
Surely, your "balance guy" isn't still waiting on 'you' to solve this! ::) If so, perhaps allow me to make a suggestion, this to aid in the endeavor: "throw something at it"! ??? He should understand what that means. :-[
Or if, we're going to adopt the position that the crankshaft 'is' "unbalanceable" (it happens), send it back to SCAT and pay to have them balance it! Just have your guy match-weight the pistons & rods "properly", supplying the bob-weight value and then re-spin the crankshaft when you get it back from them for "refinement" . . . . . . . maybe. ;)
Scott.
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update on this scat stroker crank balance.scat has agreed to exhange the crank for another thru where i bought it.took it to engine shop in torrance ca and said the same as my engine builder,it would take a lot of time ,and he said it may flip the counterweights,and may not ever get balanced.if he added weights to the front or rear,and end needing more out in space.basically he will not do it,as it is a risk of not working on either,also wth weight on flywheel he said it was iffy at best and didnt want to try it!
scat has agreed to exchange it for another crank.oh boy! what could go wrong with that one.a lot of shops will not work on scat cranks any more!
scat was sold to prvate equity in 2022 that also owns afr heads.
hope to get the the other crank next week and try again.
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After all this, . . . . . .PAY THE EXTRA MONEY AND HAVE 'THEM' (Scat) BALANCE IT! ;)
They're relatively cheap, as they (generally) don't make "match-weight" corrections on the rods & pistons, dampers & flywheels/flex-plates and the crank balance work is only what I'd call "O.K." at best; but doing such will insure that you'll get something workable, which your balance guy, along with correcting the rods & pistons might only need to "refine" anything remaining in the crankshaft after Scat's effort! :)
Scott.
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they told me they wont balnce the crank,they only sell balanced assemblys.crank is defective.
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I was referring to the "replacement" they are sending you. :)
Scott.
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scat ship crank back and shipped another crank to builder.brian balanced it today with no issues.crank was defective,even used less weight to balance it,1 small slug in front and back! happy days.thanks to scat for replacing it,pretty good customer service !
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Awesome news!