Flywheel balance SBC 1-piece rear main

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Schurkey
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Flywheel balance SBC 1-piece rear main

Post by Schurkey » Fri Jun 15, 2018 4:32 pm

I have three flexplates (flywheels) for a SBC 1-piece rear main seal engine. The engine is a '98 Vortec 5.7L, if that makes a difference. These guys have a big welded-on weight of unusual shape. I dropped them off at my favorite machine shop to have him compare the balance.

Short story: A broken GM flywheel, and two non-broken never-been-installed aftermarket replacements show WILDLY different offset weight.

First flywheel is a Genuine GM, with the center broken. It's cracked 360 degrees around the flywheel bolt pattern--a common problem. The center is loose, can be wiggled around, but won't actually come free from the rest of the flexplate. My friend says the flexplate wobbles around some on the balancer, but shows "about" 180 grams of out-of-balance. Given that the flexplate is broken, I'm not going to trust that reading to be accurate.

Second flywheel is an unknown brand, with lovely gold-colored plating and a SFI sticker that may be genuine, or may be a Communist Counterfeit. The guy I bought it from says it came included as part of a SCAT stroker-crank kit. Welded-on weight looks about the same as the Genuine GM, but not identical. This one shows 130 grams of out-of-balance. Does not appear to have been modified.

Third flywheel is a Pioneer FRA-159 I bought from Summit. Welded-on weight looks like it was carved-out on primitive equipment, the thing is generally the same shape as the others, but it's got "thicker" sections, like the machine cutting it couldn't make tight curves. It appears to weigh MUCH more than the OEM weight, and then the manufacturer drilled a half-dozen lightening holes into the flexplate on either side of the weight. This one shows 210 grams of offset.
https://www.summitracing.com/parts/pio-fra159

The machinist tossed another Genuine GM flywheel from a core 5.7L one-piece-seal engine onto the balancer, it's reading about 187 grams--fairly close to my OEM broken flexplate. My wild guess is that the OEM flexplate is supposed to be 185 grams out-of-balance, plus-or-minus 5 grams for production tolerance.

I forgot to ask if the offset weight (counterweight) ANGLE is the same relative to the dowel pin hole. Wouldn't surprise me to find that that's different among my three flexplates, too. I'll have to inquire about that next time I talk to him.

WHAT IS THE SPEC FOR THE OFFSET (Counterbalance) WEIGHT FOR A SBC 5.7L ONE-PIECE REAR MAIN SEAL FLEXPLATE/FLYWHEEL? Am I right in thinking it should be about 185 grams off?

Do all one-piece rear main SBC engines specify the same amount of out-of-balance? I know there's a difference in the big-blocks, depending on cast or forged crankshaft. As far as I'm aware, all SBC one-piece engines--5.0L or 5.7L, TBI or Vortec, '86 to current production--take the same amount of flywheel counterweight. Am I wrong?

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Baprace
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Re: Flywheel balance SBC 1-piece rear main

Post by Baprace » Sat Jun 16, 2018 3:46 am

Schurkey, if this was my repair, I would have some one tack weld your broken center to the broken flywheel and take any one of the wheels you have left and have a balance shop spin the two together and balance in the new / newer flywheel. I find the aftermarket is all over the place on balance weights BUT GM is only slightly better. Back in the day two piece crankshafts were used as a factory dealership repair for broken or burned up crankshafts, problem was the crank they keep pushing to the customer was the same part number for two different cranks, one crank was for the 305ci engine and the other one was for a 350ci engine, every thing was identical BUT one crank was over 100 grams different, and GM said that was ok. Now on crank balancing, if the dynamic balance is the same left and right the engine runs decent, so one way the engine is overbalanced and the other way it is underbalanced, the key was make the dynamic close and the engine runs decent, not great but decent on vibrations.

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Dave Koehler
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Re: Flywheel balance SBC 1-piece rear main

Post by Dave Koehler » Sun Jun 17, 2018 11:14 pm

Shurkey,
Your suspicions are correct about variation. There is a variation almost every time.
Both position and oz/in will vary. Sometimes both at the same time.
Most of the time the variation on say, a GM crank is there but usually not enough to notice in the seat of the pants.
Personally I think any genuine GM replacement flywheels available at the dealer are seconds the factory chose not to use.

Now, when it comes to the aftermarket all bets are off. I found them high, low, left and right.
The sad thing is that the vast majority are coming out of a single mfg, shine and stickers added later.
A SFI sticker means squat as far as runout and balance quality is concerned.

If I am responsible for it the first thing I look at on a flexplate is if the ring gear is welded on center. Look at the ring gear to the plate reference. It should look centered or off center as the case may be all the way around. A lot of times you can see the plate biased to one side and then the other. That will be a ring gear wobbler with or without the convertor attached.

My latest flexplate misadventure was with a stock Ford 302 with the original set of rods, Sealed Power Forged pistons, after market 50 in/oz flexplate and a 50 oz/in Balancer. My first thought was it was going to be close but I suspected the SP pistons would push it past the "gravy job" status. Yup, It was going to need a slug of heavy metal on the tail end. The numbers made me check the flexplate by itself. It was actually at 46 in/oz, not 50. And the position was off 17 degrees. Most of the time the weight variation on the sb ford is between 48 and 52. Wrong, yes but reality.

In my case I figured the better course of action was to correct the flexplate. Besides heavy metal in the CW meant it would be in the wrong spot, intersecting with a already present correction hole. Not a good scenario

After some number juggling on how to move the vector point back where it needed to be I finished it up at 51.4 oz/in and the vector dead on. I did this by adding a couple of little pieces of heavy metal to the already existing weight. This left me with the CW at 4 grams heavy. A touch up dialed it in. So, even if it was dead on at 50 oz/in the SP pistons pushed it over the edge and would have needed some weight added somewhere.
The front was a bit lite also by 9 grams. A couple of pins in the balancer fixed that.

H balancers are usually a little better most of the time.......maybe.

Anyway, flexplates are like a box of chocolates. .You know the rest.

Warning, Commercial. If this is a no no, Mods, just pull it. I won't do it again.
Over the years I developed some unique tooling and techniques to make the checking/correcting/modifying of flexplates and flywheels a quick and easy peasy deal. No more head scratching. It is designed to work with the flywheel arbor most already have. You can know exactly how much and where the vector point is. No guessing. It works with all hard bearing machines.
This is something I am in the middle of producing a base run and hope to launch sales in 3 or 4 months.
Dave Koehler - Koehler Injection
Fuel Injection - Nitrous Charger - Balancing - Nitrous Master software
http://www.koehlerinjection.com
"Never let a race car know that you are in a hurry."

Schurkey
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Re: Flywheel balance SBC 1-piece rear main

Post by Schurkey » Mon Jun 18, 2018 5:09 pm

OK, I missed an important point. The spec would not be in "grams", it's more commonly in ounce/inches, but grams/millimeters can also be used. My machinist is reading the offset weight in grams/millimeters, and he didn't tell me the millimeter part, just the grams.

At any rate, I just need to know what ounce/inches or grams/millimeters these things are SUPPOSED to be. We can weld or drill the offset weight to get more or less counterweight than they have now, to match the spec.

Is the offset angle directly in-line with the dowel pin hole?

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