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Old 01-13-2008, 08:18 AM
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Balancing Question

Hi again all,
Another question for you. While looking for cranks and such for a 350 & 400 build, I have noticed that you have to look at whether the crank requires internal or external balancing. Aren't all cranks balanced from the factory? If not, that would be like buying a brand new car and having to take it to a tire shop and have the wheels balanced before you could drive it.

Ok, say you install an "External Balance" crank. These cranks are normally sold with the flex plate and harmonic balancer as a balanced kit, correct? Ok now you assemble the motor with the pistons, rods, etc. In my view, this is not a balanced assembly anymore.
Now, if you start out with a balanced crank and install pistons & rods that have been balanced, then you end up with a balanced motor, right?

Someone please explain the whole "Internal & External" balance thing.

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Old 01-13-2008, 08:49 AM
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OK kid, i will try to explain the balance deal so you can understand. with internal balance--the crank, the flywheel, and the damper will be balanced (350SBC)--but only with the weight of the rods and pistons the engine was designed for.

external balance-the crank will be out of balance, but the flywheel and damper will have counterweights on them to be out of balance exactly opposite of the crank (400SBC) so the entire assembly is IN balance--but again, only for the weight of the pistons and rods the engine was designed for.

any substitution of the rotating parts (crank, damper, flywheel, rods, pistons, pins, rings) will requier checking the balance--this is the theory.

in actual practice you could replace a stock (damper, flywheel, clutch) with another stock one and still be in balance.

but any substitution of custom parts for stock parts will require having the balance checked.

balancing is way cheaper than having the engine shaking itself apart and blowing up in a few minutes (or seconds).

in short--IF IN DOUBT, DON'T CHEAP OUT--balance it.
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Old 06-04-2008, 06:52 PM
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to re-birth this thread.... if I'm taking a stock crate GM 350 that was in a moving van with clutch and big manual transmission, and putting on a internal balance flex-plate for a 700R4 automatic, should I have any concerns? Everything else is virtually stock, except for cam, intake, carb. Rotating assembly is all the stock stuff... including the damper that was on it that I plan to use.
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Old 06-04-2008, 09:29 PM
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A good aftermarket flex plate will come with a bolt on weight that would be used if the assembly was to be internally balanced. The guys are correct by telling you to have the compleate rotating assembly balanced before assembly. We send out a lot of engines to the mach shop for balancing and when they come back we rebalance the rods, and pistons along with the rings and bearings assembled to the rods.
The crank is balanced with added bob weights that resemble the compleate weight of the piston and rod assembly along with the oil wt calculation of .003 grahams per unit.
Occasionally they will add weight to the crank in the counterweight area with heavy metal called (Mallory) which is heaver than steel, The whole assembly is spun on a special mach that detects where weight should be added or removed. Be sure to have a trusted shop to do this work for you.
The flex plate and the damper will be balanced seperately.
I have had some problems with crate engines in the past, with oil clearences
as well as rod caps not torqued to specks. Like we say If in doubt take it apart.
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Old 06-04-2008, 09:40 PM
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But I'm not planning on doing anything to the engine. Its assembled... was running when pulled out of the truck it came out of of... believe it or not... probably only has 8,000 miles or so on it since it was new. I'm just cleaning it up. So, that said, however it was from GM is how it will be in my truck as far as internals (rotating assembly) go. Can I tell anything by looking at the cluth / pressure plate that came off of it or the damper?

I'm not planning to take it to a machine shop... there's none needed. Does that help? I've just done some mild head work (valve springs) and will install a new cam here in a couple days to give it a little more ummff.
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Old 06-04-2008, 09:41 PM
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And by the way, the flex plat I have coming is from Summit. Part # SUM-G100SFI, Flexplate, 168-tooth, Internal Engine Balance, 2 Piece Rear Main Seal, SFI 29.1, Chevy V8.
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Old 06-05-2008, 01:15 AM
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Balanced?

ALL engines are Factory balanced, but NOT to the fine degree a machine shop provides. Oversize pistons are within BALANCE of the factory standard size/weight. So, rebuilt engines are not normally rebalanced unless the customer wants it to be more "dead on". I've heard the reason rebuilder pistons have .020 off the top is to bring them to the same/closer weight of stock/standard pistons. Speaking primarily "V" style engines here; not so much inline. And, there are two components to balancing an engine, reciprocating weight (pistons, rings, pin and clips,small end of rod, and a nominal amount of oil), and rotating weight (big end of rod, rod bearing, vibration dampener, flywheel/flexplate, clutch assembly, fasteners, and a nominal amount of oil), to arrive at a bob weight that's temporarily attached to the crank, spun up, and more weight added the the crank (heavy metal), or weight removed to bring the assembly into "balance". When you start swapping cranks (ie: 400 crank into a 350 block to make a 383), balance really becomes more of an issue due the "mix" of parts, or "mismatch" of parts. Aftermarket parts further complicate the BALANCE factor.
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Old 06-05-2008, 05:27 AM
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So all that said, since I'm not swapping parts or anything, will this internal balance flex plate be ok to run with a GM 350 crate engine?
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Old 06-05-2008, 12:49 PM
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balance

dhunt00c,In a word, according to the info provided, YES.
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Old 06-05-2008, 05:17 PM
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Sorry, that was kind of rude of me. Just wanting to ensure which flex plate to buy with my setup. Does this mainly become important for racing engines and such? I'm a novice and trying to learn. Since this motor is original, and I a flex plate to conver to different transmission, just curious which one to buy. Summit couldn't really tell me anything.
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Old 06-05-2008, 06:01 PM
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External balance SBC400 & 383 use same flexplate, Internal balance 2-piece rear main seal 350 and 1-piece rear main seal 350 are different. 1 piece rear main seal crank sometimes called 1/2 internal & 1/2 external balance because the the rear of crank doesn't have enough room for the bigger counter weight and weight is added to the flexplate. So 3 different flexplates.
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Old 06-05-2008, 06:43 PM
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Cool. Thanks. I called Summit back and he said the internal balance flex plate is what i should use. He said the later model (i think vortec) and sbc 400 used the external.

Whew! Think we got that one whipped. Thanks for all the help!
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