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Old 08-02-2010, 05:23 PM
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Engine Balance

Let's say I buy a Scat 383 balanced rotationg assembly for my 350 block. I would think, I would need a nuetral balance flexplate and damper, Or could I use an external balanced flexplate and damper with my assembly? If you are not well versed in balancing please don't respond as I have had every answer I can think of from people who don't know. Some people act as though the external balanced damper and flexplate will cancel each other out, so the engine is still in balance because of it's internal balancing. I would really appreciate the right answer.

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Old 08-02-2010, 05:45 PM
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The crank says it`s balanced, but that`s not the point. It will also say if it`s internally or externally balanced. Some cranks have the opposite balance on each end. When chevy went to a 1 piece seal crank and 1986, the crank flange had to be made perfectly round so there was no way to add a counter weight to it as the old 2 piece cranks did. Without the counter weight the engine would be out of balance. So the front of the crank still used a neutral balanced balancer. The rear of the crank was externally balanced by adding counter weights to the flexplate/flywheel. As for canceling each other out I`m not so sure of, as the 400 small block came with a externally balanced crank and used a special balancer and flywheel. The issue with that was the middle of the crank would be out of balance. It`s not uncommon to tear down a 400 small block that was used for racing and the assembly never balanced and find the center main bearings have been pounded to death.
The ad you read may not have mentioned if the cranks were internally or externally balanced but as far as I recall, they offer both. Even so it`s been balanced, I would have it balanced again before I ran it, as I`ve heard horror stories about this, especially about Eagle cranks.
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Old 08-02-2010, 05:53 PM
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You can get 383 assemblies as "external" or "internal" balance..

External balance uses a crankshaft with smaller counterweights. Places counterweights in balancer and flex/flywheel. 383 uses 400 chevy components. Detroit engine producers have done this for many years. It is cost efficient and reliable. Perfectly good for street and mild race engines.

Internal balance is just that. All weight equalization is done in the counterweights within the block assembly. Balancer/flywheel are "neutral".

If you buy any assemblies or "kits", internal or external balance. I do not reccommend the "balance job" done by the sellers. A competent balance shop will produce a much closer assembly. Usually + or - 1/2 gram..
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Old 08-02-2010, 06:08 PM
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Originally Posted by Speed7
Let's say I buy a Scat 383 balanced rotationg assembly for my 350 block. I would think, I would need a nuetral balance flexplate and damper,
ONLY if the 'kit' was an internally balanced reciprocating assembly.

An assembly can still be "balanced", but also still need the external balanced damper and flex/flywheel. Just saying "balanced" isn't enough- you need to also know whether it's internal or external balanced.

BOTH internal and external assemblies are balanced, in other words. One is balanced for an external damper/flex, the other is balanced for a neutral damper/flex. Internal balancing can involve the use of Mallory metal inserted into the counterweight to give the additional weight that's required, w/o having to resort to a larger diameter counterweight that might not fit inside the crankcase.
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Old 08-02-2010, 06:29 PM
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If one orders an externally balanced recip assembly should the supplier be shipping the associated flex plate with the kit? just wondering if this is always the case.
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Old 08-03-2010, 07:52 PM
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Originally Posted by Custom10
If one orders an externally balanced recip assembly should the supplier be shipping the associated flex plate with the kit? just wondering if this is always the case.
Probably not, the crank assemblies that are sold as balanced do not include the damper or fly/flex. You will need to score them yourself.

What you'll get from in the kit will be the crank, rods, pistons and rings.

Depending on where you go these could be a kit put together by SCAT or a kit put together by a third party using SCAT parts. Or A third party making it up as they go along using some SCAt parts and other body's parts.

Balanced cranks for 383's come two ways; one for internal balanced, the other for external balance.

- Internal balance will use a neutral balanced damper and fly/flex. Single rear seal cranks take a fly/flex with a small amount of external counter-weight. Knowing this, or not, is of small concern as fly/flexes for 2 piece seal cranks and 1 piece cranks have a different bolt circle diameter so you can't fit one type to the other unless you have an electric drill and a lot of persistence.

- External balance will use an out of balance damper and fly flex such that when combined with the out of balance externally balanced crank, the assembly becomes in-balance.

Either of these can be found for internal or external balance, usually cast cranks on stroker kits will be external but not always, you've got to keep your eyes and ears open and be willing to ask dumb questions of the tech desk.

The other thing you've got to watch for is clearance to the block at the oil pan and in other than L31 engines the bottom of the cylinder wall. Also, on all engines rod to cam. Many blocks, especially the heavy duty guys like the 010 require you clearance the pan rail and bore extensions for the rods. Point oh-fifty (.050) inch is adequate and will keep you from discovering coolant. Cam clearance is more iffy but again .050 is sufficient between rod and lobe. The problem here if there's interference you either have to get a small core diameter cam, which isn't a bad idea especially for a street engine with a mild to moderate cam. Otherwise you have to take some material off your balanced rods, which of course can become a balance issue. To minimize the clearancing of the rods, I recommend using rods with cap screws from the cap into the shank rather than the bolt and nut design as the capscrew shank tends be smaller and more streamlined in this area resulting in none to less material removal and your not grinding on the bolt head making it smaller an weaker as with the bolt and nut style. The latter is done by folks with success but it gives me the sensation of gonads in my throat when I see it, so for the sake of keeping the family jewels and my tonsils separated I don't do it nor recommend it.

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