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Old 04-21-2011, 05:32 PM
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SBC harmonic balancer choice

Not sure what kind of harmonic balancer to get. I'm building an early chevy 350, 2pc rear main seal. It's an 010 block. Should i get an internal or external balanced, and should I get the 6 or the 8 inch? Not even sure what the differences are in these so if someone could clarify that with me that would be great. Thanks

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Old 04-21-2011, 05:44 PM
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Quote:
Originally Posted by Bad66
Not sure what kind of harmonic balancer to get. I'm building an early chevy 350, 2pc rear main seal. It's an 010 block. Should i get an internal or external balanced, and should I get the 6 or the 8 inch? Not even sure what the differences are in these so if someone could clarify that with me that would be great. Thanks
That should be a internal balanced engine, as far as 6 or 8 inch what are the plans for the engine. JMO And all "SBC" engines are internally balanced except the 400, Now unless someone did some work on the 350 and changed it.IMHO


Cole
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Old 04-21-2011, 05:47 PM
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ROMAC makes a nice piece, stainless, double keyed and degreed all the way around.
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Old 04-21-2011, 06:38 PM
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I went through what your are going through last week. I ended up with an ATI balancer. I called them an got their reccomendation.
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Old 04-21-2011, 06:38 PM
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Quote:
Originally Posted by Bad66
Not sure what kind of harmonic balancer to get. I'm building an early chevy 350, 2pc rear main seal. It's an 010 block. Should i get an internal or external balanced, and should I get the 6 or the 8 inch? Not even sure what the differences are in these so if someone could clarify that with me that would be great. Thanks
Since the Chinese began flooding this country with every kind of manufactured good possible, I have seen some really bad examples of harmonic dampers on the market. Couple this with the fact that most first-time engine builders are looking only at the cost of an item and things get out of whack in a hurry.

You should first understand that this piece of equipment is designed to cancel harmonics (tiny little vibrations) set up in the crankshaft each time a plug fires. The force from the cylinder firing is transferred through the piston, the rod and into the rod journal of the crankshaft, where it slightly bends the journal out of synch with the other rod journals. When it has bent as far as that particular force will bend it, it springs back the other way as far as it can (past center) until the strength of the material in the crank resists further bending. At that point, it bends back the other way, then the other way, then the other way, etc., etc. until the harmonics are cancelled or replaced with another pulse of power and a whole new bending series. If these harmonics are not cancelled in some way, all this bending will destroy the crank. That's the job of the harmonic damper.

In order for the damper to do its job properly, it has to be a press fit on the snout of the crankshaft. It is made up of three components. The inner hub and the outer inertia ring are separated by an elastomeric material that allows the inertia ring to float in relationship to the hub and cancel harmonics. The key here is the press fit and that's where the problems begin to arise with the offshore dampers on the market. The bore of the inner hub is not within specs for pressing onto the crank snout. It is either too loose, thus nullifying the validity of the damper in the first place, or it is too tight and the assembler cannot get it to press onto the crank snout.

I got tired of this Mickey Mouse arrangement and began to use only OEM dampers if I was building a street motor, or a well-known aftermarket manufacturer such as ATI if I was working on a drag motor where the motor required an SFI-certified damper at 10.99.
I like Damper Doctor for rebuilt OEM dampers. They disassemble the damper, re-clock the outer inertia ring to the inner hub so that you can properly ignition-time the motor, then press the two back together with new elastomeric material.
http://www.damperdoctor.com/Merchant...egory_Code=CHE
You'll see that there are many different dampers for a 350, covering all three ignition timing positions, Noon, 2:00 O'Clock and 2:30 O'Clock. On a street motor that is expected to have a long life, I tend to use a larger diameter damper. If I were building a dirt track motor that would get frequent maintenance and checking over, I might use a smaller diameter with a shorter polar moment so that the motor could rev a little easier.

OK, that's my best tutorial on dampers.
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Old 04-22-2011, 01:09 PM
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I really appreciate the info on this. I never knew that stuff, and now that I know how a damper works I will be able to make a more educated choice instead of an uneducated guess.
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Old 04-22-2011, 02:24 PM
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Quote:
Originally Posted by Bad66
Not sure what kind of harmonic balancer to get. I'm building an early chevy 350, 2pc rear main seal. It's an 010 block. Should i get an internal or external balanced, and should I get the 6 or the 8 inch? Not even sure what the differences are in these so if someone could clarify that with me that would be great. Thanks
Techinspector cover this really well, I'll just add the engine should be internally balanced as two piece rear seal 350's come from the factory. However, rebuilding can affect this if the replacement parts aren't within the factory's weight tolerance range. Most overhaul parts are in this zone, but hot rod parts are usually different and require a re-balance of the crank.

The 350 uses a neutral balanced damper, including the newer single piece rear seal engines which do require a bit of external balance at the rear of the shaft only.

For a street engine or a competition engine that sees mostly high RPMs and a steady throttle a large diameter heavy damper is most effective, to that end the 8 inch unit common to modern production is the way to go, or an 8 inch SFI approved unit if you're racing. The lighter weight smaller diameter dampers are best kept to engines that compete in acceleration contests or road course racing where the need to accelerate and de-accelerate the engine quickly can benefit from the reduced weight on the crankshaft, but the down side of this is less damping which reduces the life of the main bearings. For a competition engine that gets frequent maintenance bearing replacement every few races isn't a big issue, for a guy looking for 100,000 or more maintenance free highway miles it is a big deal.

Bogie
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