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Old 01-16-2004, 12:30 AM
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Internal vs. External Balance

I'm building an 8-71 383 SBC with the supplied pulley's for a boost of probably 7-9 lbs, and have already purchased the pistons and rods, JE forgings, -33 cc, for about 8.5:1 and 4340 H beam rods. I'm ready to buy the crank but am somewhat confused by the options for balancing, i.e. external or internal. I've seen cranks advertised as 'internal and external balance' for this buildup.

Isn't it my choice which balance procedure I want to use? I've read some in the KB about it and find internal is more expensive but preferred.

Do I need to specify which balance procedure to the crank vendors ahead of time? And with an after-market crank such as the Eagle 4340 piece do I use the 400 or 350 flex plate and dampener, or does that depend on the balance technique?

As ever, you guys are an asset to us all! Larry
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Old 01-16-2004, 12:55 AM
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In most cases internal balance is perferred, but I`d also say it`s up to the person doing it. you can go with either and it`ll work either way. however, 400`s from the factory were externally balanced, the 400`s balancer has a counterweight removed, the flexplate/flywheel had weights added to pacific spots to achieve external balance, if a 350 balancer and flexplate is used on a externally balanced crank, it`ll destroy the thrust surface on the crank and eat the bearings. if vice versa is done, the same thing results. the 400 small block was the only small block externally balanced. as was the 454 in the big block family. it does depend on the balance, if your crank is externally balanced, use external pieces, if it`s internally, use internal.
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Old 01-16-2004, 03:34 AM
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not to thread jack, but if my 388 stroker is externally ballanced, and I have the 400 ballancer, which flywheel/flexplate do I use?!

Thanks-

K
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Old 01-16-2004, 04:20 AM
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You'll need one for the 400
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Old 01-16-2004, 04:23 AM
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the 168 tooth 14 inch externally balanced 400 flywheel/flexplate.
it`s easy reconizeable from the weights added to it right under the ring gear. they also make a bolt on counter balance, that goes between the crank flange and flywheel/flexplate, then you can use a standard 350 flywheel/flexplate.
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Old 01-16-2004, 05:05 AM
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Hmm a quick question tho I feel dumb for asking, I emailed scat to get an answer but they never replied so I will ask here. Anyway I bought a Scat 383 Stroker Crank(9000 Series) and I assume its externally balanced? Their site lists what the other cranks in the series are, but it does not list what that crank is.
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Old 01-16-2004, 07:12 AM
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Due to the load on the snout of the crank with a blower motor you are much better off to internal balance. $200 of heavy metal is cheaper then a busted crank.

Chris
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Old 01-16-2004, 09:43 AM
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Cstraub, Hit the nail on the head. With that blower you are going to want an internal balanced crank for sure. Chevy snouts are pretty small and can be a problem with a blower if you have too much weight hanging out there. I am building a very similar setup (388ci blown) and I have mine internally balanced and I will be using a neutral balance crank hub (no dampner), the big 3" rubber blower belt will absorb the harmonics.

If you do use a balance find the lightest one you can (it will have to be steel though).

This is the reason you want to run a short water pump with a 4, 6, 8-71 setup, it reduces the stress on the crank by not having the pulley so far out.

Several blower guys have told me that using a crank hub will be bullet proof, and that with a balancer you will bend or break the crank sooner or later.

Some machinist/engine builders might tell you otherwise, but Don Hampton among others have assured me a crank hub was the way to go with a blower.

One more point if you run a balancer you will want to have a 1/4" keyway 180* from the current woodruf keys.

For internally balanced 383's (385,388) you need dampner and flexplate or flywheel from a 350 (or one that is designed for a 350).

For externally balanced you need the dampner and flexplate or flywheel for a 400.

I currently have one engine in each configuration, performance wise there is no difference. For a naturally aspirated car I don't think it really matters (unless you want to hide the fact you have a stroker). Cost wise the difference is small if any, they are so popular now they cost about the same (from my experience). If you already have the flywheel/flexplate and balancer for one or the other I would just go with what I have. If you are starting from scratch I would go with an internally balanced engine (with a blower, internally balance no matter what (IMO)).

I hope this helps some (long winded reply, sorry).

Royce
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Old 01-16-2004, 09:57 AM
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The hubs are very popular, we manufacture hundreds of them a year. Internal balance of course.

Cstraub
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Old 01-16-2004, 11:47 AM
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Wow, good stuff here! I'm not familiar with the 'Crank Hub", does it take the place of the front dampener on an internal balance, is it a dampener in it's own rite or what? Also, any direction to where I can get some info on hubs and their function? I've searched the KB and don't find anything specific. I have the gist, just need to get it all straight in my head!

Thanks, Larry
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Old 01-16-2004, 03:40 PM
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My hub is coming from Hampton Blowers, I am sure you can get one from many different blower companies (BDS, Mooneyham, Littlefield, Kuhl, etc...).

It is not a balancer/dampner, it is neutral balanced and takes the place of the balancer/dampner. It's main purpose it to give you a place to bolt your lower pulleys (blower and v-belt). It is much lighter than a dampner. The hub itself does not help with harmonics, the blower belt will do that. In other words I don't think I would run a crank hub in a non blown situation, but with a blower they work nice.

If you need phone numbers let me know.

Royce

Cstraub, might be able to tell you where to get one as well. He would know what you need.
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Old 01-16-2004, 04:56 PM
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Reading all this about internally vs externally balanced makes me wonder what I have. How can one tell the difference especially if the H/B and flywheel are not on the engine? Let say the SBC 350. I was under the impression that the later one piece rear seal were ext. and the two piece rear seal were int. balanced. Can someone elaborate. I don't know if my question should have it's own post. If so I'm sorry. Thx.
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Old 01-16-2004, 07:10 PM
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You're kind of treading a fine line. All stock 350's are internally balanced...all stock 400's are external. When you ordered the crank aftermarket, they sould have asked which you preferred. However, I'd assume that it's internally balancable. You should know though that it isn't actually balanced unless you have the whole rotating and reciprocating assembly balanced together. In addition to that, an internally balanced motor would perform better because it gives you the option of using a harmonic dampener instead of a balancer. This will take up harmonic vibration that can kill a motor quick. As for the flexplate or flywheel, you'll need the one for the block you ordered the crank for. Good luck.
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Old 01-16-2004, 08:55 PM
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I have a 600 ci. bbc with a crank hub that is external ballanced.
It is several yrs old and never a problem. Crank hubs can not be used without a blower belt.

I forgot, most crank hubs do not have timing marks so when you
assemble the engine file mark and install a pointer. You certainly
don't want to guess at the timing on a blower motor. To much
timing or a lean condition are a quick death for blown engines.

Last edited by BLOWN 1100; 01-16-2004 at 08:55 PM.
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Old 01-16-2004, 09:00 PM
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Sounds like the hub is the way to go, am I correct in assuming the hub and flex plate need to be included with the rotating assembly for balancing? Also, in researching 'hubs' I see mention of an additional 1/4" key-way requirement for something or ruther...will I need to have another key-way machined into the snout to use a hub? If so, what's that all about?

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