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Old 10-26-2010, 05:18 PM
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SBC 400 re-build balance opinions

I'm in the process of building a "509 casting" sbc 400. I'm a little confused regarding the balance procedure of these motors. I was planning on using the stock balancer and flex plate, although if that's not a good idea, I would upgrade to aftermarket units... I am going to use the stock crank, which is in great shape. My plan was to use Probe 6'' rods and Probe flat top forged pistons. My machine shop cautioned me about these rods / pistons...he said that it would not work with the external balance. Is it best to go ahead with the 6'' rods / pistons with a new balancer and flex plate, and get a new internal balance? Do new rods / pistons negate the external balance of a sbc 400? Thanks in advance.

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Old 10-26-2010, 05:45 PM
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I don't understand why they would tell you that, unless the rods/pistons require a considerably heavier than stock bob-weight. If the rods were SHORTER than stock, they could cause an interference problem with the counterweights. But they are longer, and the pistons (if designed for 6 in rods) are no doubt shorter. Unless these require a heavier bob-weight, I don't get it. Maybe one of our machine shop guys can provide a better response. If it were me, and the required bob-weight was no heavier, I'd not fear using them.

Pat
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Old 10-26-2010, 06:20 PM
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The 5.7" rods are much more easier to balance with the stock crank.

All the 6" rods do is make your wallet lighter.
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Old 10-26-2010, 08:55 PM
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Send your damper to these guys for a rebuild or as a core for one that is already rebuilt (part no. CHE400).
http://www.damperdoctor.com/Merchant...egory_Code=CHE
They'll install new elastomeric material and clock the hub to the inertia ring so that you will be able to ignition time the motor properly.

When you have the motor assembled to a short block on the engine stand, find top dead center and make sure the inertia ring TDC mark lines up with zero on the timing tab at the front cover. Here's some help on the procedure....
http://www.iskycams.com/camshaft.php
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Old 10-26-2010, 09:00 PM
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PatM is correct. Should be no problem to balance unless the new parts are heavier than the old, and if these are 6" H-beam rods you are considering they may well be significantly heavier than stock. H-beams usually are. I-beams are often very close to stock weights.

Do you know the rod and piston weights of the parts you are considering??

If you plan to drag race it faster than 10.99 1/4 mile/6.99 1/8 mile ET an SFI approved balancer and flexplate will be neccessary. If you plan to tune much over 6500 rpm it would be a wise decision even if you don't race it. Old worn out rubber in stock parts can get iffy, if you are going to resue the stock balancer you should get it rebuilt. If you just want a good new one but don't require the SFI spec Powerbond and Pioneer both are good low cost products. Stay away from Professional Products, ProComp, and Fluidampr.

Last edited by ericnova72; 10-26-2010 at 09:07 PM.
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Old 10-27-2010, 07:58 AM
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F-Bird

Quote:
Originally Posted by F-BIRD'88
The 5.7" rods are much more easier to balance with the stock crank.

All the 6" rods do is make your wallet lighter.
Could you explain the the reason that the 5.7 rods are easier to balance? I'm not an automotive machinist and don't understand all the intricacies. Please enlighten me.

Regarding the 6.0 making your wallet lighter, that is certainly true in some cases. In other cases, aftermarket 6.0 rods are equal or only a little more expensive than 5.7 in rods of the same product line. But I think the primary advantage is if you want to internally balance a 383 (and probably a 400 as well) you either need mallory metal or larger counterweights, which the six inch rods and shorter pistons will accomodate. Whether any of this is noticably "good" or not I can't comment, but the 6 inch rods do help facilitate an internally balanced 383.

But I really am curious as to why the 5.7 inch rods are easier to accomodate for balancing. Please help me to understand.

Pat
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Old 10-27-2010, 09:46 AM
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I have an internally balanced 383 with 5.7 rods. It balanced just like a 350. No mallory or counter weights needed.
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Old 10-27-2010, 01:36 PM
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No kidding? I have a Scat, internal balancing 383 crank, and they specified use of 6 inch rods, to get the necessary skirt to counterweight clearance. May I ask whose crank you are using? And, do you happen to know the balancing bob-weight that was used. Did you, maybe, trim the piston skirts?

BTW -- The VW is really cool. I get a real kick out of reading your posts.

Pat
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Old 10-27-2010, 01:55 PM
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Quote:
Originally Posted by PatM
No kidding? I have a Scat, internal balancing 383 crank, and they specified use of 6 inch rods, to get the necessary skirt to counterweight clearance. May I ask whose crank you are using? And, do you happen to know the balancing bob-weight that was used. Did you, maybe, trim the piston skirts?

BTW -- The VW is really cool. I get a real kick out of reading your posts.

Pat
Thanks, Pat.

I used an Eagle internally balance crankshaft, which I had the machine shop inspect and it was good to go out of the box. I used stock GM 5.7 rods and had 383 ARP rod bolts installed.

I'm guessing the bob weight was 639-640, going by my picture.




I do recall the skirt being ground slightly, but I don't think it was because of clearancing. It was to balance the weight of the pistons. I clearanced the rods myself.

Here you can see, somewhat, the drilled / balanced throws.




I hope this helps.


Matt
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Old 10-27-2010, 02:01 PM
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Cool Matt.

I expect that the "640" is the combined piston and pin weight for each piston, after what the shop had to do (or not do) to them. Even 1670 would be a very light bob weight. Thanks for the input.

Pat
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Old 10-27-2010, 02:09 PM
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That sounds about right, Pat.
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Old 10-27-2010, 02:10 PM
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I used an Eagle internal balance crank in my 406 5.7 Eagle rods good to go out of the box didn't need any mallory to balance it.
Shane
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Old 10-27-2010, 02:18 PM
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Apparently you, V8 Super Beetle, and I'm sure at least a hundred other forum members found a solution that I didn't find when I was shopping for my rotating components. But, I am curious, do you happen to recall what your bob-weight was? Did the Eagle crank have an anticipated bob-weight that they recommended you not exceed? Do you happen to know the weight of the Eagle crank? It's all simple physics (well, not so simple to me, really) but I'd like to know where the variables are that came into play.

Pat
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Old 10-27-2010, 02:30 PM
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On their site, Eagle, it stats a target bobweight of 1825 +/- 2% and typical total weight is 53 lbs.

http://www.eaglerod.com/index.php?op...d=21&Itemid=32


That's all I could come up with.
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Old 10-28-2010, 09:05 PM
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I don't know if you're using a cast or forged crank. For my 406 with 5.7 rods, the cast cranks were all externally balanced. For internally balancing everything I saw stated either 6.0 rods or a forged crank.
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