400 balancing question

I know that a standard SB 400 is externally balanced. My question is, on rebuild, is it possible to internally balance one? If so, what would I expect to see on the crankshaft. Weights welded on? Holes in the counterweights?

The reason that I ask is that my 400, which I purchased as a shortblock without details of the rebuild (unknown to the seller) shakes at idle like you wouldn't believe. I used the correct 400 damper and flexplate and now I'm contemplating changing these to try to see if that'll smooth it out before pulling the block. Changing the damper isn't a big deal, but I don't want to change the flexplate just for fun if this is a hopeless exercise.

73 Mike said:

I know that a standard SB 400 is externally balanced. My question is, on rebuild, is it possible to internally balance one? If so, what would I expect to see on the crankshaft. Weights welded on? Holes in the counterweights?

The reason that I ask is that my 400, which I purchased as a shortblock without details of the rebuild (unknown to the seller) shakes at idle like you wouldn't believe. I used the correct 400 damper and flexplate and now I'm contemplating changing these to try to see if that'll smooth it out before pulling the block. Changing the damper isn't a big deal, but I don't want to change the flexplate just for fun if this is a hopeless exercise.

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There would be an large amount of mallory metal added to the first and last counterweight.

Yes, mine is internally balanced. It's a 427 on a 400 block.

Doing a little internet searching, it looks like it may make a difference on 2 piece/1 piece rear seal too.
Let's see what Marck and Grumpy say. 400 is not really my thing.

big2bird said:

Doing a little internet searching, it looks like it may make a difference on 2 piece/1 piece rear seal too.
Let's see what Marck and Grumpy say. 400 is not really my thing.

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It is a 2 piece. I know this very well because there was no seal installed when I got the motor. I didn"t check and did the cam break in without the seal. It leaked some :banghead:

Tim helped me replace the seal without removing the crank. Not a drop leaked since. :wink:

Jsup said:

Yes, mine is internally balanced. It's a 427 on a 400 block.

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Funny that you mention this. I've noticed that the larger displacement engines built on the 400 blocks tend to be internally balanced. This is based on reading and not any knowledge of the process.

big2bird said:

Doing a little internet searching, it looks like it may make a difference on 2 piece/1 piece rear seal too.
Let's see what Marck and Grumpy say. 400 is not really my thing.

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Mine is a 2 PC in case it matters.

Going to need a new crank, rods, flywheel, and damper.

For the size of the rod throw on a 400 the counterweights are too small, big enough oens don't fit the crankcase so to make it internal balanced you need to have mallory metal added to it.

1 or 2 pc seal won't matter, apart from that the flywheel/flexplate is different because of the bolt pattern (87 up has 3" bolt circle, earlier is 3.58").
Also I don't think GM ever made an ext. balance flexplate for the 1pc main seal bolt circle, did they? (grumpy??) But since yours is a 2 pc it doesn't matter. If it's a GM flexplate.

Are you 100% sure you have a counterweighted flexplate and damper? You can buy bolt on weights for the flexplate. Do not mistake a weight on the flexplate for a counterweight, it could be just a small coutnerweight to balance the flexplate itself.

If you pull the pan and there is no sign of metal smugs welded to the counterweights then it's not internally balanced.

Here's a pic of some mallory metal welded to the rear crank counterweight: ( know, it's a ford FE... shame on me )



It usually it looks this sloppy. It can also be done very clean but thats something you usually see on high $ circle track cranks and nascar stuff.

73 Mike said:

I know that a standard SB 400 is externally balanced. My question is, on rebuild, is it possible to internally balance one? If so, what would I expect to see on the crankshaft. Weights welded on? Holes in the counterweights?

with a neutral balancer and a neutral flywheel yes it most likely can be balanced by drilling holes in the counterweights, depends on piston and rod weight of course.

The reason that I ask is that my 400, which I purchased as a shortblock without details of the rebuild (unknown to the seller) shakes at idle like you wouldn't believe. I used the correct 400 damper and flexplate and now I'm contemplating changing these to try to see if that'll smooth it out before pulling the block. Changing the damper isn't a big deal, but I don't want to change the flexplate just for fun if this is a hopeless exercise.

If it is externally balanced you must use the damper/balancer that was installed when the assembly was balanced.... you can't just slap another damper on there and expect it to run smooth... same for the flywheel/flexplate

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pull the pan and see if there are holes drilled into the counterweights or if holes were drilled and heavy metal plugs were installed to increase counterweight weight. If there are no weights added or holes drilled it's ext. balanced and you cannot change flywheel/flexplate or balancer without disturbing the balance.

could the motor have been rebuilt and use an eagle internally ballanced unit?

Twin_Turbo said:

It usually it looks this sloppy. It can also be done very clean but thats something you usually see on high $ circle track cranks and nascar stuff.

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Man that is ugly.:shocking: I have to say that after viewing this photo it shows that there was absolutely no attention paid to the cosmetics of the job and that makes me wonder about the quality of the balance job in total. It may be perfect on balance but I would have to take it to a different shop to have it checked to assure myself.

I have seen a ton of internally balanced engines and have never seen one that looks like that. The Mallory metal should be added by drilling the counterweights from front to rear (or rear to front) and the surfaces of the counterweights machined smooth after the metal is added.

Rich:beer:

Thanks for the input guys. Now I know what I'm looking for. I'll pull the pan and have a look on SUnday, but I suspect that I would have remembered something odd looking like the pic in TTs post.

Having said this, I added a standard couterweight flexplate and balancer. The weights on both are large enough that they are definately not just added to balance the parts. These parts were definately not used to balance the rotating assembly in the first place, if it was indeed done at all on rebuild. The pistons were stock replacements and I assumed the weight would be similar to stock. Perhaps not.

Take some pictures Mike. It very well could be the Eagle crank was installed. They are available internally or externally balanced as stated.

big2bird said:

Take some pictures Mike. It very well could be the Eagle crank was installed. They are available internally or externally balanced as stated.

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Scat has both internal and external balanced cranks also.:drink:

SORRY I MISSED THIS SOME HOW UNTILL NOW!

Ive found that buying complete ballanced rotating assemblies from a single source is usually the best route, you can get either internal or externally ballanced assemblies, it helps a great deal to use light weight tool steel piston pins, pistons and (h) rods to lower the rotating/reciprocating weight, if you want to internally ballance the assembly.

PHONE: 310-370-5501

http://www.scatcrankshafts.com/

http://www.adperformance.com/index.php?main_page=product_info&cPath=71_231&products_id=669

call and ask if they have a complete ballanced assembly available
if your not sure of how an engine was assembled or the components or clearances that were used your choices are, run it as it arrived and pray.....or take the time to disassemble and verify everything in the assembly is as it should be.
most 400 sbc engine use a externally ballanced assembly, simply because that cheaper to produce and it costs less, but you can certainly build an internally ballanced 383-400-406 sbc from easily available components, but its usually going to cost more to do it that way.

My 400 is internally balanced too. The engine builder suggested doing this so that "normal" external equipment like the damper and flywheel could be used.