Ions91Z28

tried doing a search, but didn't have much luck.

When you internally balance a 400sbc, what all is needed? Machine work, parts, and so fourth. Just need the basic run down, and what parts are needed.

Thanks.

ede

a boat load of mallory metal and nutral balance dampner and fly wheel

Vader

Or if your wallet is really phat you can buy an internally balanced rotating assembly.

Ions91Z28

That's bascially what I'm asking.

Do they make a crank for 400s that use the 350 balancer and fly wheel and such? Or how do they do it?

RB83L69

I keep having to post this, maybe I should write a book....

The deck height of a SBC is 9.025" nominal, from crank centerline to deck surface. Unlike some other mfrs (Brand "F" and Brand "C" for example), GM decided to keep the deck height of all these motors the same, so that all their brackets and accessories would be the same, and their intakes will interchange, and the push rods are the same, etc. etc. .... all of these differences are costs that drive the price of the finished product up without giving the end purchaser any increased value. Therefore, the sum of: ½ the stroke (rod journal offset from crank centerline); plus rod length, center-to-center; plus "compression height" of the piston (center of pin bore to top of piston surface; plus "deck cleareance" (how far down in the bore the piston is at TDC), must equal 9.025", within a few thousandths. So, let's look at some of the parts.

The rings need to go between the top of the pin bore and the top of the piston. That means that if there is going to be some material left between the bottom of the oil ring and the top of the pin bore, and some material between rings, and some material above the top ring, and the rings are going to be 5/64" - 5/64" - 3/16", then the pin bore can only be moved up just so far in the piston. There also has to be some minimum amount of material left below the pin, to make the pin bore. So, there is a minimum height from the top of the piston surface, to the bottom of the bosses that the pin bore is drilled through.

The stroke of a 350 is 3.48". ½ of that is 1.74". So, 1.74" + 5.7" + .025" (deck clearance), gives you 7.465". That leaves 1.56", which coincidentally is the pin height spec for a stock 350 piston. That also happens to be the minimum space that you can fit that ring package as described above into, which probably figured heavily into the decision to make the 350: it was as far as they felt they could go, given the restrictions they were working with.

The stroke of a 400 is 3.75". Hmmmmm..... ½ of 3.75" is 1.875". So, 1.875" + 1.56" + .025" = 3.46"; subtract that from the deck height of 9.025", you get 5.565". In other words, given the other dimensions of the 400 engine, there's only 5.565" left for a rod. By coincidence, that just happens to be the length of a stock 400 rod.

Now let's look at how a crank is made. It has rod journals, which are offset from the center; each of those has 2 rods on it. Rods have mass.

Consider how you balance something. Let's take the simplest possible multi-cylinder engine; let's assume that it is has 2 cylinders, each with its own rod journal, one at each end of the crank. If the rod journals and the big ends of the rods are of equal weight to each other, then this motor would be statically balanced; i.e. if you could suspend the crank in gravity with no friction, it would not tend to turn. But... start it spinning, and one end of it will be trying to fly off one way, and the other end will be trying to fly off the other way. It would vibrate horribly even though it appears to be "balanced".

The solution is of course to "dynamically" balance it. This is done by putting a counterweight as close as possible to each rod journal, of exactly equal mass and with its center of mass at exactly the same radius as the rod journals' center of mass. If you really wanted this thing to run smooth, you could even split these counterweights in half, and put half of each one on either side of each rod. Now, it will still "statically" balance, but it is also "dynamically" balanced, meaning that each individual incremental unit length of the crank is balanced with itself, as closely as practical, so none of it has a tendnecy to vibrate at all. Unfortunately the crank now weighs more, but at least it runs smooth.

In a motor with more cylinders, there are more counterweights. But the principle remains the same.

Now look at the relationship of the pistons to the counterweights. The counterweights are exactly "opposite" the pistons on the crank, so when the piston is all the way up in the bore, the counterweight is all the way down. And likewise, when the piston is all the way down, the counterweight is "up".... right immediately underneath the piston.

Remember the rod length we came up with to make the 400? Well it's too short to allow the necessary counterweights to fit under the piston. That is, given the space limitations inside the block of how much room is available for its width, if the counterweight is made large enough to completely balance the rod journals, its outer diameter will be so large, that the bottom of the piston will hit it. So a stock 400 crank's counterweights can't be made large enough to balance the motor. The solution is to cram as much counterweight inside the block as will fit, and then hang any extra required that won't fit, on the ends of the shaft. This means non-neutral-balanced harmonic vibration damper and flywheel.

The factory even lightened up the rod journals by drilling a big hole through them. That reduced the amount of external "unbalance" required to a reasonable amount.

So, the conclusion we came to is, it's impossible to internally balance a 400 without doing one or more of several things, any of which was unacceptable to the factory in terms of longevity and warranty cost. Either: drill the rod journals really thin; or lighten the big end of the rods; or remove iron from the counterweights, and substitute plugs of some heavier material; or move the pin bore up in the piston to accomodate a longer rod, which might invlove allowing the oil ring to partially overlap the pin bore, or using narrower rings spaced closer together. All of these possibilities have problems; either cost, or durability, or both.

Most of us who build large small blocks are willing to accept the compromise of narrower rings spaced closer together with the oil ring invading the pin bore. The factory wasn't, for good reason, but when we build a motor like this, we aren't going to put it in a fleet of thousands of trucks heading to construction sites, that will get their oil changed maybe once in its lifetime if they're lucky; or a fleet of phone company vans numbering in the tens to hundreds of thousands, belonging to somebody that will cut their phone service off if they have too mch trouble with GM's product; etc. etc. etc. We build for people who are willing to make all sorts of sacrifices in order to go faster. So we use longer rods, which provide space for larger counterweights, which allow fully internal balance, at the expense of rings that are somewhat more fragile over the long term.

Make sense?

ede

i wish rb was here he says this so much better than i do, but basically to internally balance an engine the counter weights on the crank have to equall the mass of the rods, pistons, and pins. on the short rod (5.56) 400 SBC the counter weights would crash into the pistons at BDC so the counter weights are made smaller and the extra mass is added to the flywheel and dampner. i'd suspect a internally balanced crank for a 400 would have counter weights full of tungston or mallory metal.

85yellowiroc-z

Hmmmm.. are you trying to disguise(?)a 400 as a 350?Been there,done that!

Ions91Z28

RB83L69, good information, I was bascially looking for the end, but good stuff none the less.

So if I do internally balance this, I'm going to need to run longer rods, and possibly use expensive metals to be able to balance the crank? When running longer rods, I run into clearance problems as well though right?

Maybe I should just stick with the externally balanced version.

ohyeahjay

how could one look at a 350 and 400 and tell the difference (unless the motor was tore apart) If it was in the car and running

85yellowiroc-z

By the size of the balancer,400 has the largest of all small block chevys.(largest outer diameter)

ohyeahjay

very good but unless you are racing in stock class how many other people would know that and notice it?

RB83L69

The 400 balancer is the same outer diameter as alot of other SBC ones.

Most people who race, especially circle track people, know stuff like that. They're not a bunch of idiots like you'd find in the local McDonalds parking lot with a bunch of Hondas or something.

The way to tell, is that the 400 one has a chunk missing out of it...

DTL504

iTrader: (1)

I'm not sure if I over read something here, but if you have a SBC 400 Internally Balance Crank, you can use a SBC 350 Flexplate and Flywheel. Then the only way they will be able to tell if they really got into your engine bay and look at the freeze plugs. Most 400 blocks have 3 on each side or the (397817) block have 2 but the boss is still visible where a third freeze plug could have been install. Im in the process of building a 400 and I will be using a internally balance crank w/ 6in rods. I probably will have to get a small base circle cam. However if I decide to get them 4340 H Beam Rods they have clearance up to 3.800 and will not require a small base cam.
Hope this help: David

RB83L69

You are correct, with a neutral-balanced motor, you would have to use a neutral-balanced harmonic damper.

The single most common 400 block is the 509 casting. It ran from 73 up. The only ones that have the 3-freeze-plug thing are the 711-72 ones, which also mostly have 4-bolt mains. Here's a 509 casting. Talk about stealth to the eye...