4mul8r

I have done a search, but can't find iformation on this specific example.

I have a 406 short block block that has been externally balanced. The guy I bought the block off of gave me the balancer and flywheel. I have a 700r4 that I want to bolt onto the engine. Will I have to get the flexplate balanced to the rotating assembly?

Here's some more specifics. The 400 flexplate I got came off a friend's car that had a 700r4 behind a 400. Here's the pic. I measured the diameter and it was like 14" You can see some kind of weight placed on it.


Here's my banacer (note the holes drilled in it for balancing?)


Here's the flywheel (same thing with the holes)


Am I screwed???

84 Z-28 350

it will work, but I would reccomend having it ballenced to the engine anyways....I don't think a 168 tooth (14") flexplate will work our cars...I know for a fact that it woun't fit under a stock T-5 bellhousing, I'd think you'd need a 153 tooth (11.5") for it to work properly, mabe one of the auto guys can clarify this, I just thought I'd bring it up.

4mul8r

To have it balanced to the engine, the shop wants me to pull out the crank, pistons, rods, etc... and bring that all in and charge me $260.

That seems unpractical in many aspects to me. Because what if I want to get an sfi flexplate down the road. I have to pull the engine and tear it apart???

But I'm afraid that this whole setup I have was balanced to specific parts.

HalfInchWrench

Your balance depends on your whole rotating assembly. You won't be able to put those parts on unless they are all in balance unless you want to rebuild your engine again real soon. Get it done, you have to.

sellmanb

lol @ externally balancing.


Just take out your rotating assembly, get a new balancer and flexplate, and have them INTERNALLY balance it for yah. Yes it's a PITA, NO there's no way around it.

Sorry man, it's just one of those things we all have to do when we get a new engine.

HalfInchWrench

He is talking about a 400. They have specific requirements that don't apply to your run of the mill 350.

sellmanb

damn, missed the 400 part. My bad. Ignore my post then lol.

contactpatch

4',
have you thought about...
having a shop 'copy' the amount of imbalance
in the flywheel, over to a flexplate?

ede

don't balance a flex plate/flywheel or dampner to an engine. if you'd have to replace one you'd need to rebalance everything. they should be nutral balanced

RB83L69

Think for a second about how the factory builds a motor.

Rods come out of the production process, and are measured in various important properties. Big end weight, small end weight, total weight, length, whatever. They are placed in an n-dimensional array of bins, that each accumulates rods of a certain total weight range, a certain big end weight range, etc. Presently a bin fills up to the point that it has 8 rods. At that time, they take those 8 rods, and get a crank off the crank shelf; since they know the properties of the rods (and of course the pistons they're hanging on them) they know exactly what balance the crank needs, without having to "measure" the rods again. They balance the crank using the bob weight of the parts that they know they're going to assemble. All that stuff then goes into a block. And the most amazing things about this are: (1) nobody ever touched a rod with a grinder; (2) the rods and the crank never met until they were assembled; and (3) most importantly, by doing it this way, every single motor that comes out of the process is balanced exactly the same.

So, now we have a motor that's completely balanced, and more importanly, it's balanced to a specific spec: every motor that comes off that production line, is balanced exactly the same. Furthermore, they know what that balance is, because they designed it. They have it written down somewhere, and they (or we) can go back and look at it if we want to. That's very important.

Next, the motor continues on down the production line, and they grab a random balancer out of a bin of those which have been previously balanced to the same spec as the motor, and they grab a random flywheel out of a bin of those which have been previously balanced to the same spec as the motor, and they assemble them to the motor. No further balancing is required.

They DO NOT go through the same sort of monkey-motion that a machine shop (as opposed to a balancing shop) always seems to want to do, with the flywheel and damper; where they "match" all the parts together. That's just plain stupid.

The correct way to balance a motor, is to set it up to the factory spec, whatever that is (be it neutral, or the 400 spec, or the 1-piece RMS spec, or whatever); then balance the flywheel to the same factory spec, and the damper to the same factory spec. That way, they still interchange with the rest of the world. You don't end up with the exact situation you are in now.

You can't internally balance a 400 if it has short rods, without the use of Mallory metal. Here's how that works:

Imagine a 1-cyl crankshaft with a rod and piston hanging off of a journal, and no counterweight. Obviously, it will be unbalanced, and will vibe like hell. So what do you do?

You add a counterweight, right? OK, so in the simplest possible configuration, we have a crank with main bearings at the ends, a rod journal next to one main bearing (for the purpose of the thought experiment, let's say it's at the left end), and a counterweight near the other main (the right end) that exactly counterbalances the rod & piston. If all this is sitting still, and we hang it horizontally in space in perfectly frictionless bearings and cylinder walls so that we can observe its balance, we can adjust the counterweight until it doesn't tend to turn any more. This is the condition known as "static balance". Seems like it would run smooth now, right?

Wrong. It will still vibe like hell; maybe worse. The reason is, once it's spinning (i.e. no longer in the "static", or standing still, condition) the end with the piston is going to be trying to fly off one way, in the direction the rod & piston is pulling it; and the end with the counterweight is going to be trying to fly off the other way. The 2 "centrifugal" forces are NOT directly opposed to each other, because they're acting on 2 different points on the shaft; so the shaft is still going to try to wobble.

So what do we do now? We can't possibly put the counterweight directly opposite the rod & piston, can we? Clearly that's not an option.

Should be pretty obvious. We take the counterweight, split it in half, and put half of it on each side of the piston.

Now, the moment of inertia of the 2 smaller counterweights acts on the crank at a point in between them; and if that point is arrange to be the exact same point that the moment of inertia of the rod & piston acts, then they will exactly cancel each other, and the shaft will run smooth. Now, it's not only "statically" balanced, it's also "dynamically" balanced.

Make sense so far?

OK, so now we've mastered the art of building a 1-cyl engine. Let's look at a 2-cyl, with 2 rod throws.

In its simplest possible setup, it will have exactly the same problem as the 1-cyl did, when we put one large counterweight next to one end and the R&P next to the other end. So what to do?

Again, it's pretty simple: we put counterweights next to cyl #1 to counterbalance that journal, and counterweights next to cyl #2 to counterbalance that one. But wait: they're exactly opposed, so there will be 2 counterweights at the center of the crank that are exactly equal and opposite to each other! Do we need those? No. Or at least, not if we manage the other counterweights properly. We'll need a counterweight at the #1 end that exactly half-balances the #2 R&P, and a counterweight at the #2 end that exactly half-balances the #1 R&P. And once again, if we arrange the locations of those such that the point at which the left CW and the right rod balance together is the same point at which the right CW and the left rod balance together, it runs smooth; with the least possible amount of metal, and the lightest possible shaft for lowest rotational inertia (in a car, translates to better "throttle response"). But, if we can't locate the weights at that perfect spot (for instance, if the shaft has to be as short as possible, and there's not enough room left at the ends) then we might still have to put at least some small weights at the middle, to keep it dynamically balanced.

Continue the thought experiment by adding more cylinders. In particular, consider the case of a V8 motor, where there are 4 rod throws, 90° apart.

Remember about the 2 center CWs in the 2-cyl motor. We were able to minimize them, because the 2 rod throws mostly balanced each other. Well a V8 does the same thing, if the 2 center rod throws are 180° apart. So that leaves the 2 end rod throws as the ones that mostly need CWs, since they can't possibly have a cyl next to them that's 180° off. (Incidentally, the rod throw arrangement is ingredient #1 in creating a firing order... since it determines in what order the pistons successively reach TDC.)

So let's look at the end counterweights. They have to be kind of big, compared to the center ones, remember. That's going to lead to the next problem we'll have to solve.

Remember that the counterweight has to be exactly opposite the rod journal it's counterbalancing. Duh. The means that when the piston is at TDC, the CW is at BDC so to speak, right? Well it also means that when the piston is at BDC, the CW is at TDC. That means that when the piston is at BDC, there has to be enough space between the bottom of the piston and the center of the crank, for the CW to fit in there. Otherwise, the bottom of the piston will crash into the CW.... and if that happens, it won't matter much whether it's balanced or not. It won't run smooth since it can't run at all.

For a given "deck height" of the block (distance from crank centerline to top of piston), 4 numbers must add up. The "deck clearance" (how far the piston lacks making it all the way to the deck), plus the piston "compression height" (the distance from the pin centerline to the top of the poston), plus the rod length (from piston pin center to rod journal center), plus half the stroke, must all equal the block's deck height. Which means, if we take blocks of the same deck height and put cranks with different strokes in them, some one or more of those other numbers has to change.

In the Chevy small block, the factory started out with a 3" stroke. Later on, when they introduced the 327, they used a 3¼" stroke, with the same rod length, which meant they reduced the "compression height" of the piston.Still later, they went to the drawing board, and figured out that the lowest that they dared to make the compression height was 1.56", because that was the bare minimum required to fit a ring package that they considered adequate. If you plug that number, plus the same rod length as the 265, 283 & 327 into the formual above, you get the longest possible stroke as 3.48". That's why the stroke is that odd number that gives a 350, instead of a nice round 3.500" x 4.000", which would be a 351.

A few more years went by, and they decided to invent the 400. The same restriction existed for the rings: can't raise the pin any higher in the piston, because the rings are in the way. So instead, when they lengthened the stroke some more, they shortened the rod, to make it all still fit in the block.

Remember about the CW having to fit under the bottom of the piston when the piston is at BDC? Well, with the shorter 400 rods, there is no longer enough room to fit enough counterweight in the available volume in the crankcase. Some of the counterweight had to be located in less-than-ideal places, out beyond the "perfect" spot where it exactly dynamically balances the 2 end rod throws. As long as you run the short rods, there's no way around this, unless you drill holes in the CWs and fill them with some kind of much heavier material. We use "Mallory metal", which is an alloy that's very heavy, but has a simialr heat expansion rate to the crank material, so it'll stay put. That stuff is EXPENSIVE.

So now that you know how and why a 400 is balanced like it is, it should be obviouos what the right way to get it balanced is.

1. If it has the short 400 rods, it has to be "externally" balanced, because it's impossible to "internally" balance it without the use of Mallory metal.

2. There is a factory 400 balance spec. The motor should be balanced to that spec, by itself, independent of the balancer and flywheel. I REFUSE to supply my balancer or flywheel to a balance shop at the time the rotating assembly is being balanced, for that reason. If they won't balance it without them, I pick my parts up and take my money elsewehere.

3. The flywheel and damper should be individually balanced to their factory specs.

If you do those 3 things, you come out with a motor that is correctly balanced, yet you can still replace the external parts, without having to do what you're about to have to do because somebody somewhere along the line took the "easy" way out. Seemed easy at the time, but now, you're paying the price. In fact, I've seen more than one 400 for sale REAL CHEAP, too cheap, impossibly cheap, precisely because they were in the situation you're in now.

Looks to me like you have a factory 400 flex plate, a factory damper, and a factory 350 flywheel that's been "unbalanced" to the 400 spec. That flywheel is the 14" one (168 tooth) and WILL NOT work in one of these cars with the stock bell housing. You MUST use a 12.8" one (153 tooth). The easiest way to get one is to use the right one for a 83-85 305/T-5 Camaro/Firebird, and have it "unbalanced" like the one in your pic. Problem with that is, your block may not have the roight starter hole; and it is NOT POSSIBLE to make a starter, ANY STARTER, that will fit to a small flywheel without the late-model hole; because the early-model hole would put the inner bolt RIGHT THROUGH THE MIDDLE of the starter drive. You can look at your block and tell. This pic shows a 400 block that I drilled the hole in. The one that's circled is the one I drilled. The location of the 2 cyan lines is where some early blocks have a hole. That hole will not work with the T-5 starter; nor will the one at the left end of the cyan line. You MUST have the circled one. Get it installed BEFORE you drop the motor in. Do not believe anybody that tells you that there's some starter you can get that will work, even if you want very badly to hear that; it's not true.

Now my fingers are tired.

4mul8r

contactpatch,
That was suggested to me by the person who game me the flex plate. The shop that I called told me that is would be very difficult to do that and that it would cost "more than I can afford" (as if they know how much I can afford). But it's probably true, because they want $260 for the balance. But, if they do the assembly and reassembly of the short block, they want between $500-$600. So, it probably would be more than I can afford or is practical for that matter. If you know a shop that can do this for a reasonable price, I am all ears.

ede,
It looks like the person I bought the block off did balance the block to specific parts. Which doesn't make sense and is pissing me off now.

RB83L69,
Was some good information. Thanks. I am a little confused with the last paragraph though, where you start talking about the flywheel. I would like to use the felxplate, as I will be running a 700r4 with this engine. Supposedly this flexplate came from a 400/700r4 combo. And the starter thing I am interested in too, but don't know if what you are talking about is dealing specifically with a t-5 or auto. If I just get a flexplate matched (or unbalanced) to the factory 400 spec, will it work, or do you think that my specific engine is balanced in some other kind of dumb configuration (hence the holes in everything)?

At this point, what are my options here? If I ultimately have to dissasemble everything to get it all balanced, will I still be able to use that damper? And will they be able to balance it the correct way so I won't have to go through again this if my flexplate cracks down the road?

Or at this point, considering the fact that my short block is probably going to have to come apart, and (considering that I don't feel comfortable dissasemling and reassembling my short block....meaning the cost of the balance would be $500-600) would I be better off going with all new better internals?

RB83L69

The starter thing has to do with ANY 12.8" ring gear, whether flex plate or flywheel (auto or manual).

The 700 torque converter will bolt to either size flex plate, and a starter for that size flex plate will bolt to any 400 block; that being the stock size for the 400. However, the larger flex plate moves the starter outward about 5/8", which can cause extreme header and chassis fitment problems. The chassis is REAL tight right there.

I'd recommend getting the hole drilled, having a small flex plate "unbalanced" to the stock 400 spec (requires welding on a weight), and using a mini-starter. That way, you have the max possible clearance for everything, and basically you solve all your problems before they happen.... instead of after you've already got the car on the road and you have to walk for a few days in order to solve issues that come up, and spend more money dealing with it when it's hard than it would have been when it was easy. "Put a fence at the top of the cliff, instead of putting an ambulance at the bottom".

I ran a CVR mini-starter on the 400 block in the pic, with a 83 T-5 flywheel. I ran the stock T-5 starter for a long time, and finally got real sick of it being too weenie to turn the 400 over reliably. The stock one will work, barely; but I can guarantee you won't like it.

Lo-tec

For starters, I would check out another shop if they want $260.00 to balance a flexplate or say they can't do it without the rotating assembly.

If the manual flywheel in your pic was balanced to the motor, get a small flexplate like RB83L69 suggested and have that balanced the same as the flywheel. C5 vettes have problems with flywheel imbalance all the time because the flywheel is balanced to the engine, and when changing from a stock to an aluminum flywheel there are shops balancing the new flywheels to the same imbalance as the old one. Who knows, your flywheel might be balanced to the stock spec for a 400.

Good luck.

contactpatch

Out of curiosity, how much,
is the imbalance of,
traditional 400, front, back, ,
newer 350?
.
The answer, I think, would be something like:
the 400 front, needs 40 ounce-inches of weight added
at 18 degrees crank angle.
{I just made those numbers up}

anyone?