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broken crankshaft

13K views 41 replies 10 participants last post by  1Gary 
#1 ·
well i have a 400 sbc and i pulled my pan to figure out were all these metal shavings in the oil was coming from and found the crank is broken right at the hole in the front of the crank the journal for #1and 2 need help as i have never replaced a crank the right way i was talking to the guy at summit about getting an internally balanced crank and he stated that i should get the whole rotating assembly balanced ? anyone have any input

crankshafthttp
://www.summitracing.com/parts/ESP-10400375057I/
 
#5 ·
You can choose either a internally balanced rotating assembly Eagle Specialty Products B13470030 - Eagle Street Performance Rotating Assemblies - Overview - SummitRacing.com or a externally balanced rotating assembly which includes main & rod bearings and a balancer and flexplte. Eagle Specialty Products B13460030 - Eagle Street Performance Rotating Assemblies - Overview - SummitRacing.com Have the block cleaned, bored .030, new cam bearings, freeze plugs, and screw-in oil gallery plugs you have abrand new short block. You then what heads to use. New or rebuilt.
 
#7 ·
there are two types(generally,theres actually more) close enough is a 2 gram or less accuracy,then a precision zero balance which is as close as possible to perfect. dont worry about under balance,,,,
costs vary depending on time spent. I pay about $500.00.Ive seen it done for $200.00. some builders just static balance which isnt all that great,
 
#11 ·
If I were in your shoes, I would go with the ballanced rotating assembly, no waiting, no hassle, but with that much metal in the engine I would be concerned about metal in lifters, passages, etc. I have rebuilt enough motors to know that starting with everything clean, saves wear down the road.

Pull the block have everything cleaned, throw in the rotating assemble and put it back in. I am not a professional, but I am experianced and I would hate to see you throw 700 bucks into a motor that gets damaged due to old shavings in the motor.
 
#12 ·
The entire rotating assembly does NOT need to be "rebalanced", only the crank itself. When they advertise "internally balanced", they don't mean it's already balanced, they mean it's suited to the internal versus the external. Since this is a 400, it IS externally balanced "stock". If you get an "internal" crank, you'll need a new balancer and flywheel/flexplate.

The imported cast cranks have shown to be a lot of trouble. The only "cast" crank we use for a 383/400 is the genuine Chevrolet. That nodualr iron crank is pretty good. The forgings from Eagle, etc. are quite good.

A balance "job" is $200 here (Richmond area). When converting to "internal", it can get expensive due to the cost of the "heavy metal" (tungsten) and the labor to install it. I've never heard of varying "degrees" of balancing costs, based on "how far". All the shops I know balance to within 1 gm. SAE standards are 1/10th of 1% of the entire rotating mass. In the case of the SBC, that's about 25 gm.

FWIW

Jim
 
#14 ·
No. You MUST get it balanced. Internally balanced mean that you do not need a special flywheel/flexplate and harmonic damper to achieve balance. Factory SBC400's had additional weights on these components to make this work. Internally or externally balanced rotating assemblies both need to be balanced, the difference is how it is achieved. It is two different styles/methods.

You can get this kit but you must:
1) have it balanced

2) buy a new flywheel/flexplate for internally/neutral balanced SBC's, like a SBC350 . Aside from the fact of the balance issue, the link you posted uses a 1 piece rear main seal, which uses a different crank so you old flywheel will not even come close to physically bolting on.

3) buy a new harmonic damper for internally/neutral balanced SBC's, like a SBC350
 
#17 ·
Yes sorry, I was looking at the pic which is 1 piece, the description says 2 piece. They do come with a "balance" from the factory, but from what I hear they are not a good/accurate balance. More than likely they have a target weight/balance for each part and if they fall into a certain tolerance it is good to go, meaning the entire assembly/collection of parts you get is not holistically balanced. You can chose to trust the Chinese balance, but it is not suggested for any performance or RPM use. Low RPM grocery getter you are OK.
 
#16 ·
QUOTE=atcoscrazyest69;1597869]Well i planned on gettin her all cleaned up. Ok just so i got this right if i get this kit Eagle Specialty Products B13470030 - Eagle Street Performance Rotating Assemblies - Overview - SummitRacing.com i will not need to get it rebalnced? Just put inu and go?
Thanks for all your help guys![/QUOTE]

This is an internally balanced kit it will require you add the proper for internally balanced crankshaft the zero balanced damper and flywheel or flexplate.

Let’s talk internal and external balance for a couple sentences. The definition is that the internal balanced crank carries all the counter balance on that portion of the shaft that is inside the crankcase. This is not to say that the balance is by cylinder to cylinder but is overall balanced within the crankcase. Externally balanced crankshafts typically do not have sufficient space within the crankcase for all the counter balance so some portion of the needed counterbalance is hung outside the crankcase. This usually applies the needed counter balance to either of both the front damper and the rear mounted flywheel or flexplate.

In the case of the internally balanced crankshaft the damper and flywheel or flexplate are what is called neutral balanced. That is they have no offset balance and may be used on any engine they will fit to that also has internal balance.

The externally balanced engine uses a damper and flywheel or flexplate that is offset balanced. That is it is out of neutral balance and may only be used on matching engines for which they are intended.

The failure of a crankshaft through the forward rod journal faces is mostly caused by the failure of the damper to perform its job. The damper is intended to absorb the alternating energies on the crankshaft caused by the power adding or power using strokes of the many cylinders. These cause sizable vibrations in the shaft that need to be absorbed which is what the damper does. If it fails in the case of a factory unity it will be a hub and ring that are separated by a rubber ring. When the rubber fails or if the hub doesn't fit snugly on the crankshaft, it stops absorbing these wild vibrations and the crank snout starts to orbit rather than rotate. This motion will bust the shaft usually just behind the number 1 main through the cheek that supports the number 1 rod journal. Carving out the number one main bearing thus starving the number one rod bearing of oil and the subsequent failure of the rod is also a common event.

There's more to say but I need to run, this shoud give you some food for thought.

Bogie
 
#27 ·
That "kit" is already balanced according to the ad. Eagle DOES sell pre-done "drop in" kits. I would guess it's one of them. Call Eagle and ASK.

I would not recommend that kit. It has a cast steel crankshaft. Those are the ones we've seen problems with. They're not as rigid as a factory nodular crank, and not as elastic as a forging. In our industry, when the word "steel" is used to describe a crankshaft, "forged" is implied. Engine builders, except those trying to mislead for a sale, won't call a cast steel crankshaft a "steel" crank. They'll include the word "cast".

If you DO buy it, as SS says, you'll need a new balancer and flywheel/flexplate for "internally balanced". The stock 400 stuff WILL NOT work.

Jim
 
#28 ·
I understand the external balance points with 400s and 383 as it is on a 4 speed trans and i had to buy that plate that bolts between the flywheel and crank. I understand that the balancers are different for the externally balanced 400 and all the other internally ballanced motors. The whole reason for trying to make it internally balanced is i do not like having to run that balance plate. I just dont understand why the add says its balanced and a couple fellas on here are saying send all the brand new stuff out to get balanced. If i have to send it ourmt to get balanced either way then theres no need for me to get a whole rotating assembly the only reason i said id get the assembly is because it states its balanced. I just donot know what to do. But i do not want to do it again
 
#29 ·
That's reasonable; I'm not enamored of externally balanced engines, therefore, work hard to keep 400's and 383's internally balanced.

The kit you sight is internally balanced and will, therefore, require that you purchase a netural balanced flywheel and damper.

The raging argument about how well balanced is one revolving around how good is good enough. Some of these pre-balanced kits have been miles off. Sometimes this is the result of the manufacturer other times it's been the fault of the builder taking to much material from the rods or piston skirts in the pursuit of clearance. Frankly, I prefer SCAT to Eagle when it comes to budget bottom end parts and assemblies. But since you're dealing with Summit, they have a very good customer service reputation so I'd mentally lean on them with this provisio; if you have interference between the rods and cam or piston skirts DO NOT FIX IT. Call Summit and make arraingements to return it in the original packaging in a clean and unmolested state. That eliminates any argument over who did what and has blame. Grinding on the rods and or pistons will alter their weight destroying what ever level of balance they came with. Mayby not by much, but you don't want to get caught in "if you touched it, its yours".

One thing we see a lot of when sending parts to our balancer is that many things that are sold as balanced (including flywheels, flexplates, and dampers) are often well off any reasonable standard for a performance to competition level engine. So we do not even bother with pre-balanced kits. We procure things in pieces, make up assemblies, proceed to a mock-up build that trims parts if necessary, mark them as to where they go and who goes with whom, then have them balanced. This is tedious and adds some cost but the end result is a correctly balanced engine, whether that's spot on or over/under balanced by end use or customer desired requirement. For a mild performance street engine that doesn't go over about 6 grand of RPMs, spot on is the place to be no over/under balance games needed.

Bogie
 
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