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383 external balance question

4K views 24 replies 8 participants last post by  71C10 
#1 ·
i bought a 383 shortblock w/o cam from a guy at a machine shop, friend of a friend kind of deal.

i was told it was a 400 crank turned down for the 350 journals. did not know at the time of purchase that 400 cranks need to be external balanced.

of course, how can you balance a 400 in shortblock? you cant, cause youre missing a flexplate/flywheel and harmonic balancer. of course, i didnt know that at the time.

so now, i buy all my parts, including external balance flywheel and balancer. i do some more reading and hear that everything needs to be pulled out of the block to be balanced.

i go and talk to my friend about this. he talks to the machine shop guy. guy says he balanced rod/pistons to same weight, crank was balanced when turned down to 350 journal size, and block is ready to be assembled with a 400 crank flywheel and balancer w/o the balance weights.

sure enough, when i go home and look at the block, there are grind marks on the crank and there is a hole drilled and some of the rods have been ground on. no heavy metal anywhere.

my question is, is it really balanced? is this a correct way to balance it? i thought it was called external balance cause its gotta be external balanced?

i got a summit harmonic balancer and it has a weight on the inside of it, it is pretty big, and spans about 1/4 of the balancer. do i take this off before i put it in? and do i just put the flywheel on without a weight on it?

i'm just sorta comfused, cause all the threads i have ever read say you gotta get crank, rod, piston, balancer, flywheel all balanced together.

Thanks,
Jerome
 
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#2 ·
ljjjrome said:
my question is, is it really balanced? is this a correct way to balance it? i thought it was called external balance cause its gotta be external balanced?

i'm just sorta comfused, cause all the threads i have ever read say you gotta get crank, rod, piston, balancer, flywheel all balanced together.

Thanks,
Jerome

It depends on how much faith you have in the honesty of that machine shop guy that supposedly did the work. :rolleyes:

flywheel and damper should be standard issue 400.
 
#8 ·
An externally balanced crank can be made to be internally balanced. This is done with slugs of "heavy metal" set into the crank counter weights. There should be big round plugs in a few of the crank counter weights if the 400 crank has been internally balanced.
 
#9 ·
im goin through the exact same thing

take the external crank to a machine shop and tell them to convert it to an internal balance they may have to add a slug into it but if ur flexplate or damper goes you dont have to take it all out. this is explained well in the balancing and blueprinting book by cartech.
the weight depend on the weight of your pistons and rods. if you are usin aluminun pistons you may get away cheaply.
or just get the balancing and blueprinting book and read. http://www.amazon.ca/Step-Step-Guid...ef=sr_1_1/702-2967619-2632051?ie=UTF8&s=books
 
#10 ·
taking all the stuff out of the block is basically out of the question. i dont have the resources or the budget to pull everything out and have it professionally balanced. I would be looking at at least $500 to do that, and risk cracking a piston while pressing the pins in and out. keep in mind that this shortblock is completely assembled and torqued down.

i just want to know what the recommended course of action is regarding the harmonic balancer, assuming that the shortblock is "balanced".

i dont understand how a harmonic balancer works, and i would like to know whether the weight that is attached to it is for rotating assembly balancing purposes, or for harmonic balancing purposes. obviously i would remove it if it was for balancing the rotating assembly, but keep it if it is what keeps the harmonic vibrations away.

the machine shop guy basically just says that it is ready to go, just bolt on your stuff, and offers no relevant technical information. he is very unclear about everything, which is why i am asking you guys here.

Jerome
 
#12 ·
right, but i only bought the shortblock (block, crank, rod, piston) from him. i purchased the flywheel and balancer from summit and they are in boxes in the basement.

i read another thread that says that if everything has been balanced to the same weight, that the balance can be very very close without having to pull everything out and balance with flywheel, harmonic balancer, crank, rod, piston all together. i guess that this is what the machine shop guy is assuming.

Jerome
 
#13 ·
ljjjrome said:
the machine shop guy basically just says that it is ready to go, just bolt on your stuff, and offers no relevant technical information. he is very unclear about everything, which is why i am asking you guys here.

Jerome

Install whichever type flexplate and damper he advises. Is he tellilng you to use a 400 set up or a 350 set up..... you need to know.. Go for it :thumbup:
 
#15 ·
400 setup without balance weights is what he says.

i bought 400 flywheel for external balance and 400 balancer for external balance

http://store.summitracing.com/partdetail.asp?autofilter=1&part=SUM-C4266&N=700+0&autoview=sku

that is what i bought. on the opposite side of that shown by the picture, there is a weight that sits in the inner ring and has a bolt anchoring it i assume it can be put in any of those numerous bolt holes in the inner ring the picture. im not sure whether this weight is what prevents harmonic vibrations, or whether this weight is for balancing the rotating assembly.

Jerome
 
#16 ·
bobscogin said:
" block is ready to be assembled with a 400 crank flywheel and balancer w/o the balance weights."

400 crank and balancer WITHOUT the weights? What's that mean?

Bob

see, i have no idea how a external balance flywheel and balancer work. i am guessing by what he said, that you put a weight on the flywheel and a weight on the balancer to balance the rotating assembly, with the weight and position of these determined by you crank/rod/piston? but since this assembly is already "balanced" i dont need those weights on the flywheel and balancer?

i really dont know how this external balance stuff works.

Jerome

Jerome
 
#18 ·
ljjjrome said:
see, i have no idea how a external balance flywheel and balancer work. i am guessing by what he said, that you put a weight on the flywheel and a weight on the balancer to balance the rotating assembly, with the weight and position of these determined by you crank/rod/piston? but since this assembly is already "balanced" i dont need those weights on the flywheel and balancer?

i really dont know how this external balance stuff works.

Jerome

Jerome

On a 400 the crankshaft counterweights cannot be cast heavy enough by conventional casting methods to balance the big pistons and longer stroke,

so weight is added to the flexplate and damper to make up for the balance.

A 350 does not need these weights.

edited: When a cross breed is made, it can be balanced by either method. So you have to know exactly what he did. Do you need a 400 set up or a 350 set up?
 
#19 ·
ljjjrome said:
i go and talk to my friend about this. he talks to the machine shop guy. guy says he balanced rod/pistons to same weight, crank was balanced when turned down to 350 journal size, and block is ready to be assembled with a 400 crank flywheel and balancer w/o the balance weights.

Thanks,
Jerome
Jerome, I read through your original post carefully this time. What the machine shop did is balance the rods and pistons, calculated the bob weights and internally balanced the rotating assembly. What this means is you will use a neutral balanced flywheel and dampner. You can remove the weight from the dampner and purchase a neutral balanced flywheel that is designed for an internal balanced rotating assembly.

No need to have the rotating assembly rebalanced.
 
#20 ·
thanks rick, i already purchased an external balance flywheel. would it be possible to use that flywheel?

heres a quote from ram clutches site:
"The balance situation gets more interesting when the flywheel must be balanced to an external balance specification. External balancing requires putting a specified ounce inch imbalance within a one-degree radial on the flywheel."

so is a external balance flywheel just an internal balance with provisions for a weight to be attached? if that is the case, i can just use the flywheel as is, without attaching any weights right?

Jerome
 
#21 ·
I have read this thread over closely and I advise once again.

Find out exactly from the balance man if you need a 400 external balanced set up, or a 350 internal balanced set up.... (I've seen 1/2 and 1/2 too)

He should have a file card with a detailed description of every part used in the balance procedure.

After you put it in the car and start it is NOT the time to discover severe vibration. :nono:

Yes I personally have balanced lots of engines.
 
#23 ·
ljjjrome said:
thanks rick, i already purchased an external balance flywheel. would it be possible to use that flywheel?

No, you can't use that one, you need a flywheel for an internally balanced rotating assembly.

so is a external balance flywheel just an internal balance with provisions for a weight to be attached? if that is the case, i can just use the flywheel as is, without attaching any weights right?

The weights are welded onto the existing flywheel you purchased.

Jerome
Hope this helps.
 
#25 ·
ljjjrome said:
right, but i only bought the shortblock (block, crank, rod, piston) from him. i purchased the flywheel and balancer from summit and they are in boxes in the basement.

i read another thread that says that if everything has been balanced to the same weight, that the balance can be very very close without having to pull everything out and balance with flywheel, harmonic balancer, crank, rod, piston all together. i guess that this is what the machine shop guy is assuming.

Jerome
You really NEED to talk the to MACHINE SHOP that did the work. Nobody on this forum knows what was done to your short block. Your friend could be misquoting what he was told by the machine shop. Go to the SOURCE!
 
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