[zildjian4life218]

A friend of mine got a 383 built by a local guy who builds engines. The guy said its balanced and such but its an externally balanced rotating assembly and he told my friend that he has to get a weight to bolt to the flexplate to get the balance correct.... how did the engine builder have it balanced if he didn't have the weight in hand? The guy asked me if I knew I had never heard of this before. I told him I would pull the bottom end and send it to someone else to have it balanced correctly with the appropriate externally balanced flexplate. Will any externally balanced sbc flex plate do? Was there a standard size weight for these? Can you get away with using the little weights that bolt on? He said he has the weight but it hits the starter or something so I told him hes probably better off just having it redone somewhere else with the appropriate flex plate. Any input? Im not sure on this one... Thanks!

[bigdog7373]

Use a 400 external balance flexplate.
You need the harmonic balancer too

383 cranks can come internal or externally balanced.
If you use an actual 400 crank it's external balance.
Any external balance 383 uses a 400 flex plate.
Internal balance uses a 350.

[cdminter59]

The 400 flex plate has the weight welded to plate and ring gear on the transmission side. As far as I know there is no bolt on weight for a flex plate. The 400 balancer will have a weight bolted to the inside of the hub.

[oldbogie]

Originally Posted by zildjian4life218

A friend of mine got a 383 built by a local guy who builds engines. The guy said its balanced and such but its an externally balanced rotating assembly and he told my friend that he has to get a weight to bolt to the flexplate to get the balance correct.... how did the engine builder have it balanced if he didn't have the weight in hand? The guy asked me if I knew I had never heard of this before. I told him I would pull the bottom end and send it to someone else to have it balanced correctly with the appropriate externally balanced flexplate. Will any externally balanced sbc flex plate do? Was there a standard size weight for these? Can you get away with using the little weights that bolt on? He said he has the weight but it hits the starter or something so I told him hes probably better off just having it redone somewhere else with the appropriate flex plate. Any input? Im not sure on this one... Thanks!

There are two ways of getting there:

1) Use a 400 flexpate that has the counter balance welded to it.

2) Use a 350 neutral balanced flexplate along with bolt on balance weight, go to Summit Search Results for balance plate - SummitRacing.com

I have to agree that you need a different balance shop as if this guy has a problem with the weight hitting the starter then something wasn't done correctly. If your friend wants, the crank can be made to balance internally by the addition of heavy "Mallory" metal into the counterweights. That would use a netural balanced damper and flexplate. Ask your new machinist about his/her recommendations.

Bogie

[2old2fast]

You can get 383 rotating assy's that are internal bal, ext. bal. & internal frt. ext. rear . There is so many posibilty's now. I'd still want someone else to assure proper balance . You can get a generic/aftermarket external bal. flexplate.

dave

[zildjian4life218]

I believe it's an eagle crank. So a 400 flexplate would work?

[bigdog7373]

Yep. As long as its and external balance engine.

[zildjian4life218]

Originally Posted by bigdog7373

Yep. As long as its and external balance engine.

Alright cool. Thanks for all the input everyone.

[PatM]

If the engine is using 5.7 rods, it's most likely external balanced. Most of the internal balanced cranks require 6.0 rods.
Patm

[vinniekq2]

how did they balance the engine with out all the rotating parts? including balancer and flywheel/flexplate,,,clutch or convertor should be sent in also.Dont forget to balance your new driveshaft too

[68NovaSS]

Originally Posted by PatM

If the engine is using 5.7 rods, it's most likely external balanced. Most of the internal balanced cranks require 6.0 rods.
Patm

Why do you say that? Rod length has nothing to do with a motor being internally or externally balanced, particularly when using an aftermarket crank.

[bigdog7373]

Originally Posted by 68NovaSS

Why do you say that? Rod length has nothing to do with a motor being internally or externally balanced, particularly when using an aftermarket crank.

^ X2 on that.

[CNC BLOCKS NE]

Originally Posted by 68NovaSS

Why do you say that? Rod length has nothing to do with a motor being internally or externally balanced, particularly when using an aftermarket crank.

OHHHHH yes it does as the 5.7 rod crank uses shorter counter weights and in most cases they are extrernally balanced.

A 6 inch rod crank uses bigger or taller counter weights and can be internally balanced.

Thats why all OEM 400's with their short rods are externally balanced.

[CNC BLOCKS NE]

Originally Posted by vinniekq2

how did they balance the engine with out all the rotating parts? including balancer and flywheel/flexplate,,,clutch or convertor should be sent in also.Dont forget to balance your new driveshaft too

You don't balance clutches!! You migh blanve the flex plates and clutch covers!!!

If you use a neutral balancer they are fine from what I have seen.

A convertor is filled with liquid no need to balance.

[Mr. P-Body]

Most aftermarket forgings ARE "internal". Most castings are not. All of them can be "converted" using Mallory Metal (tungsten) as stated. Not cheap to "convert".

Many shops with a balancer have "on hand", stock 400 balancer and flexplate. We have a Pontiac flexplate that "lives" at the balance shop for this purpose.

The "bolt on" weight is popular among the 383 "crowd" because it allows the use of the "zero" flexplate or flywheel (they usually already have a 350 piece).

The clutch "cover" (pressure plate) is often attached during balancing for manual transmissions. Not used in automatic applications. With modern manufacturing techniques, this has become less an issue than it was 20 years ago.

Lastly, some "myths" about balancing. SAE "standards" are that the rotating assembly is balanced to within 1/2 of 1% of the total rotating "mass". In a 383, that amounts to about 28 grams. Most balance shops routinely balance to within 1 gram, FAR exceeding standards. The slight variations from using different "stock" balancers and flexplates/flywheels are not significant.

For higher revving engines (over 6,500), often "over-balance" is included. That's where the factor for the bob weight is changed from 50% (reciprocating) to a little higher percentage to offset the downward "thrust" under hard accelleration (engine speed, not car speed). High-end "big" engines usually have an over-balance of 4-5%.

FWIW

Jim