I see the terms used > External balance and Internal balance used with harmonic balancers.

Can someone briefly explain the difference > AND which would describe a stock 1974 model 454?

Thanks....still learning here.........

 

When a crankshaft is made, there are certain sized counterweights that are considered normal for any given forging/casting. As pistons/rods get heavier with larger bores and rod lengths and strokes get longer, it gets harder and harder to have enough weight on the counterweights of the crank to balance it all. They want it to be heavier than needed and then they can drill small holes in counterweights to lighten it to make it perfect.

The 396's and 427's were able to be internally balanced. When the 454 was designed it was found that the crank counterweights weren't heavy enough. So they added some weight to the balancer and the flywheel. Since these are *outside* the crankcase..these are externally balanced motors.

Small block Chevy's were internally balanced until you got to the 400's. Same thing happened, and the cheapest easiest way to handle it was to add weight to balancer and flywheel again. So that's why 400's and 454's take special balancers and flywheels. For years the 383 stroker small blocks were made by using a crank from a 400 and a block from a 350. So they still required the special balancer and flywheel.

Ford has externally balanced just about everything for years.....even the famed Cobra Jet's had an extra weight slipped on behind balancer to make up for heavier rods etc. So do many others.

The theory behind internal being better is that the weight is concentrated back *inside* the motor and there are less bending loads on end of crank than an external arrangement. A gazzilion race motors have been built as external balanced and do well, but if you have a choice internal is better for really high RPM.

These days you see aftermarket cranks that have larger counterweights to allow internal balancing easier. If that is still not enough, then there is Mallory metal. It is heavier than steel and they drill a hole in counterweight and insert a slug of it. This adds weight to counterweight to allow you to lightly drill it back to the correct amount. Mallory is expensive and can easily run into a $400-$900 balance job. The easiest way is to use lighter pistons and rods...it always makes it easy to balance and usually can get away without Mallory metal.

So your 454 is externally balanced. When you look at balancer there will be a weight cast into the back side of it between outer ring and neck area. You must replace it with another external balanced balancer unless you tear down whole motor and go through whatever it takes to make it internally balanced.

Hope this helps some..

JIM

 

Thanks Jim....That was very good and I think that helped a lot.
(Let's see!)


So...If I noticed that the little rubber bead - "circular gasket" has deteriorated on the front side of my 8" balancer.....I can replace the balancer with any new balancer for an external balanced 1974 454. (which means-?) The original harmonic balancer on my motor has not been balanced for "my motor" and has no unique weight or characteristics - It is only a method to add weight to the rotation of the crank. I was afraid (ignorant) that a new balancer would need to be custom balanced to my particular motor. I think I got it.....Let me know if I'm still mixed up here.

Anyone out there got a Big Block Harmonic balancer that not missing most of the rubber bead?