Whats the difference between external and internal balace...

And how do you tell the difference?

 

Boy, I'm anxious to follow this thread. I've had it explained to me by a machine shop who does balancing and I still don't understand. Logically, to me anyway, it would seem that if the internal rotating assembly (crank, rods, pistons) are balanced, and the dampener and flyweel are each independently balanced, you'd be completely balanced. (Or they could all be balanced together as an assembly.) If any one of those three things are out of balance, it would seem that the engine is not balanced. College education, with some physics and mechanical engineering, yet somebody's gonna have to dumb this down for me. I just don't get it.

 

Internal means crank does not need flywheel or balancer to be balanced......the flywheel and balancer are neutral. In other words the engine would not vibrate if either piece was not on the engine.

External means there are weights on the balancer and flywheel because there is not enough counter weight on the crankshaft.
If either of these pieces are left off in an externally balanced engine....severe shake!


How do you tell? What's engine in question?I have a chart that tells me what engines are done each way from the factory..........
If your using aftermarket parts........that's what we get paid for when it goes in the machine.

In our line of work we put EVERY assembly in the machine wether its stock parts or performce

 

Goat -

It was more of a general question, although I have a Ford FE rebuild on the project list, although it's a ways down...for now.

Chris

 

So you couldn't even replace a damper on a externally balanced engine without tearing down completely and balancing the whole rotating assembly as a unit?

 

If you're doing an FE, only the flywheel will have a counterweight, except the Super Cobra Jet with LeMans rods, and even then the dampener itself is neutral and the "cookie cutter" weight is on the spacer sleeve that fits on the crank snout. Only the 410 and 428 came externally balanced, with the grocery-getters (usually) getting the 1U crank, later "regular" CJ's should have a 1UB but some (and the early ones) got 1U's- 1U's and 1UB's have an extra counterweight at #3 that was adequate with the weight of the nut and bolt rods in those engines. The SCJ piston was beefed up and heavier, and so was the LeMans rod, so on the 1UA the small #3 counterweight was deleted and replaced with the "cookie cutter" on the dampener spacer sleeve- so a 1UA and cookie cutter should be kept together (and are both fairly valuable). The 1UB "Cobra Jet" crank has a slightly different balance factor than the regular 410/428 1U crank, the change was made so that the beefed-up SCJ piston could be used in both the CJ and SCJ. Factory 410 and "regular" 428 pistons were made the same weight, and used the same rods and flywheel and balanced the same.

 

You can replace a balancer without tearing the engine down .....Provided its the same unaltered style.

Thats why when I balance ...say a stock assemble ....I wont alter the factory end pieces....I just work on the carnk....that way if you were to tear up a flywheel you can put the equivalalet one back on.

There are cases I will set up a mock up assembly in the machine and duplicate the weighted part being replaced.

Now most(99%) of our performance and race assemblies are internal just for the same reason.
If I build an engine and it were external and the customer changes his mind and goes from auto to manual trans for example....kind of hard to take and exchange an aftermarket auto to a manual flywheel thats external and just bolt it on and expect it to be correct....thats just guess work...not good...and more often than not have issues.

When they are nuetral pcs....its easy to just spin the flywheel or balancer by itself to double ck....then bolt them on.

As I said we balance all assemblies.....You never know what could have been changed in an engine you know nothing about....And during a rebuild....oversize bore pistons will weight different...crank journals have been machined.....these things alter the bobweight......and in aftermarket parts....you starting from scratch.

I hope I made it understandable.

Tony

 

Well it wouldn't be perfect, but it would do. I sourced a 1990 350 flywheel to make an 87 305 a stick and had no bad vibrations.

1 piece rear seal small block chevys are external, but not weighted the same as older 400 sbc which were external. I replaced a flexplate on a 400 and it used a separate balance weight, no vibrations.

Also Ford windsors changed somewhere in time and not all 302/351 external balanced parts interchange. I believe they have different bolt patterns to prevent mixup.

 

Here's a type card that's used to figure what the "bobweight" will be for a STD V8 engine.

Measured in grams.

 

For those that don't know what a bobweight is.... Here they are simulating rod and piston weight in the machine.

 

If you look close you can see how heavy or Mallory metal is pressed/installed into counterweights to internal balance a crank, that would other wise need help with the end pcs.

 

"How do you tell? What's engine in question?I have a chart that tells me what engines are done each way from the factory.........."

GOATROPER02 - Any chance you could tell us where we might be able to obtain that chart?

 

TagMan ... mine came from the machine manufactor....Im sure there is something printed from the AERA or similar organizations.

Tony

 

Thanks for the explanation.
I am still mystified by the fact how pistons and rods can be simulated by bob weights. Once spinning, the weights will exert a force radially from the axis of the crank, whereas the direction of force in the engine will be in direction of the rod, which obviously constantly changes in respect to the crank as it rotates.

Are bob weights calculated differently for inline and V engines?

 

Yes, they are calculated differently based on engine configuration. Primary balance in an inline engine with an even number of cylinders really isn't an issue, because for every piston/rod that is moving "up", another is moving "down" so the primary forces cancel. This is also true in horizontally opposed engines like the first generation volkswagens, Corvairs, a lot of aircraft engines, etc. None of these types require counterweights for primary balance. A "V" engine configuration doesn't have forces working against each other 180 degrees apart, so balancing is a bit more complicated.

 

When they started making engines... they put weights on the crankshaft to counterbalance the weight of the piston and rod... look at a crankshaft, that's what those big lumps of steel are for. Sort of like the gobs of lead they stick on your wheels to make them stop vibrating.

Then the manufacturer made the motor bigger. Chev 265 => 283 => 327 => 350 => 400, Dodge 277 => 301 => 318 => 340 => 360.

Eventually they made the motor so big, there was no room for bigger balance weights inside the engine (without hitting something).

So they stuck a gob of weight on the flywheel, torque converter or harmonic balancer. In other words the last bit of balance weight went on the outside of the engine instead of the inside.

There is no hard and fast rule about which motor is externally balanced. You have to ask which models are which. But you will find, it is the biggest version of that engine, that has to have the extra weight. The original small version won't need it.

 

So...... Would you be in trouble switching from a flex plate to a flywheel on an externally balanced engine without tearing everything down and going to the machine shop?


Posted from the TJJ App for iPhone & iPad

 

Rusty. I guess that explains the additional external counter weight on the Chevy 400 small block???

 

Excuse me. We're talking about ratational balance here...internal vs external.

Isn't what some call the "balancer pulley" or the "harmonic balancer" more accurately referred to as the harmonic damper? And am I correct in thinking that it has nothing to do with balancing the engine...only controlling harmonics by absorbing the resonance produced by the power pulses?

As I understand it, an engine is either balanced totally internally by balancing the internal rotating parts, or partially internally balanced with the rest taken care of by the flywheel. But the harmonic damper itself is not weighted. Even in the case of a removable weight being present at the front end of a crankshaft, it's separate from the damper.

 

Ricky
Not always,
A harmonic balancer can be weighted..... Think factory balancer on a 400 sbc....the outer ring is undercut on approx half a side....if the rubber goes bad and the outer ring "walks" you all of a sudden have a bad vibration.

 

The big advantage to external balance it uses less weight because it's farther from the center line of the crank.

 

Thanks Goat.

 

if you want to feel some vibration.....put a 350 or 396 flex plate on a 454. I bought a rebuilt 454 once to put in my 66 Impala. They guy "threw in" a new flex plate. I never checked it and installed it in the car. Had to drive it on a 300 mile trip a couple days later. Felt like all 4 tires were out of balance. For every couple hours of running I had to tighten a bunch of bolts under the hood. Exhaust bolts were falling out. Finally figured it out when I pulled the trans to fix a leaking front seal.

 

Respectfully correcting you sunbeam, this is the DIS-advantage of external balance.

The internally balance of an engine is supported " internally " meaning inside the block and supported by the main caps and main webbing of the blocks structure. The external balance of a engine is NOT supported structurally by the block and it's mains and webbings, but outside of the block without support. This design uses counterweighted front and rear " balancers " whether flywheel/flexplate or damper/balancer that roatate with the crankshaft OUT-side of the support of the block and it's structure. Therefor this EX ternal balance is of the IN ferior or SUB ferior design to a Internally balanced engine. This is only 1, but arguably the primary aspect of the discussion as well.

 

There will be no problem swapping flex plate to flywheel on an externally balcanced engine. It's not the total weight of the rotating add on, but the counter weight that is added. You can even buy bolt on counter weights for the neutral balance flex plate/flywheel to make it external balanced.

 

I agree internal is a better deal but for the factory external was easer and CHEEPER. Allthough there million of small block Fords that have done just fine.

 

SO if to say I was building an engine (406 chevy) that I plan on running the shit out of and have a balanced assembly (external). Would I be better off to get a crank that is internal balanced. I have been colecting and building this thing for over 26 years now.Not for any particular reason. Guess thats why it has been taking so long.

 

I was just trying to help the O/P and those following to understand the original question, " what is the difference between internal and external balance ". I don't know how this seemed to get changed into what's better or worse, cheaper or more expensive.

There are also millions of race engines externally balanced doing fine as well, both winning and equally reliable too. I have been balancing for over 30 years and just wanted to help with the thread. For some of us, " done just fine " is not even close to being enough, TR

 

SO if to say I was building an engine (406 chevy) that I plan on running the shit out of and have a balanced assembly (external). Would I be better off to get a crank that is internal balanced.

Depends. You have more flywheel and balancer size options if you get it internally balanced. I wanted to run a 153 tooth flywheel and small bellhousing on the 5.7" rod 406 for my A roadster, so I spent the money of a few slugs of Mallory and had it internally balanced.