[GTOLou]

Can one of you smart guys explain to me how a motor can have an internal balance harmonic balancer and external balance flywheel - at same time? I thought it was an all or none type deal - balance/flywheel are either both neutral balance or both external balance.

(Its about a SBC - don't shoot me!)

[25stevem]

I believe Pontiac did that same thing.
The final balance (tweak) was done by spinning up the assembled short block with the flywheel or flexplate on the motor as needed.

Then the overall balance was corrected by the drilling of balance correction holes on either of these parts.

[Kenth]

Pontiac flywheels and flexplates have an assymetrical bolt pattern so that the flywheel will bolt on the crank in only one position. There is a slight external unbalance bult into factory parts to aid in the internal balance of the engine. This amounts to about 6 inch-ounces wich if not compensated for will show up as a pronounced vibration over 2500 rpm. Since all Pontiac flywheels/flexplates have the same amounts of external unbalance they may be interchanged freely.

The harmonic balancer is designed to dampen torsional vibrations in the crankshaft. Without it the crankshaft may break. The harmonic balancer is not designed to compensate for engine balance and can only affect if the damper itself is out of balance. Pontiac does NOT use any of the "unbalanced balancers" like the 454 Chevy units, so Pontiac dampers/balancers may be interchanged freely.

FWIW

[mgarblik]

If your question was specifically about SBC, this may help. Most of the Gen 1 SBC engines up to 350 Cu In. were internally balanced. When they went to a one-piece rear main seal, they lost a small counterweight that was on the end of the crankshaft. At that point they went to a internal balance/neutral on the front and a flex plate/flywheel with a counterweight added to the rear. (like a Pontiac), but much larger. The pilot diameter also changed on the Chevies. The 400 Chevy is the only weird exception. That abomination is completely externally balanced, different balancer with counterweight and flex plate with counterweight. Hope this helps. (They should have left 400's to Pontiac!)

[Schurkey]

Originally Posted by mgarblik

Most of the Gen 1 SBC engines up to 350 Cu In. were internally balanced. When they went to a one-piece rear main seal, they lost a small counterweight that was on the end of the crankshaft. At that point they went to a internal balance/neutral on the front and a flex plate/flywheel with a counterweight added to the rear. (like a Pontiac), but much larger. The pilot diameter also changed on the Chevies. The 400 Chevy is the only weird exception. That abomination is completely externally balanced, different balancer with counterweight and flex plate with counterweight. Hope this helps. (They should have left 400's to Pontiac!)

Mostly correct.

Because there was an offset-weight flange on the crankshaft behind the rear main bearing, they were NOT "internally balanced", but they did use a "neutral balance" flywheel/flexplate. Calling them "internally balanced" is a VERY common mistake.

Any number of Fords hang an offset weight behind the damper, but ahead of the front main bearing.

Having a neutral-balance damper and flywheel does not mean the engine is "internally balanced"; although an internally-balanced engine will have a neutral-balanced damper and flywheel.

[GTOLou]

Great explanations - appreciate the responses here.

[mgarblik]

Originally Posted by Schurkey

Mostly correct.

Because there was an offset-weight flange on the crankshaft behind the rear main bearing, they were NOT "internally balanced", but they did use a "neutral balance" flywheel/flexplate. Calling them "internally balanced" is a VERY common mistake.

Any number of Fords hang an offset weight behind the damper, but ahead of the front main bearing.

Having a neutral-balance damper and flywheel does not mean the engine is "internally balanced"; although an internally-balanced engine will have a neutral-balanced damper and flywheel.

That's a great point. I've been "schooled" ! Could you expand on what constitutes an internal balance engine if a neutral balancer and flywheel are not the only factors? Learning new stuff here. So much of engine balance is treated as "black magic", even most of the text books and seminars I have been to haven't been the best.

[Schurkey]

"External" balance is when any correcting counterweight is hung ahead of the front main bearing; or behind the rear main bearing. In other words, the correcting out-of-balance is not straddling a pair of bearings. The correcting out-of-balance is overhanging one or both of the end bearings. As the Small-Block Chevy--and other engines--have a small counterweight at the flywheel flange (overhanging the rear bearing) which makes them not internally balanced; yet the damper and the flywheel/flexplate have no counterweight on them. When Chevy went to the one-piece rear main seal, they couldn't have a counterweight at the flywheel flange and a neutral-balanced flywheel, so that correcting weight got tacked-onto the flywheel.

For "most" purposes, neutral balance of damper and/or flywheel is "good enough" at the respective end of the crank; as evidenced by decades of folks claiming the SBC (except for the 400) is internally-balanced

As more weight is needed for counterbalance; and especially as the RPM goes up (Double the weight, double the stress. Double the RPM, stress goes up by...four times? Ten? I forget the math) hanging a counterweight unsupported on one side, tends to flex the crankshaft. It's no big deal for grocery-getters, or mild engines; but it's totally not acceptable for any engine getting run to the fun side of the tach often or long.

A genuine "internal" balance has all the counterweighting done on the crankshaft; therefore the damper and flywheel are going to be neutral balanced; but there's no overhanging correcting weight. All the correction will be supported on two sides; generally between the first and second main bearings, or between the fourth and fifth (V-8) although depending on how the engine is configured, there may be counterweights between any pair of main bearings.

The 400 Chevy in my 'Camino ended-up with a standard offset weight in the damper, but a neutral-balanced flexplate and some heavy-metal on the counterweight between #4 and #5 main bearings.

Ford used a weighted spacer between damper and the front main bearing. I think the damper itself is neutral-balanced. Photo shamelessly stolen from https://www.428cobrajet.org/id-damper-spacer

[mgarblik]

More great info. I am going to share this with my class tomorrow. We are balancing crankshafts right now. I have a 383 stroker SBC in the balancer right now for a student and it is being very difficult. It has an Eagle crank and the counterweights are weird shaped and the OD factory cut down for clearance in the block. The counterweights look like they were formed with a chop saw. Already has Mallory metal in 2 locations from Eagle. It needs 21 grams removed on the front. (no issue). But on the back, it needs 113 grams removed where there is no counterweight and the area where 113 grams could be added, eagle has already clipped off the counterweight. I think our only choice is going to be to weld additional weight to the flex plate. I hate Eagle junk and 1piece seal SBC's.

[Schurkey]

IF (big IF) you modify the flexplate, use a marker or something to note how much weight was added. When the flexplate is replaced years from now, they'll know how much and where to add.

Can't add material to the counterweight on the crank? No clearance? Cast crank?

My favorite Treasure Yard had an Armasteel crank laying in the dirt. Had additional steel straps as weights welded to the counterweights to make it balance. Seemed somewhat scary to me--welding to cast iron is not a trivial matter.

[mgarblik]

Point taken. Can't add weight where it's needed because Eagle already had a slug in the same place. IMO, they made the rear counterweight too small. Kind of chopped it off. I will take a few pics tomorrow.