Question applies to the forged crankshaft, not the periodic cast ones they were using a while ago.

 

They are internally balanced, and require the little bat-wing weight on the flex plate like other 1-pc RMS internally-balanced motors.

 

So I want to match that with an internally balanced damper correct? My old one was starting to dryrot and seperate (circa 1989) so I figure the safe bet is to get a new one.

 

Incorrect. 86 and later smallblock chevy's with the 1 piece rear main seal are externally balanced and require a specific flywheel flexplate and torsional damper. It isn't externally balanced like a 400, but instead the weight is located on the inner hub of the balancer. The flywheel/flexplate, is easily recognized by the additional weight and the holes that are drilled in them. DO NOT interchange internally balanced components with the flexplate and balancer or the engine will suffer a sever imbalance condition that could create catastrophic results.

 

 

Not completely correct either.

The one piece is internally balanced on the front but not the back.
So....a regular balancer on the front and a weighted flywheel (or flexplate) on the back.

 

 

There is extra weight on the flywheel, or there are holes drilled in the flywheel to balance out the rear of the rotating assembly. The front inner hub of the balancer has weight. So, the front and rear weights are controlled by the balancer and flexplate. Last time I checked that was the definition of externally balanced.

 

This is correct.They are internally balanced,and can use the same dampener as any 350.The flexplate is weighted simply to make up for the weight loss of the rear mounting flange being smaller than the earlier,2 piece seal cranks.

 

The how do you explain the weight on the inner hub of the balancer?

 

I will stand by my original statement.

They are internally balanced, and like ALL 1-pc RMS motors, require the little bat-wing weight on the flex plate to substitute for the half of the #7 & 8 throw balance weight that used to be on the flywheel flange.

The torsional damper is neutral-balanced, just like any other internally balanced motor.

"Internally" vs "externally" balanced, is NOT a function of which side of the oil seal that the balance weight is on. It is a function of whether the rod length allows sufficiently large counterweights to exist on the crank, to offset the journal bob weights, without the bottom of the pistons running into them when they're at BDC.

So as Dyno Don also put it, neutral balance torsional damper, and the correct flex plate or flywheel with the small weight on it, is correct. But just because your eyeball says the weight is "external" to the motor, doesn't make it "externally" balanced.

This question ALWAYS creates massive confusion and flame wars. It's the wrong question to ask anyway, because you don't buy your parts that way. You buy a flex plate for a 86-up 305/350, and it's what you need. Period. Applying the confusing verbage about "balance" is not helpful.

Yes Token, a stock 86-up damper is the correct choice.

 

Thank you all very much!

 

BUT,the flange IS the reason that the "bat wing" weight is there in the first place.Noone said otherwise,I was simply stating WHY the flywheel is weighted and balancer is nuetral.

 

I'm not disagreeing with the 86-up balancer being correct. Sorry, but I'm still going to also disagree about the balance of the motor. It is externally balanced. That's why the weight is there. Continue to disagree if you like, but before you do, check out the chapter on crankshafts in John Lingenfelter on modifying small-block chevy engines. I have a feeling you'll agree with me afterwards.

 

Token: what did you end up doing???

I ask, as I'm in the process of putting together a ZZ4 bare block for my 88 IROC. I'm wanting it to be as balanced and longevity minded as possible. With so many balancers and flywheel manufactures out there it's hard to know what's the difference between a great part and a really-really great part. What did you decide to do for your timing chain?
Thanks, Nitro

 

Stock GM 1 PC RMS cranks use a weighted flexplate/flywheel and a neutral balance harmonic balancer(doesn't balance at all, rather dampens), PERIOD.

 

"Now that's the correct question. . ."

(Where have I heard that before???)

The damper is a neutrally-balanced type. Doesn't have to be '86-up, just the size you want and neutrally balanced.

Flexplate is for one-piece rear main seal, '86-up. As sofa has said time and time again, it has a weight on it - Doesn't make it externally-balanced, just makes it one-piece rear main seal type.