Any damper over 20 years old should be replaced. There are 3 reasons for this:

(1) The rubber sleeve has hardened, and the harder the rubber becomes, the less the ring is capable of moving independently of the hub; the three piece damper behaves more like a one piece "weight" hanging on the nose of the crankshaft. Thus the damper is not dampening the crankshaft (absorbing twisting) as it was designed to do. 

(2) The 3 pieces of the stock damper (hub, rubber sleeve, and the ring) are unbonded, they rely upon the "stiction" of the rubber sleeve to hold the damper together. As the rubber sleeve hardens (resulting from age and heat cycling) it no longer grips the hub and ring as tightly as it should. That loss of "stiction" means the positioning of the crankshaft damper’s outer ring has most likely shifted on the hub to some degree, and will continue to shift over time. Thus the timing marks are not reliable. I've witnessed this on many older cars.

(3) The OEM H code (351 2V) and R code (Boss 351) dampers are counter-weighted via the hub. But the OEM Q code (351 cobra jet) damper is partially counter-weighted via the ring; thus as the ring shifts the Q code reciprocating assembly shall no longer be balanced properly.

Turns out the Romac damper I used to recommend is not fully bonded, an Australian member made me aware of this. You had to read the small print. Thus I no longer recommend it. A quality replacement damper, made in Australia, that is fully bonded, made of steel rather than iron or aluminum, is the PowerBond p.n. PB1082SS. It is far better quality than OEM. It is SFI approved. And it costs only $215 at Summit Racing.

Choice quality stuff.

cqs

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George is giving good advice. I replaced mine with the PowerBond unit. While you’re in there I recommend changing your timing chain and gear set. Mine (original) was surprisingly stretched for the 23,000 miles on it. Then engine actually feels tighter as a result. 

Tom, what are your goals here? OEM M-code 351-C in Panteras were redlined at 5900/6000; that's about all the cam, stock hydraulic lifters and OEM valves could reliably deliver under load. For under 6K max revs, about any balancer will work. It's when we push the revs to 6500, 7000 or beyond, that Plan A needs modification.  

The near-stock 351-C needs a 28.2 inch-ounce balance factor on both the harmonic balancer and flywheel to work correctly. 50 inch-oz balance factors were used starting in the '80s after Clevelands were out of production. If used, 50 inch-oz on either end of a crank will quickly tear a near-stock engine apart, sometimes breaking the crank or motor mounts.

Some references say OEM Ford balancers were bonded, some say they were only press-fitted and some say the Boss & HO series was both. The outer ring on old assemblies has been known to get so loose that it spins off and hits the pavement or walks back and wears a hole in the oil pan. A few balancer rebuilders use RTV to glue the two iron parts together and I doubt anyone knows what the real vibration-absorbtion effect is with that technique.

I believe the most important thing for you is, your balancer of choice should be new since rubber deterioration with age (and oil leaks) is the killer here. Not expensive at all. 

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