Two Reasons

As far back as the 1970s we considered the M code & Q code dampers inadequate for high performance, they are basically 5000 rpm dampers. If you want to wind that motor out you'll want to beef up the damper to bolster the durability of the motor. In the 1970s we drove to a Ford dealership and ordered the R code (Boss 351) damper new from Ford. You don't have that option today.

Ford stopped manufacturing the 351C in 1974, and the 351M & 400 in 1982. Which means any Ford damper you may want to use is 3 to 4 decades old. The damper consists of an outer ring and an inner hub bonded to one another because each is bonded to a rubber ring between them. After 3 to 4 decades the bonds to that rubber ring are questionable. And when an outer ring comes loose it can come apart and cause damage and injury.

Those are the two reasons for replacing the OEM damper.

If Larry has you questioning the ATI damper, look into using a BHJ damper instead. #FO-EB351C-7 for external balance.
The BHJ damper is choice quality stuff.



-G
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If you use a 351C 4V powered vehicle for a grocery getter ... the eggs aren't going to make it home!