What is the difference between a 16721-84c and a 16721-84d other than breather . Can they be unchanged?

 

You can use top breather heads up to 1999 on the older bottom breather engines with no problem, but you can't use the older 1984-1991 or 1992 I think non breather heads on a newer 93 and up motor. They switched to the top breather heads around 92 or 93 although the casting numbers are similar where both end in 84A, C, D etc. The head castings are the same with revisions made to the later designated letter codes by making provisions for ports to allow the heads to breathe thru the tops with umbrella valves and vent out thru the two bolt holes and into the air cleaner or aftermarket breather kit.

So in short...............newer big twin heads can be used on all evo single cam motors and the older big twin heads can't.

Sportster heads, big twin single cam heads, and big twin-twin cam heads are totally different and are not interchangeable.

 

You can also machine non-breather heads, and convert to breathers, via drilling/tapping, drilling a bi-secting hole, to accomodate the use of breather rocker covers.
Scott