|One of the tubs in my shop had not one, but two sets of Thunderheads in
them. These are aluminum heads you can bolt on to an Iron Sportster.
Duncan Keller told me that they make an Iron Sporster a different bike and I can vouch for that, since my 1980 show bike has a set on it.
These heads leverage what Jerry Branch taught the Harley racing team-- smaller ports with higher velocities will fill the cylinder better.
Smaller ports mean smaller valves, so you can narrow the 90-degree angle in Iron Sportsters to 60 degrees. It allows a flat-top piston with better flame propagation.
The really cool thing is that the Thunderheads on the right are machined for a 900cc bike. I want to mount them on my 1962 Sporty. Its a big job setting up the valves and getting all the parts. Thankfully I have a set of Thunderheads on my 1980 show bike
to crib off of, and that 1000cc set on the left side of the tub to get
parts from. Thunderheads make an old Iron Sportster into an Evo, but
without the tall motor due to increased deck height. It also uses stock
exhaust and carb, although the manifold is different and hard to find.