From the Boutique:

Sportster and Buell Heads

Aaron Wilson

Screaming Eagle Sportster heads are what we call a "Lightning" head. They came out in 1996 on Buell S1 Lightning models, and a dual plug version came on the 1200S as well. They have the same size valves as a regular 1200 head: 1.715" intake & 1.480" exhaust, and the ports are the same. HD claims 8% better flow from the Screaming Eagle heads. But I've flowed a lot of heads, and I haven't seen it. Even identical heads vary pretty significantly from one to the next, and the range of numbers I get from 1200 heads and SE or Lightning heads is pretty much the same.

The big difference between a 1200 head and a Lightning head is the chamber. Where the 1200 head is a 67cc hemispherical chamber with no squish band, the Lightning head is a 62cc chamber with a pair of 10 degree squish bands. Note that these 10 degree squish bands don't do anything for you with a stock flat top piston. But the smaller volume of the Lightning chamber bumps the compression ratio up to about 10:1 with flat tops. That's really where the performance improvement of a Lightning head comes from.

The problem with Lightning heads, besides the ports and small valves, is that the extra material they put in the chamber to get the volume down tends to shroud the valves. The shrouding gets really bad when you start sticking big valves into the heads. When we do a 1.850" or 1.900" valve in a Lightning head, we always have to unshroud the valves quite a bit to get the low lift flow back. And that ends up taking the chamber bigger, often all the way back to 67cc where the 1200 head is. So then we have to either cut the hell out of the deck to get back to 62cc, or generally we recut the squish band to 15 degrees and put the customer into a set of Thunderstorm pistons, which is a better solution anyway because he gets a squish band. So essentially, we end up converting the Lightnings to Thunderstorms.

In 1998, Buell came out with the Thunderstorm head. It has improved ports as well as larger valves: 1.810" intake and 1.575" exhaust. The chamber is 67cc like the 1200 head, but it has a pair of 15 degree squish bands. So this chamber doesn't have so much material in it and they're able to put in big valves without shrouding them. It's a big improvement. If you put the Thunderstorms over flat tops, you'll have 9:1 compression with no squish just like you'd have with a 67cc 1200 head. But Buell paired these heads with a small 15 degree domed piston that gives 10:1 compression. All in all, it's a much better setup than the SE/Lightning heads. And the ports have some extra material where the Lightning heads really need it, so we're able to make a better port out of them. One downside to Thunderstorm heads on Sportsters though is that they're only available in black finish, no silver or black w/polished fins (not that some people don't convert them). Lightning heads have been made in all 3 finishes. Also, Thunderstorms have 7/16 NC front mounting holes, your Sportster uses 3/8, so you've got to drill the holes in your mount and swap your hardware, but that's pretty easy.

In 2003, Buell came out with the "XB" head. Same size valves as the Thunderstorms (1.810/1.575), but the valve stems are 7mm for less weight and the springs are conical style. They can easily support .550 of lift right out of the box, where all the older heads were only safe to about .500. They also have a 62cc bathtub chamber with a pair of flat squish bands. So now you can get 10:1 with your flat tops and have an effective squish band and big valves that aren't shrouded, and the best ports they've ever made (they tend to flow about the same as Thunderstorms but it's a less turbulent flow). In 2004, they put these heads onto the 1200 Sportsters as well, and they made them black with polished fins (the Buell XB heads are silver). Putting XB or 04XL1200 heads onto an 86-03 model isn't too bad, the only real issue is the front mounting bolts (front head), they moved the right front one inboard by a quarter inch and like the T-storms, the hardware is 7/16" as used on most Buells. So you've got to open up the holes in your front mounting bracket and elongate the right side one. If you go the XB (silver) route, you'll also have to mill/drill/tap your breather holes, the XB heads have the bosses but they're not prepared because the XB breathes out the top of the rocker box.