High Performance Factory Heads are Available from HAMMER PERFORMANCE!
HD has offered a lot of heads for XL & Buell models over the years, some good and some not-so-good. If you're looking to upgrade to the best ones,
Hammer Performance can set you up. We offer factory performance heads either as bare castings with stock seats and guides (which make a great basis for your
Smash or Sledge porting service), or fully assembled and ready to bolt-on.
Buell Thunderstorm Heads - No Longer Available
As of July 2013, we're having trouble sourcing new Thunderstorm heads. The last bikes that had them rolled off the assembly line 11 years ago, so this is no
big surprise. We'll leave the information below for reference purposes, but we can no longer supply new Thunderstorm heads
Buell Thunderstorm heads were the first real performance head for XL's offered by the motor company. They came out in 1998 on Buell S1W and S3 models,
and instantly became enormously popular as an upgrade for XL's and other Buells.
The improvements over the previous XL heads came in the form of much larger valves, improved ports, and an improved chamber. Valve sizes were increased from
the earlier 1.715" intake & 1.480" exhaust all the way up to 1.810" intake & 1.575" exhaust. This alone results in dramatically better flow. But the ports were
improved as well, with a higher floor and a more gradual radius in the roof through the bowl area. Finally, the chamber was improved to add a 15 degree squish
band while not shrouding the valves terribly, and the heads were paired with an all new matching piston. Squish band effectiveness is limited, however, by
the cast-in squish shelf, which is uneven and not very accurate. Hammer Performance can machine this area for a precise and effective squish band.
Retrofitting these heads to 91-03 Sportster models is very straightforward, as they are almost a bolt-on. The only issue is that the front head's front mount
bolts are 7/16-14 thread, instead of the 3/8-16 as found on 91-03 Sportsters. Some drilling of your front motor mount bracket will likely be necessary, and you'll
need to fit a short stud onto the left side (spark plug side) or use a very short bolt.
Retrofitting these heads to 86-90 Sportster models is not recommended, however, plenty of people have done it. The main issue is the spacing of the pushrod
holes. Four-speeds have slightly different geometry because the tappets are offset from the cams slightly. When you put
five speed heads onto a four speed,
you risk causing a pushrod rub problem where the pushrods pass through the heads. The same bolt hole size issue described above also exists
when retrofitting these to four-speeds.
Retrofitting these heads to 04-up Sportster models is recommended even less. First off, if your 04-up Sportster is a 1200, you already have better
heads than these, so swapping them would be a downgrade. But even if you're looking for something better than your 883 heads, Thunderstorms are generally not
the best choice for an 04-Up. Thunderstorm heads have smaller fins than the 04-Up Sportster head & cylinder fins, and they may look funny sitting over
the top of your late model cylinders. Also, the front head's right front mount hole (pushrod side) sits 1/4" more outboard on a Thunderstorm head than on
a late model XL head, so a mount modification would be needed. It's not impossible to put these on a late model XL, but most people opt for the Buell XB
or late model XL1200 heads instead.
Thunderstorm heads are available in a straight black finish only. The fins can be highlighted manually but this seldom comes out looking as good
as a factory highlighted fin. If you're looking for highlighted fins, we recommend going with some 2004-Up XL1200 heads as listed below. If you're looking
for silver, you can either paint a set of Thunderstorms, or go with a set of XB heads listed below. You could strip and powder coat a set of
Thunderstorms, but by the time you got done, you'd be into them about as much as a set of XB heads.
Thunderstorm heads are available from HD either bare or fully assembled. However, as of their latest price increases, they're charging the same amount
either way. Therefore we are only offering them in assembled form.
Despite the introduction of the Buell XB and 2004-Up XL1200 heads, Thunderstorms remain popular.
The bikes that Thunderstorms came on as original equipment are long out of production, though, in fact Buell doesn't even
exist anymore, so who knows how long these heads will be available. If you want a set, get them while you can.
Buell XB & 2004-Up XL1200 Heads
The best factory heads for high performance are the 2003-Up Buell XB and 2004-Up XL1200 heads. These heads feature improved ports over any of the previous
factory heads, with large 1.810" intake and 1.575" exhaust valves featuring lightweight 7mm stems and a beehive spring pack that can handle up to .550 lift
(previous XL heads were only safe to .500 lift). Although they flow about the same as the Thunderstorm heads, they do it with a smaller port and the flow is
less turbulent, both of which result in more power. In our testing, comparing stock to stock, these heads are worth 4-5 extra horsepower over Thunderstorm heads,
in situations where the heads are the thing limiting the power. Stock Thunderstorm heads start becoming a big bottleneck at around 90hp and these heads
in stock form start becoming a bottleneck at closer to 95hp.
Another improvement in these heads over the Thunderstorms is the compact 62cc bathtub chamber. This chamber gives a nice, even squish area when used over flat top
pistons (for a 9.7:1 compression ratio) or our premium rectangular dome Sledge Hammer forged pistons (for a 10.5:1 compression ratio). They also provide
plenty of material for machining angled squish bands, which improves performance even more by reducing the shelf the air must flow around when entering and exiting
the cylinder, as well as by directing the fuel coming out of the squish band more directly at the flame front. Hammer Performance offers both 15 and 30
degree angle dome Sledge Hammer forged pistons and can CNC machine the heads to match. These heads work especially well with our
30 degree reverse dome pistons described above.
Be aware that there was a significant change beginning in 2007 when the factory went to fuel injection on the Sportsters. 2007 and newer XL heads have their manifold mating
surface milled down about .062" versus the carburetor heads, and also they have the manifold flange bolts moved apart to accommodate the larger EFI throttle body, with wider
manifold flanges to match.
The 2006 and earlier earlier XL heads, made for a carb manifold, cannot be used with an EFI throttle body without significant machine work. The 2007-up heads however
can be used with a carburetor by using a
special wider intake manifold available from HAMMER PERFORMANCE.
Recently some of the replacement carb style heads from HD have been coming with the wider bolt spacing to
match the EFI style manifold flanges. However, the manifold surfaces have not been milled down and a standard carb manifold still fits fine.
Buell XB heads have manifold surfaces for a carb type intake manifold, so they cannot be used with the XL's EFI throttle body.
The black versions however use the new style manifold flanges, so be sure to pck up a pair of flanges if you don't already have them.
Also, be aware that Buell XB heads as delivered do not have the breather bolt/carb mount holes drilled and tapped. However, if you buy the heads from HAMMER PERFORMANCE
and let us know you'll be using them with a carburetor, we will machine and tap the holes for you for no charge. This is one advantage to buying these heads from
Since all Buells are now out of production, we can't be sure how much longer XB heads will be available. If you're thinking about getting a set, this might
be the time.
Which Heads to Choose?
Use this guide to help choose which heads are right for your bike:
See Note 2
See Note 1
in Black Highlighted
Note 1: The EFI heads will also work in this application when combined with a
special wider intake manifold available here.
Retrofitting late model heads to 91-03 XL models is not difficult and many people have done it with great results. As with the Thunderstorm heads, you have to deal
with the larger 7/16-14 front mount bolts. You also need to deal with the fact that the right front mount bolt is relocated inward 1/4", but this is easily
resolved with an inexpensive front mount available below. Also, be aware that your head fins will overhang the cylinder fins somewhat, although
most people don't find it objectionable. Finally, be aware that these heads are .020" shorter than stock 91-03 heads, which by itself should not cause an issue.
However, if you're lowering your stack further with head milling and/or thin gaskets, you need to factor this in when considering pushrod length.
Read more about
pushrod length considerations here.
Retrofitting these heads to 86-90 (four speed) models is not recommended for the same reasons cited above for Thunderstorm heads, primarily the valvetrain
geometry and the potential for pushrod rub issues.
These heads are not available from the factory in an assembled form, you must buy the heads and all the componentry separate and do the assembly
yourself. However, Hammer Performance will sell you the heads completely assembled and ready to install for the same price as all of the pieces cost
when buying from your dealer. We'll even upgrade the valves to a set of our premium quality stainless steel swirl polished Sledge Hammer valves at
no extra charge!
|Buell XB Heads - Pair
|Buell XB Heads - Pair
Note: wider manifold flanges required, order below
|2004-2006 Sportster 1200 Heads - Pair
Black with Highlighted Fins
|2007-Up Sportster 1200 Heads - Pair
Black with Highlighted Fins
|2007-Up Sportster 1200 Heads - Pair
|Intake Manifold Flanges - $13.98/Pair
for 2007-Up Sportster Heads
|Front Mount Bracket - $14.99
(to Adapt XB/04-Up XL1200 heads to 86-03 Models)
|Machine Angled Squish Band
and Size Chamber -
|Machine/Drill/Tap XB Head Breather Bolt Holes