1991-2003 (5-Speeds) and 1995-2002 Buell
For 1991, the Evolution Sportster got a new 5-speed
transmission. HD also made changes to the valvetrain and
crankcase breathing that affected the cylinder heads. The
cams were relocated to sit directly under the lifters (4
speeds had them slightly offset) and the holes in the head
where the pushrods pass through were moved to match. Also
the crankcase breather was moved from the cam box cover to
the bolts in the heads that had previously just held on the
carb and air cleaner. This creates two issues when fitting
5-speed heads to a 4-speed:
Since the holes for the pushrod tubes are in a
slightly different place, there's a potential for
pushrod rub when 5-speed heads are put on a 4-speed or
vice-versa If you're going to take advantage of the 5-speed
crankcase breathing, your 4-speed lower and middle
rocker boxes will need to be swapped to 5-speed
versions. Likewise if you're putting 4-speed heads onto
a 5-speed, you'll need to modify your heads to allow
breathing out the carb bolt holes (Hammer Performance
can provide this service) or alternatively, use XB style
rocker boxes.
This above picture shows the differences between a 1991-2022 (5-speed) Sportster head (top)
and a 1986-1990 (4-speed) Sportster head (bottom).
91-03 XL883 Heads
49.5cc chamber, 1.585" diameter intake valves, 1.350"
diameter exhaust valves
There are no significant differences between an 87-90 and
a 91-03 883 head other than the pushrod hole locations and
the crankcase breather provisions as described above.
91-03 XL1200 Heads
67cc chamber, 1.715" diameter intake valves, 1.480"
diameter exhaust valves
There are no significant differences between an 88-90 and
a 91-03 1200 head other than the pushrod hole locations and
the crankcase breather provisions as described above.
This is one of the most common Sportster heads you'll
come across, as it came on every 1200 Sportster from 1991
through 2003, except the 98-03 XL1200S model. Unfortunately,
for performance applications it's not a particularly good
head, as the ports are turbulent and the chamber is not,
which is just exactly the opposite of what you want. See the
pictures below for more information.
A stock 88-03 1200 head has a 3.5" diameter open hemispherical chamber and a 67cc nominal volume
The lack of any squish band results in poor chamber turbulence and causes detonation issues at high compression ratios
For this reason, many people prefer to prepare their 883 heads when doing an 883 to 1200 or 1250 conversion,
rather than convert to this vintage of 1200 heads.
This picture also shows the very squared off roof area through the bowl in both ports.
Notice the sharp angle near the stamped numbers.
The stock intake port on an 88-03 XL1200 head is essentially the same
as the 883’s intake port, except the bowl does not have to squeeze
down to such a small valve. This is an improvement but it’s still not
a very good port due to the low floor and squared off roof through the bowl.
95-96 Buell S2 Heads
67cc chamber, 1.715" diameter intake valves, 1.480"
diameter exhaust valves
These heads were identical to the 91-03 XL1200 heads
described above except that the spark plug side front motor
mount bolt hole was 7/16-14 instead of 3/8-16. So be aware
that not all Buell heads are higher performance than all
Sportster heads.
96-03 Buell Lightning Heads
62cc chamber, 1.715" diameter intake valves, 1.480"
diameter exhaust valves
In 1996 a new cylinder head was introduced on Buell S1
Lightning models. They were standard equipment on 1996-1998
S1 models (not S1W), 1997 S3 models, 1997-1998 M2 models,
and 1998-2003 1200S Sportster Sport models. The Buell Blast
uses a version of this head too. They were also available in
the Screamin Eagle catalog, initially with a "Lightning"
script in the fin area just below the rocker box, and later
with a "Screamin Eagle" script. Versions were offered in
silver as well as black with polished fins. Many of the
black with polished fins versions (including those on the
1200S) had dual spark plugs as well. Buell versions of these
heads use 7/16-14 bolt holes for the front mounting bracket
while Sportster versions used 3/8-16.
The Lightning head has the same ports and valve sizes as
the XL1200 heads of the same vintage and as such they flow
exactly the same. The Screamin Eagle catalog at one time
claimed 8% more flow but I've flow tested lots of them and
the range of results I get is the same as the standard 88-03
XL1200 heads.
The one big difference in a Lightning head is that extra
material was added to the chamber to reduce the size and
raise the compression. Over a 1200cc flat top piston these
heads give about 10:1 compression. The chamber material is
cast into a 10 degree squish band, however, all of the bikes
that had these heads from the factory used flat top pistons,
so the squish band was ineffective. Hammer Performance
offers an angled dome piston to match these heads and can
even machine the squish band into a nice, even shape for
maximum effectiveness - as cast, it's very rough and uneven.
The big down side to a Lightning head is that the extra
material used to make the chamber smaller also ends up
shrouding the valves badly. This hurts low-lift flow, and is
especially bad when used with larger valves. When we do a
SMASH or SLEDGE porting service to a Lightning head, we
always unshroud the valves heavily to cure this problem.
98-02 Buell Thunderstorm Heads
67cc chamber, 1.810" diameter intake valves, 1.575"
diameter exhaust valves
In 1998 a new cylinder head was introduced on Buell S1W
and S3 models. In 1999 these heads were made standard
equipment on all Buell models and they remained that way
through the 2002 model year.
The Thunderstorm head has multiple improvements over the
Lightning head. Not only are the valves significantly
bigger, but the ports were improved as well, with a higher
floor and a more gently radiused roof through the bowl area.
The chamber was enlarged back to 67cc to unshroud the larger
valves, and the squish band angle was changed to 15 degrees.
These were paired with a factory piston to match and utilize
the squish band, which HD had never done on Lightning heads.
A stock Thunderstorm chamber improves on the Lightning chamber by unshrouding the much larger valves.
The squish band is also taken out to 15 degrees although it's still a rough, uneven casting with a lot of overhang
around the perimeter, making it difficult to achieve a good squish clearance.
The unshrouding resulted in a 67cc volume and requires a matching domed piston to get 10:1.
Also, you can see in this picture how the roof through the bowl area is more gently radiused,
as compared to the picture of the XL1200 chamber above. Look near the stamped numbers and compare.
The stock Thunderstorm intake port improves on the Lightning & 88-03 XL1200
intake port with a somewhat higher floor and more gentle radius
through roof area in the bowl. Valve sizes were increased as well.
These are very popular heads to retrofit to Sportster
models. They're almost a bolt-on; the only issue you
have to deal with is that the front motor mount bolt holes
are 7/16-14 instead of the Sportster's 3/8-16. It's
generally fairly simple to drill out your factory motor
mount however.
Contrary to what you might hear, the springs, retainers,
locks, guides, locaters, and valve stem seals are all
standard HD pieces in a Thunderstorm head, the exact same
part number pieces that you'll find on every other Evolution
Sportster and Big Twin head of that era. The Thunderstorm
heads do not have any special hardware other than the larger
diameter valves.
Thunderstorm Heads are
available direct
from Hammer Performance.
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