Motorhead Memo: It's all in your head

Knowing as we do that cylinder head breathing, exhaust pipe and cam choices are the three major ways to bump power, and that the latter two have remained relatively static, I thought it might be interesting to review just how much of that improvement (and virtually all of the latent potential) in Sportsters and Buells today can be traced through developments in their cylinder heads over the years. I guess you could call it a “heads up” for hot X-engines.

All Evo Sportster/Buell heads offer combustion efficient enough that catalytic converters have never been necessary. Still, there are several iterations: 1986 only (883), 1987–2003 (883), 1986–1987 (1100cc), 1988–2003 (1200 Sportster), 1997–2003 (1200 Sportster Sport “Dual Plug”), 2004–2006 (883), 2004–2006 (1200 Sportster), 2007–to present (883), 2007–to present (1200 Sportster), 1990–1995 (Buell), 1996–1999 (Buell), 1998–2002 (Buell “Thunderstorm”/Screamin’ Eagle), and 2003–2010 (Buell).
Within a given generation, generally speaking, Sportster heads are mechanically interchangeable between the different models, i.e. all bolt holes line up and no needed features are missing. Valve geometry is the same: 27-degree intake and 31-degree exhaust for a 58-degree included angle, and the location of the valve tips is the same.
The rocker box bolt pattern has not changed across any of these generations; a 1986 lower rocker box will bolt right up to a 2006 head and vice-versa. The only thing you need to be aware of is that crankcase breathing arrangements have changed at each generation. Early models provided no facility in the head for a crankcase breather. 1988-and-later models incorporated crankcase breathers into the carb/air cleaner mount bolts, and used a different middle rocker box that had a check (a.k.a. “umbrella”) valve incorporated. Rubber-mount models use an air/oil separator type check valve unit bolted into the lower rocker box and do away with the middle rocker box, using a different top cover. Buell XB and XR1200 models have external breathers.
Heads from ’91–’03 models have been successfully fitted to ’86–’90 models. The major difference is that the spacing between the pushrod holes is different between a four-speed and a five-speed. This is because the cam box geometry is different and the pushrods are at a different angle, so there’s potential for pushrod “rub” where they pass through the heads into the rocker box.
Heads from ’04–up have been successfully fitted to ’91–’03 models and theoretically could be used on earlier models. The new heads have the right front bolt hole relocated inboard 1/4″, and also use 7/16″ fasteners as opposed to the 3/8″ fasteners used on the ’91–’03 models. You can either modify your existing front mount or, more recently, a new mount has become available that solves this issue. Also, be aware that the ’04–up heads have larger fins and will overhang ’91–’03 cylinders somewhat, most noticeable on the left side. You can use your stock ’91–’03 rocker boxes or use the ’04–up rocker boxes; either will work fine.

Buell Heads
The ’90–’02 Buell heads are all but identical to ’91–’03 1200 Sportster heads except ’95–’96 S2 models, which use a smaller 3/8″ fastener on one side and the standard 7/16″ fastener on the other.
2003–present Buell XB heads are very similar to ’04–present 1200cc Sportster heads in that they use large fins and 7/16″ front mount fasteners with the right front mount hole moved inboard 1/4″. The main difference is that XB heads do not provide head breather/carb mount bolt holes. The bosses are cast in, however, and the holes can be drilled and tapped. (Buell XB models don’t need these bolt holes due to their downdraft fuel injection system.) Crankcase breathing is provided on stock XB models through the tops of the rocker box covers.
Some specifics…
The 883 Heads (all generations) have a small volume (roughly 49cc) chamber that’s approximately 3″ in diameter to match the 3″ bore of the 883. The chamber is also shallower than the chamber in any other Sportster head. Valve train geometry is the same as the other heads except that small 1.580″ intake and 1.350″ exhaust valves are used, and the valves are longer to work with the shallower, smaller-diameter chamber.
In stock configuration these are the worst ports of any XL head, primarily due to the way the bowl area squeezes down to the small diameter valve. This arrangement causes a lot of turbulence. The heads respond well to a good porting and seat-blending job, particularly when used with a larger valve that better matches the bowl size. The seats can accommodate the standard XL1200 valve sizes (1.715″ intake/1.480″ exhaust) and special longer “conversion valves” are available to do this upgrade.
When used in an 883 to 1200 conversion, reverse dome (dish) conversion pistons are available to give a workable compression ratio. Alternatively, the chamber can be relieved and the heads used with flat-top pistons.
These heads have one advantage over 1200 “hemi” open-chamber heads in that the 3″ diameter chamber provides a “squish band” around its perimeter when placed over the larger 1200 bore. The squish band results in more turbulence and a more efficient chamber, as well as better resistance to detonation and/or pre-ignition. However, the longer valves in these heads are also a disadvantage in that piston-to-valve contact issues are much more likely. Be concerned about this if the cams you’re using have a high TDC lift figure on one or both valves. In other words, the “limit” for 883-based performance heads is big valves and big cams. All are easily identified by the “883” script cast into them adjacent to the chamber.
1986–’87 1100 heads were sand-cast and featured extra large and extra heavy (1.840″ intake, 1.615″ exhaust) valves and a “bathtub” shaped combustion chamber with squish band, offering 9.0:1 compression. In late 1987 these heads were changed to more uniform and less expensive die-cast construction and round combustion chamber and valve sizes that were continued through to the 1200 heads and valve sizes shrunk to 1.715″ intake and 1.480″ exhaust. (FYI: sand-cast 1100 heads cannot be mixed with die-cast heads!)
1988–’03 1200 heads have a 67cc hemispherical chamber, i.e. a round bowl with no squish band, which gives about 9:1 compression with flat tops at 1200cc. Hemi chambers provide maximum valve un-shrouding, but have poor chamber turbulence. Domed pistons are readily available to raise this number up to 10:1 or higher. The pistons designed for this chamber have a small radius on the edges of the domes in order to fit well inside the “hemi” chamber shape, but the irregularity of the chamber (as cast) makes it virtually impossible to achieve really good squish. Valve sizes are 1.715″ intake and 1.480″ exhaust and the stock seats can support only slightly larger sizes.

The ports on these heads are not particularly good; they have something of a squared-off bowl and a low floor. In other words, since they have turbulence in the ports and not the chamber (the opposite of a “good” head), these are areas where they need more material and a ton of reshaping and porting to be anywhere near as good as Thunderstorm or XB heads for performance applications… usually not worth the effort.
1995–’96 Buell heads are identical to the ’88–’03 Sportster 1200 heads in all respects except that one of the front mount bolt holes is 7/16″ instead of 3/8″ thread.
1996–’02 Buell heads were introduced on the Buell S1 Lightning model, as well as (in dual-plug guise) the 1200S Sportster “Sport” model—the second plug being accessible through the “chimney” hole in the top of the rocker box. Single-plug versions of this head were also sold as Screamin’ Eagle heads. Early versions were silver and carried the “Lightning” script above the pushrod area. Another version was sold in black/polished with no script. And, finally, a black/polished version was sold with the “Screamin’ Eagle” script and dual plugs.
Valve sizes, at 1.715″ intake and 1.480″ exhaust, are identical to the ’88–’03 Sportster 1200 head, as are the ports. The only place this Buell head is different from the ’88–’03 Sportster head is in the chamber. Extra material was added, as well as a 10-degree squish band, to bring the volume down to about 62cc. This gives around 10:1 when paired with stock flat-top pistons. The extra material in the chamber somewhat shrouds the valves, however, hurting low lift flow. It gets especially bad when/if oversize valves are fitted. Since the squish band does almost nothing with those flat-top pistons, any performance increase from this head comes almost entirely from an extra point in compression ratio.
1998-’02 Buell Thunderstorm heads were the second-generation high performance head—introduced in 1998 on the S1W and S3/S3T (the S1 and the M2 retained the “regular” Lightning heads in 1998). In 1999, all models came with “Thunderstorms” and it remained that way through the 2002 model year. Although this head performs substantially better than the Lightning or XL1200 head… it isn’t without issues.
Ports were improved a bunch, particularly in the bowl and the chamber reverted to 67cc of volume and used a slightly domed piston to raise the compression ratio. The advantage to this is that it un-shrouded the valves—so valve sizes were increased to 1.810″ intake and 1.575″ exhaust. A 15-degree squish shelf was incorporated into the chamber and matched to a 15-degree dome angle on the “pop-up” piston.
But… the squish band on a Thunderstorm head is still part of the casting and, as such, it’s not very accurate. There’s a good-sized overhang between the deck and the perimeter of the squish “shelf,” and core shift often makes the squish area uneven from side to side. So while it’s nice to have a squish band and the turbulence it generates, its effectiveness is limited unless you re-machine it, which generally requires 0.030″ to 0.040″ to be removed from the deck and practically mandates special pistons thereafter. Also, contrary to rumor and mystique, the springs, retainers, locks, guides, keepers and valve stem seals are all standard H-D pieces in a Thunderstorm head, the exact same part number pieces that you’ll find on every other Evolution head of that era. The Thunderstorm heads do not have any special hardware other than those larger diameter valves.
2003–’10 Buell XB and ’04–to-present rubber-mount Sportster heads all got new 7mm valve stems, a “beehive” valve-spring pack and a new seal assembly that affords more retainer to guide seal clearance and can safely accommodate up to 0.550 lift cams, as compared to 0.500 in the earlier heads.
2004–and-later 883 heads come with 49.5cc chamber volume, 1.585″ diameter intake valves, and 1.350″ diameter exhaust valves. There are no significant differences between a ’91–’03 and a ’04–up 883 head beyond the changes mentioned above. The 2007-present (EFI) 883 heads received the same intake manifold area changes described (below) for the XL1200 heads. Other than that, there are no significant changes from the 2004–2006 version.
There are some minor ones though… common to all “XB-XL” heads, as we’ll see.
A stock XB/04-up XL1200 chamber has a pear-shaped bathtub and a 62cc (nominal) volume design that creates a squish band on each side of the chamber and leaves plenty of material for cutting 15- or 30-degree angles to match angled dome pistons.
Despite the compactness of the chamber and the large (Thunderstorm-sized 1.810″ intake and 1.575″ exhaust) valves, they are not shrouded and the ports have been improved significantly with a floor that’s been raised higher than ever and an even more gradual radius through the bowl. These two changes work together to greatly reduce port turbulence, because they reduce the difference in the length of the floor and the roof, thus reducing the difference in velocity. Here’s the trick (and a bit of a tricky) part: On a typical flow bench, these heads appear to flow about the same volume as a Thunderstorm head. Thing is, this improved velocity and its related interplay with effective turbulence often adds up to a 4–5 horsepower advantage over the Thunderstorm heads… on a dyno, or in the real world!
But…
If you try XB heads on a Sportster, you’ll quickly discover that XB heads do not have the carb mount holes machined or tapped, although the bosses are there. The 2004–2006 XL1200 heads are identical to the Buell XB heads except carb mount/breather bolt holes are already drilled and tapped; no need to do it yourself or have it done.
In 2007, with the wholesale switch to fuel injection on the Sportster line, H-D changed the machined depth of the intake manifold flanges by 1/16″, as well as the bolt hole spacing. If you’re using 2007-or-later (EFI) heads on a 2006-or-earlier (carburetor) motor, you’ll need new intake manifold flanges to match. You’ll also need to take care to properly center your manifold on installation to avoid causing an intake leak.
2004–2006 XL heads do not have a provision for a head temp sensor. Buell XB heads and the 2007-and-later XL1200 heads are equipped for a temp sensor on the rear head.
2007–present (EFI) XL1200 heads are identical to the 2004–2006 version except for a small change to the floor of the intake port. The change actually makes the flow through this port slightly less laminar than the 2004–2006 head. It’s not an enormous difference, but in stock form this head is capable of supporting a few horsepower less.
As delivered from the factory, these XB/XL heads are 0.020″ shorter than the previous generation heads, at 3.670″ instead of 3.690″. Normally this is not enough to worry about with respect to pushrod length and manifold fit. However, if you’re shortening the motor further via thin gaskets or further decking, if you’ve shortened the stack by 0.045″ to 0.050″ overall—it’s something to be aware of and checked closely.
Lastly, none of these late-model stock heads are available from the factory in an assembled form. You have to buy the heads bare and then buy the valves, springs, retainers, locks, and guide seal/locaters separately and do the assembly yourself.
Or…
Buy the (assembled) Screamin’ Eagle heads—based on this XB/XL head. The SE version uses the same casting, and the ports and valve sizes remain the same. The SE version has been decked an additional 0.050″, making it a full 0.070″ shorter than an ’03 or older head. It also pushes the limit for any further decking or valve size increases, as the valve seats are right at the edge of the chamber. Be sure to use shorter pushrods with SE heads and also check valve-to-piston clearance carefully, as well as your intake and exhaust fit. A stronger spring pack is included on these heads, but unfortunately, the SE spring pack sinks the spring pockets by 0.120″. This makes the roof of the ports thinner and limits what a head porter can do with it.
Doesn’t much matter for most of us, though, because the simple fact is XB/XL1200 heads (or the SE equivalent) are the best performance heads for Sportsters and Buells ever offered by Harley-Davidson.