4597 Calle Del Media

Ft. Mohave, AZ 86426

+1 (928) 763-7600


Kawasaki STX 1100 – Big Bore Hammer Kit

Group K Modifications

This literature gives background information on a our “recreational use” Big Bore Hammer modification kit for the Kawasaki 1100 engine models, as well as some recommendations for the best bolt on parts compatible with those kits. We strongly recommend that you read this entire pamphlet before deciding which modifications are right for you and the kind of riding you intend to do. Our kits are as follows:

Stock 97 – 99 Kawasaki STX (1070cc)

51-52 mph

7100 – 7150 rpm

Group K Big Bore 1125cc Hammer

59-60 mph

7250 – 7350 rpm

Overview – Over the ’97and ’98 endurance racing season, we conducted on going testing of various prototype engine arrangements for the Kawasaki STX 1100. The earliest arrangements were simply based on our Sleeper kits (originally developed for the 2 seater Zxi 1100). Unfortunately, the big STX hull consistently runs much slower (about 5mph) than a similarly modified Zxi two seater. The larger hull and much greater water contact surface area of the 3 seater STX makes for much larger horsepower requirements. To this end, we began prototyping engine formats that employed larger bores and higher rpm levels to achieve the power we were looking for.

It didn’t take long before we had reached the “beneficial” upper limits of displacement and peak rpm. While we had initially made some significant gains in speed, there seemed to be a point at which we “hit a wall”. That is, a point where significant increases in torque and rpms were not resulting in “any” increases in peak waterspeed ability.

After spending a lot of time studying this problem from many sides, the apparent cause was the sheer thrust abilities (or rather limitations) of the stock 148mm pump design. In short, we were trying to push a very large heavy boat with a pump of limited thrust abilities. The speeds our STXs were able to attain (while very impressive for a 3 seater) were not on a par with the top Yamaha 1200 (155mm pump) race machines…and never would be, no matter how much power we generated.

While this wasn’t a pleasant realization, we still wanted to make some use of all the testing data we had gathered. With hindsight being 20/20, we realized that our “race gas STX” was likely making a lot more power than it was delivering to the water. Knowing this, we decided to see how much this arrangement could be “softened” before there was any significant reduction in peak speed or acceleration. Our subsequent testing in this direction would yield a reliable pump gas machine, that delivered performance numbers within a “hairs-breath” of our earlier 110 octane race boat.

Setting the Baseline – Under ideal conditions, stock STXs run about 52 mph @ 7150 rpm. Our best “race gas” formats ran 61 – 62 mph @ 7450-7600 rpm (depending on impeller choice and air conditions). We had always considered the rpms of our race formats to be “a little too high for our liking”. Furthermore, we seriously questioned the speed benefits that we were getting for the very apparent reliability risks we were taking on for these high rpms. As our testing showed, the significant reduction of these rpms (with a slightly steeper prop) led to a nearly insignificant loss of peak speed ability. Furthermore we regained 92octane compatibility along with a big safety margin on long term reliability. Given all this, we decided to maintain a peak rpm of no more than 7300 rpm for our BB Hammer kit.

Cylinder Porting – Cylinder porting is an absolutely mandatory modification for the BB Hammer kit. During the course of our race testing, we prototyped port layouts that used somewhat “radical” port timing combinations (in an effort to increase high range power). These formats performed very well, and offered good high range performance. Unfortunately, the high rpms generated by these layouts didn’t result in more speed…just more rpm. As a result, we chose a more conservative “broad range” port layout for our BB Hammer. By doing so we gained a big measure of low end and mid range acceleration, while giving up no peak water speed at all.

Since the cylinder displacement is increased with our modification, we must also increase the size of the exhaust port passages themselves. Exhaust port passage enlargement and manifold matching are a mandatory part of the BB Hammer porting modification, and each cylinder and manifold are prepared as a matched pair. Your exhaust manifold must be supplied along with your cylinder for the porting modification.

Our cylinder porting for the BB Hammer is available in two different finish modes. Both the “Recreational” and “Competition” finish modes include all the same port timing, port shaping, and specification work. The Competition mode also includes additional exhaust port finishing and a transfer port “rough finish” that further improves fuel atomization. This additional finishing work accounts for 25% more shop time, yet contributes to only about 10% of the total performance gain (about .5 mph and a little bit of acceleration). The recreation finish is standard in this kit, the “Competition” finish is available as an option. The cost difference…$700 vs $550. All ported cylinders are also honed.

Engine Displacements – The stroke of the Kawasaki 1100 engines is already 3mm longer than the Yamaha 1200s. Given that, and the very high cost of stroker crankshafts, we considered stroke increase to be a very unwise choice for both our endurance race engines (as well as our BB Hammer kit).

There is no doubt that the BB Hammer kit performs much better with a bore increase. However our testing did cast some doubt on the “bigger is ALWAYS better” mentality that most folks apply to displacement. The stock Kawasaki 1100 cylinder castings can accommodate over boring up to 83mm (1151cc) without resleeving. Increasing the stock 80mm (1071cc) bores to 81mm (1097cc), makes for a very big increase in overall power that is well worth the cost. Further increasing the bore diameters to 82mm (1125cc) makes for an increase that is only slightly better than the 81mm. Over-boring to 83mm (1151cc) makes for no increase at all over the 82mm. We suspect that the transfer ports in the 1100 cylinder have difficulty properly scavenging the cylinders as displacement is significantly increased. Since the 1100 cylinder casting does not have enough base material to accommodate any significant increases in transfer port area, additional port work is not an optional solution for this problem.

All in all, we found the power of the 81mm and 82 mm bores to be very comparable, and both very good. We would tend to recommend the 81mm bores so that there is room for a future overbore. For the 81mm bore diameter, either OEM Kawasaki pistons or Wisecos can be used with equal results. At 82 and 83mm the Wisecos are the only available option, and we have found them to offer excellent long term wear.

Compression – Stock 1100s typically have about 110 psi compression…that’s very conservative. While there are many builders using extremely high compression on the 1100 engines, none of our combinations gave best results with indicated compression over 160 psi. Our endurance racing packages, in particular, yielded much better overall acceleration with compression just below 160 psi. Unfortunately, those higher compression ratios do not allow for the use of 92 octane pump gas when revved into the 7450 – 7600 rpm range. However when we reduced the peak rpms of the BB Hammer kit, the reduced temperatures once again allowed for the safe use of 92 octane pump gas. This reduction in peak rpm would normally result in a loss of peak speed. However the additional torque provided by the milder port layout, along with these compression ratios, allows this STX to pull a steeper pitch impeller that makes up the speed difference.

Inlet Systems – The ’98 Kawasaki 1100’s all come with the new “CV” type carburetors. These carbs differ from all other pwc carbs in that they have a vacuum operated slide (as in many street motorcycle carbs) in addition to the conventional butterfly. This design offers big advantages with respect to emissions and fuel economy. Unfortunately, these carbs also use jets that are very unavailable, and very unlike anything else. Since these CV type carbs do not lend themselves very well to high performance applications, the BB Hammer must be fitted with different carbs. We got the best results with two different setups:

Modified 96/97 Zxi/STX Carbs – The carbs that came on the earlier model 1100s were conventional “pwc design” type Keihin 38’s. These earlier carbs bolt right on to the later style manifolds and accommodate the same flame arrestors. To purchase a set of these carbs brand new is a bit pricey. However there are plenty of sets sitting on the garage shelves of 96/97 owners that installed other aftermarket carbs. Most of out BB Hammer owners obtain their carbs from those owners.

While the butterflies of these older 96/97 carbs are 38mm, the actual throat diameter, in the center of the carb, is about 28mm. For our engine kits, we bore the throats of these carbs to help feed the modified 1100 engine. These bored carbs offer instant acceleration from all speeds, along with easier starting and smoother idling. In addition, the fuel economy of these modified carbs is virtually the same as the stockers.

42 Keihin Carb Kit – For owners less concerned with fuel economy, The Aqua Sports 42mm Keihin carb kit is also an effective option. Like the modified 96/97 carbs, these 42’s can bolt directly on to the stock inlet manifold (however the manifold must be opened up slightly). The 42’s have a larger top diameter that requires the use of different flame arrestors (we recommend the R&D Pro Lock arrestor).

On our full blown high rpm race engines, the performance difference between the 42’s and the bored 96/97 carbs was very noticeable (in the high range). However on the lower rpm (pump gas safe) BB Hammer kit, the performance difference between the two sets is much smaller.

All the Kawasaki 1100 engines come stock with reed stuffers and fiber reed petals, so we recommend only the stock inlet system parts. We conducted much of our testing (on the bored 96/97 carbs) using the stock flame arrestor with outstanding results. However the 42’s still require the R&D arrestor.

Ignitions – During all our testing, we found the reliability of the stock ignition components to be excellent. The 8000rpm limiter of the stock ignition is high enough to be out of the way at all times…that is, no rev limiter mod is needed. The only ignition modification that we consider mandatory is the advancing of the ignition timing. There are aftermarket “advancer plates” available that offer seven degrees of additional advance though the entire rpm range. These plates worked well on some of our race gas formats, however we had great difficulty controlling operating temperatures (with this much advance) on our pump gas BB Hammer. Our best results came from the more conservative 3 degree spark advance that is available by modifying the stock wiring. (instructions for this are included with each kit, please do not call our technicians for this info).

Exhaust System – There are several aftermarket exhaust systems available for the Kawasaki 1100 machines. Most of these pipes do not increase torque, and so they rely heavily on big increases in peak rpm to generate their power increases. This high rpm bias does not make these pipes “bad”, but it does make them bad for our particular modification arrangement on the STX hull. The same holds true for the aftermarket waterbox/mufflers we tested (intended to replace the stock unit). On the BB Hammer kit, the stock pipe and waterbox offered the best overall performance results. Our kit does include a modification to the plumbing of the stock exhaust pipe, and some internal modifications for the waterbox/muffler as well. The info for these mods is also included with each kit.

Lower End Modifications – There are no mandatory lower end modifications required for the BB Hammer to perform properly. Truing and welding of the stock crankshaft is a good idea for owners that will be doing endurance racing, however none of our un-welded test boat cranks experienced any difficulties.

The transfer port bases on the crankcases of the 1100 motor mismatches the ports on the cylinders very badly. Despite this mismatch, the power increase that comes with case porting and matching is not “huge”. Just the same, case porting and matching is advisable for the owner who is looking to get “every performance advantage” that’s available from the big Kawasaki motor. This case porting requires that we have both the cylinder and the top crankcase half.

All of our BB Hammer testing was conducted with the stock oil injection in place. This system always worked flawlessly, and we consider it safe to utilize with the rpm increase at hand.

Pump Options – The high rpm engine arrangements of our endurance race machines yielded the best peak water speeds with the short overlapping Nu Jet or Solas Concord impellers. However with the terrific torque of the BB Hammer kit, the rough water hook-up of these props (or lack there of) is in a category that we considered “unacceptable”. After much testing, we made huge inroads toward reducing the rough water cavitation by using the Skat Trak 14-20 Swirl design impeller. Ordinarily the outstanding “hook-up” of these Swirl design impellers comes along with a noticeable decrease of peak water speed ability. However the BB Hammer STX is the first machine we’ve seen that accepted a swirl prop with no peak speed loss at all (we don’t know why, but we don’t question it).

For all the improvements offered by the Skat Trak Swirl prop, the rough water cavitation was still somewhat noticeable under high-speed rough-water operation. This last amount of cavitation can be almost completely eliminated by the installation of a “Worx Racing” top loader scoop grate. Here again, top loader grates are normally notorious for causing losses in peak water speed. The Worx top loader does account for a .3 mph loss of speed on smooth water conditions. However all our test riders agreed that .3 mph was a small price to pay for the huge gain in rough water hookup (and the increase in rough water speed).

For owners looking to get every bit of rough water hook up that’s available, we can also “blueprint” the pump of the 1100’s. Pump blueprinting eliminates all the casting drafts and manufacturing imperfections. This blueprinting does not increase peak speed on smooth water, however it is a noticeable benefit in high speed rough water conditions.

Handling – From the first day of testing, all our test riders agreed that the STX hull had a very “nose heavy” attitude in the water. As the horsepower and speeds of our test boat increased, so too did the punishing ride of this nose-heavy characteristic. Besides making the rough water ride even rougher feeling, this nose-heaviness also scrubs off a considerable amount of peak water speed by significantly increasing the wetted surface area of the hull. To resolve this problem, Group K offers a combination of a pump nozzle wedge and a “negative angle” aftermarket ride plate. Both of these mods serve to literally lift the nose of the hull off the water. Installing the nozzle wedge alone is a quick and easy modification. Unfortunately there is no aftermarket plate available for the STX, so we chose to machine the needed angles on to the stock ride plate. All our test riders said that these two modifications made the biggest single improvement in the speed and “ride” of our STX test boat. Besides the acceleration and peak speed improvements that came with the reduced water contact area, the rough water ride became “much” smoother and more manageable. Often this type of change in the ride plate and nozzle angle reduces the precision or responsiveness of steering, however the STX hull (with these two mods) turns easily and predictably, as well as holding high speed turns with impressive security.

The only down side to this ride plate modification is that the reverse system cannot be retained. The ride plate mounting points for the “reverse-bucket” become dangerously weakened to the point that reverse engagement would not be safe. Even with this, all our test riders agreed that the loss of the reverse was a small price to pay for the huge improvement in overall handling and ride.

Hull Finishing – When you take delivery of any new machine, the bottom surface of the hull is very smooth and shiny. While this finish looks real nice, it is totally non-functional. The ideal bottom surface finish is a non-shine finish created by numerous full-length front to rear “scratches”. The best way to accomplish this finish is with a piece of very coarse emery (40 – 60 grit is best). We use sanding disks (on hand not a disk sander) because the disk pads easily accommodate the radiased curves of the hull chines. All the sanding strokes should be front to back (continuous) for the full length of the hull. The deep, full length, scratches should eventually eliminate any part of shiny surface. While this preparation may not look attractive, these scratches will act as thousands of small rudders that will make the hull track “a lot” straighter in all water conditions. These scratches will also allow for much better surface holding in high-speed turns.

We consider this preparation to be mandatory for our Hammer kit machines. The added nose lift, offered by our ride plate modification, means that there is slightly less “steering” hull contacting the water surface. A shiny hull, at this attitude, can tend to “seek” at high speeds. This sanding preparation allows the same machine to run arrow straight at peak speed, along with much more responsive steering control.

Assembly Information – All Group K kits are accompanied by a step by step instruction pamphlet that outlines the assembly, break-in, fine tuning, and maintenance of your kit. For any further questions you may have regarding your kit, you’re welcome to contact us directly for assistance.

Big Bore Hammer Kit

Group K Price

Big Bore Hammer Top End Modification Set
Includes cylinder boring, piston assemblies, recreation finish porting, head modification, enlarged diameter head gasket, and enlarged diameter exhaust gaskets.


Competition Finish Cylinder Porting Option


Cylinder Head Modification


Optional Crankcase Cylinder Porting & Matching


96/97 Carb Throat Boring & Jetting (include cable bracket for modification)


42 Keihin Carb Kit (includes carbs, linkage)


42mm Inlet Manifold Modification


R&D Pro Lock Arrestor Kit (for 42 Keihin carbs)


Primer Kit (for all carb kits)


Driveline Labor & Parts

Group K Price

Pump Blueprinting (send vain case and impeller housing)


SKAT TRAK “Swirl” Impeller 14-20


Worx Toploader Scoop Grate


Group K Ride Plate Modification (send ride plate and steering nozzle of STX)


Group K Nozzle Wedge



Group K Price

Cylinder Boring, Hone and Chamfering (one oversize)


*prices subject to change based on manufactures pricing


GROUP K • 4597 CALLE DEL MEDIA • FORT MOHAVE, AZ. 86426 • (928) 763-7600

GETTING THE WORK DONE – Most customers send GROUP K the parts needed for modification via UPS, and then do the engine assembly work themselves. We also do complete engine and pump assemblies for customers who want a finished unit ready for installation. The 150-lb. UPS weight limit makes engine shipping practical and affordable. NOTE: Group K will bill an additional $25.00 handling charge for complete engine assemblies. All orders prepaid with a cashiers check or money order will be returned freight free via ups ground service anywhere in the continental United States. All other orders will be billed to a visa/master card or sent freight collect cod cash. If you would like to pay additional for 3 day, 2 day, or 1 day return shipment, please specify your preference in a cover letter with your parts. Be sure to include your return address and day phone information in case we have any questions regarding your order. PACK YOUR PARTS CAREFULLY !!