The rules do not say1 part
No boost article.
- Boost relief / overboost control valve · Forced induction
UIM · ski
Ski Division GP1
UIM2026, 501.04. An open class. It carries no clause requiring the craft to stay stock, so anything the rules do not restrict appears to be permitted. That reading is ours rather than the rulebook’s.
Places where the rulebook contradicts itself, or where a figure could not be settled. Nothing in this list is decided, and it sits here rather than in a footnote because it is what you would want before spending money.
- The 1630 cc four-stroke figure conflicts with 1620 cc in the 302.01 table.
- 501.04 is silent on engine internals, exhaust beyond turbo water cooling, ignition, carburetion, pump internals, handlebars, seat and hood. The natural reading is that unlisted areas are FREE, but the rulebook does not say so in terms.
- No minimum rider weight or combined weight is stated.
Confirmed absent by a targeted read rather than merely unnoticed. In a closed-list class an absent article is why a part reads as not permitted; in an open class it is why one reads as open.
- engine internals
- cylinder and head
- porting
- carburetion
- fuel injection
- reeds
- ignition and ECU
- pump and impeller and nozzle
- handlebars
- seat
- hood
What the rules say (66 parts)
16 permitted outright, 48 answered by our reading of the class rather than by a quoted rule, 50 the rules do not settle. Grouped by the answer, because an open class gives one answer to most questions and printing it once per part would say the same sentence forty times.
No rule, this is an open classOur reading, not a rule we can quote at you. This class carries no stock clause at all, so anything unlisted appears to be open. The rulebook does not say so in terms.
The rules do not say1 part
Not restated for GP1. Whether the prohibition carries over into an open class is not stated.
Legal, within limits1 part
Free, EXCEPT that on forced-induction craft the exhaust manifold must be of the full circulating water type at all times when the engine is running. 501.04.07
- Aftermarket exhaust manifold / head pipe · Exhaust
Legal, within limits1 part
Fuel tanks may be modified or aftermarket. The filler neck must be located outside the engine compartment and the tank must present no danger to the pilot or other users. 501.04.02.08/.09
Legal, within limits1 part
Must fit the OEM battery box and be securely fastened. GP2 additionally allows relocation of the battery, box or housing limited to the strict minimum needed to fit an aftermarket exhaust.
- Replacement battery · Electrical
Legal, within limits1 part
Turbocharged four-stroke up to 1000 cc. Turbo housing AND exhaust manifold must be full circulating water type at all times when the engine is running. NO BOOST LIMIT IS PUBLISHED. 501.04.01.01, 501.04.07
- Aftermarket turbocharger or supercharger · Forced induction
Legal, within limits1 part
Max 100 mm beyond THE END OF THE PUMP ITSELF. 501.04.02.04
- Aftermarket pump cover plate / extension · Driveline
Legal, within limits1 part
Full-length type with at least one bar parallel to the drive shaft, all leading edges radiused, extending no more than 12.00 mm below the flat plane of the pump intake area. GP1 adds that the projecting member must be filed so as not to create a hazard.
- Aftermarket intake grate · Driveline
Legal, within limits1 part
Max 2 sets. Max 153 cm each, 210 cm total. 501.04.02.02
- Aftermarket sponsons · Hull and handling
Legal, within limits1 part
Permitted. May not exceed the width of the planing surface or extend more than 100 mm rearward beyond the transom. All edges radiused. No fins, rudders or skegs. 501.04.02.05
- Aftermarket hull trim tabs · Hull and handling
Legal, within limits1 part
PROTOTYPES EXPRESSLY PERMITTED via a UIM-issued logbook, valid 4 years, presented at every event. GP2 and GP3 homologated craft are also eligible. 500.02.02
- Aftermarket or prototype hull · Hull and handling
Legal, within limits1 part
75-degree movement requirement applies. No other pole article. 501.04.01.06
- Aftermarket handle pole and mounting bracket · Hull and handling
Legal, within limits1 part
Replacement bumpers allowed. A soft flexible water-spray deflector may attach to the hull sides or bond flange, not exceeding the perimeter of the original bumpers or the external hull edge measured with a plumb line. SAFETY BUMPERS ARE COMPULSORY in both GP1 classes.
- Replacement bumpers and water-spray deflectors · Hull and handling
Legal, within limits1 part
Constant mass only, within the normally exposed areas of the hull, no fluids, requiring no modification or relocation of other parts. Weight control is performed with an empty fuel tank and all other liquids at race level. 501.04.05/.06
- Ballast weight kits · Hull and handling
Legal, within limits1 part
All cooling system valves must be of FIXED OR AUTOMATIC type - thermostats, pressure regulators, solenoids. Manually controlled devices that alter cooling water flow while under way are prohibited in every class, as are electronic water injection systems unless of origin.
Legal1 part
Aftermarket spark plugs of a different heat rating are allowed in every class that addresses them.
- Aftermarket spark plugs of a different heat rating · Electrical
Legal1 part
Padding and mat kits may be added and custom painting is allowed. Metal components above the bond flange may be polished, shot peened or painted.
- Padding, mat kits and custom paint · Hull and handling
Legal1 part
Cooling system flush kits are allowed in every class that addresses them.
- Engine flush / rinse kit · Cooling
No rule, this is an open class31 parts
No article covering this part. Free under the open-list reading of GP1.
- Through-hull exhaust outlet flap removal · Exhaust
- Carb jet kit, needle valve, needle valve spring · Fuel
- Aftermarket or modified rotary valve · Fuel
- Aftermarket intake manifold · Fuel
- Ducting between flame arrester and throttle body / compressor · Fuel
- Aftermarket crankcase vent catch-can / blow-by oil system · Fuel
- Aftermarket electric fuel pump · Fuel
- Aftermarket fuel filler cap · Fuel
- Long-distance fuel filling system (stopper and tank connection) · Fuel
- Aftermarket vapour / air separator · Fuel
- Aftermarket or modified throttle body · Fuel
- Aftermarket flywheel or alternator (2-stroke) · Ignition
- Aftermarket charging system / stator · Ignition
- Aftermarket cylinder (2-stroke) · Engine
- Aftermarket cylinder head (2-stroke) · Engine
- Aftermarket piston assemblies · Engine
- Crankshaft rebuild components (counterweights, pins, rods, bearings) · Engine
- Aftermarket four-stroke crankshaft · Engine
- Aftermarket camshafts · Engine
- Aftermarket valve springs, washers and retainers · Engine
- Aftermarket valves · Engine
- Aftermarket head gasket · Engine
- Port chamfering service · Engine
- Replacement engine mounts / engine bed · Engine
- Replacement starter motor and bendix · Electrical
- Replacement wear ring · Driveline
- Aftermarket pump nozzle and directional nozzle · Driveline
- Aftermarket driveshaft, couplers, bearing housing · Driveline
- Aftermarket nozzle trim system (pump-mounted) · Driveline
- Aftermarket switches and switch housings · Electrical
- Aftermarket bilge draining system · Hull and handling
No rule, this is an open class3 parts
No article.
- Aftermarket waterbox · Exhaust
- Aftermarket reed valve assembly · Fuel
- Aftermarket intercooler · Forced induction
No rule, this is an open class2 parts
No induction article.
No rule, this is an open class2 parts
No ignition article.
- Aftermarket CDI / ignition box (2-stroke) · Ignition
- ECU reflash / software reprogramming · Ignition
No rule, this is an open class2 parts
No driveline article.
No rule, this is an open class1 part
No exhaust article beyond the turbo water-cooling rule. Free under the open-list reading.
No rule, this is an open class1 part
No carburetion article.
No rule, this is an open class1 part
No ignition article at all.
- Aftermarket standalone ECU · Ignition
No rule, this is an open class1 part
No engine article.
No rule, this is an open class1 part
No hood article.
- Aftermarket hood · Hull and handling
No rule, this is an open class1 part
No steering article.
- Aftermarket steering shaft, holder, handlebar holder, cables · Hull and handling
No rule, this is an open class1 part
The handlebar padding requirement of the lower classes is NOT restated for GP1.
- Aftermarket handlebars, throttle, cable, grips · Hull and handling
No rule, this is an open class1 part
No cooling article beyond the turbo water-jacket rule.
UIM 2026, 501.04, read from the rulebook on 2026-09-08, not verified by a racer. Read the rule yourself, Ski Division GP1. Every UIM class · where the rulebooks differ.
This is one class. Every part below also has a page of its own showing where it stands across every class this body runs, and legality by part is the index of them.
66 parts read · 16 permitted outright · 2026 rulebook