Not legal2 parts
Only atmospheric engines are eligible, so the class admits no turbocharger or supercharger for this part to serve. 501.01.01.01
- Aftermarket intercooler · Forced induction
- Boost relief / overboost control valve · Forced induction
UIM · ski
Ski Division GP4
UIM 2026, 501.01. A closed-list class. The craft must stay stock except where the rules allow otherwise, so anything unlisted is not permitted.
Read out of the published UIM rulebook on 2026-09-08, so confirm anything that decides a purchase. The full note is at the foot of the page.
Running the class
- Displacement limit
- 1100cc four-stroke atmospheric (501.01.01). The main criteria state the capacity and the engine section carries no allowance to bore, so the figure describes the engine as the class admits it. 501.01.01, with the engine rules at 501.01.03.
- Minimum weight
- 160 kg dry.
Read from the same class section as the part answers. The rider side of the sport, tiers, ages, licences and what gets inspected, is set out across the bodies at Racing.
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.
- No overall height limit and no handle-pole 75-degree movement requirement was seen for GP4, unlike GP3, GP2 and GP1. Cannot confirm whether genuinely absent.
- No GP4 exhaust article and no GP4 fuel-system hardware article were found.
- No explicit statement on whether GP4 four-strokes may be bored - the boring permission sits under the two-stroke engine heading in other classes.
- No ballast article.
What the rules say (55 parts)
8 permitted outright, 14 permitted with stated conditions, 10 refused by a rule we can quote, 3 held to the original part by a rule we can quote and 20 refused by our reading of the class rather than by a quoted rule. Grouped where the same rule gives the same answer, so two "Not legal" groups can sit side by side under different rules.
What each answer below means
- Not legal
- This comes off. The rules refuse it in this class.
- OEM part required
- This comes off. Only the part the manufacturer fitted is allowed here.
- Not permitted, not listed in the rules
- Treat it as coming off. The class requires the craft to stay stock except where the rules allow otherwise, and this part is not on the list, so this is our reading rather than a rule we can quote at you.
- Legal, with conditions
- Keep it, but measure it: the rules permit this part only inside stated limits.
- Legal
- Keep it. The rules permit this part in this class.
Read the refusals as a list of work. Anything below marked Not legal, OEM part required, or Not permitted has to come off the machine, or go back to the standard part, before it races in this class.
Not legal1 part
ECU must be original. Aftermarket ECU prohibited. 501.01.05
- Aftermarket standalone ECU · Ignition
Not legal1 part
No internal engine modifications of any kind; ports may not be cleaned with abrasive material. 501.01.03.01
Not legal1 part
Not a listed configuration; barred by the closed list.
- Aftermarket turbocharger or supercharger · Forced induction
Not legal1 part
ANY internal modification - grinding, surfacing, polishing, machining, shot blasting - of any transmission component, including the stator and exit cone, is prohibited. 501.01.06.02
Not legal1 part
The driveline article opens only the impeller and wear rings, and bars grinding, machining or any other modification of transmission parts, the exit cone included. 501.01.06.01, 501.01.06.02
- Aftermarket pump nozzle and directional nozzle · Driveline
Not legal1 part
Nothing in the driveline article opens the couplers, bearing housing or driveshaft, and it bars any modification of transmission parts, so they stay stock under the closed list. 501.01.06.02, 501.01-02
Not legal1 part
Moot - no transmission modification at all.
Not legal1 part
Homologated production craft only - 50 specimens, marketed, conformity certificate.
- Aftermarket or prototype hull · Hull and handling
OEM part required1 part
Crankshaft must remain stock; replacement bearings or bearing shells only, of original type and dimensions. 501.01.03.02
- Aftermarket four-stroke crankshaft · Engine
OEM part required1 part
Camshaft(s) must remain stock; replacement bearings of original type and dimensions only. 501.01.03.03
- Aftermarket camshafts · Engine
OEM part required1 part
The hull may be repaired but not otherwise modified, and a repair may not alter the original configuration by more than 5 mm. 501.01.02-02
- Modification of the OEM hull · Hull and handling
Not permitted, not listed in the rules10 parts
Not listed in this class. Illegal under the class's own stock clause.
- Through-hull exhaust outlet flap removal · Exhaust
- Aftermarket fuel filler cap · Fuel
- Long-distance fuel filling system (stopper and tank connection) · Fuel
- Aftermarket charging system / stator · Ignition
- Aftermarket valves · Engine
- Aftermarket head gasket · Engine
- Replacement starter motor and bendix · Electrical
- Aftermarket nozzle trim system (pump-mounted) · Driveline
- Hull extension · Hull and handling
- Supercharger internals, clutch and pulleys · Forced induction
Not permitted, not listed in the rules1 part
The class is four-stroke only and prints no exhaust article, so the original water box stays fitted under its own stock clause. 501.01-02
- Aftermarket waterbox · Exhaust
Not permitted, not listed in the rules1 part
The class is four-stroke only and prints no exhaust article, so the original manifold and head pipe stay fitted under its own stock clause. 501.01-02
- Aftermarket exhaust manifold / head pipe · Exhaust
Not permitted, not listed in the rules1 part
Only the flame-arrester-to-throttle-body ducting allowance.
- Aftermarket intake manifold · Fuel
Not permitted, not listed in the rules1 part
No fuel-system hardware article.
Not permitted, not listed in the rules1 part
No fuel-system hardware article; treat as stock under the closed list.
Not permitted, not listed in the rules1 part
Only the ducting allowance.
Not permitted, not listed in the rules1 part
Valve spring, washer and retainer changes first appear at GP2, and Ski GP4 lists no valve train change, so the originals stay. 501.01-02, 501.01.03
Not permitted, not listed in the rules1 part
No trim article.
- Aftermarket hull trim tabs · Hull and handling
Not permitted, not listed in the rules1 part
No hood article.
- Aftermarket hood · Hull and handling
Not permitted, not listed in the rules1 part
No ballast article.
- Ballast weight kits · Hull and handling
Legal, with conditions1 part
Equivalent aftermarket air filters available on the market. 501.01.04.01
Legal, with conditions1 part
Excess oil or fuel leaving it must be caught and must not spill into the engine compartment. 501.01.04.03
Legal, with conditions1 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, with conditions1 part
The FRONT ENGINE SUPPORT may be reinforced or replaced. 501.01.02.13
Legal, with conditions1 part
Modified or aftermarket impeller provided THE ORIGINAL DIAMETER IS MAINTAINED. 501.01.06.01
- Aftermarket impeller · Driveline
Legal, with conditions1 part
Replacement wear rings within OEM internal diameter specification.
- Replacement wear ring · Driveline
Legal, with conditions1 part
Max 100 mm beyond the end of the OEM plate; extension no wider than OEM; sides connected to the radiused portion; no fins, rudders or skegs.
- Aftermarket pump cover plate / extension · Driveline
Legal, with conditions1 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, with conditions1 part
Max 2 per side. Max 126 cm each, 153 cm total with two sets. Max 100 mm lateral protrusion, 52 mm vertical channel, 50 mm below the hull side/bottom intersection, 50 mm below the lower bond flange. Minimum 6 mm thick if aftermarket.
- Aftermarket sponsons · Hull and handling
Legal, with conditions1 part
The handlebar must be padded at the mounting bracket, or the crossbar padded if fitted. Original switches required.
- Aftermarket handlebars, throttle, cable, grips · Hull and handling
Legal, with conditions1 part
Switch HOUSINGS may be modified or aftermarket and the switch position may be changed, but ORIGINAL SWITCHES are required.
- Aftermarket switches and switch housings · Electrical
Legal, with conditions1 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, with conditions1 part
Allowed. NO WATER EXIT may be added to the cylinder head, cylinder or casing. Bypass fittings must be directed downward and/or rearward. 501.01.03
Legal, with conditions1 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
The ducting between the flame arrester and the throttle body or intake manifold inlet may be modified or aftermarket. 501.01.04.02
Legal1 part
ECU must be original; SOFTWARE may be reprogrammed. 501.01.05
- ECU reflash / software reprogramming · Ignition
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
Handle pole and mounting bracket may be modified or aftermarket provided they function as originally designed. The attaching point may be reinforced. The spring may be modified or aftermarket. 501.01.02.09/.10
- Aftermarket handle pole and mounting bracket · Hull and handling
Legal1 part
Plus quick-turn steering modifications. 501.01.02.09
- Aftermarket steering shaft, holder, handlebar holder, cables · Hull and handling
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
The original bilge pump may be modified, disconnected or removed and aftermarket bilge draining systems are allowed if no hazard is created. 501.01.02.12
- Aftermarket bilge draining system · Hull and handling
Legal1 part
Cooling system flush kits are allowed in every class that addresses them.
- Engine flush / rinse kit · Cooling
UIM 2026, 501.01, read from the rulebook on 2026-09-08. Read the rule yourself, Ski Division GP4. Every UIM class · where the rulebooks differ · how this site reads a rulebook.
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.
Cite this page
“Ski Division GP4, UIM class rules”, BaddJet archive, https://baddjetarchive.com/legality/uim/ski-gp4/
Add the date you read it.
55 parts read · 8 permitted, 14 with conditions · 2026 rulebook