UIM · Driveline

Aftermarket impeller, UIM

Legal in 11 of the 16 UIM classes that publish rules, with the original diameter kept below GP2. The GP1 classes are open by this site's reading, and 8 classes publish no rule.

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This is one body's answer, not the sport's. You are bound by whichever body sanctions the event you enter. If you race more than one series, build to the stricter answer. The differences are set out in the rulebook comparison.

For every class below GP2, publish the impeller diameter next to the OEM diameter. That one number decides legality.

Where it stands, class by class

24 classes, grouped where the same rule gives the same answer rather than listed one by one, because an answer that holds for a class usually holds for every class that borrows its rules. 8 of them borrow another class’s rules and are marked below; for those, the link to the parent class is read from the categories table rather than stated in the rulebook. 8 publish no rule for this part and name no parent to borrow one from, and are grouped together at the end.

What each answer below means

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.

No rule, this is an open class

Probably keep it. The class carries no stock clause at all, so anything unlisted appears to be open. The rulebook does not say so in terms, and this is our reading rather than a permission anybody wrote down.

No rule published for this class

Unanswerable here. This class publishes no rule for the part and names no parent class to borrow one from, which is an absence in the rulebook and not a permission.

Legal, with conditions 7 classes

Original diameter maintained.

• Runabout GP4 · 502.01.06-01
• Runabout GP3 · 502.02.09-01
• Ski Division GP3 · 501.02.06.01
• Runabout Juniors GP4 · 502.01.06-01 · borrows another class
• Runabout Ladies GP4 · 502.01.06-01 · borrows another class
• Ski Juniors GP3.2 · 501.02.06.01 · borrows another class
• Ski Juniors GP3.3 · 501.02.06.01 · borrows another class

Legal, with conditions 2 classes

Modified or aftermarket impeller provided THE ORIGINAL DIAMETER IS MAINTAINED. · 501.01.06.01

• Ski Division GP4
• Ski Juniors GP4 · borrows another class

Legal 1 class

The impeller may be modified or aftermarket on both engine types, and the class's own clauses carry no original-diameter condition. · 502.03.09-01, 502.03.10-04

• Runabout GP2

Legal 1 class

Diameter constraint lifted for two-stroke; impeller, housing, stator, plate and shoe all modifiable. · 501.03.09

• Ski Division GP2

No rule, this is an open class 3 classes

No driveline article.

• Ski Division GP1 · 501.04
• Ski Ladies GP1 · 314 · borrows another class
• Ski Veterans GP1 · 302.01.05 · borrows another class

No rule, this is an open class 2 classes

No article covering this part. Free under the open-list reading of GP1.

• Runabout GP1 · 502.04
• Runabout Veterans GP1 · 302.01.05 · borrows another class

No rule published for this class 8 classes

No component rules are published for this class and no single parent class is named, so this part cannot be answered for it. That is an absence in the rulebook, not a permission.

• Endurance · 317.03
• Freestyle Amateur · 312
• Freestyle Free · 312
• Jet Raid · 317.01
• Offshore · 317.02
• Parallel Slalom · 313
• Ski Future Electric (SFE) · 302.01
• UIM Hydro-Fly · 900

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Aftermarket impeller: if the rule your class applies reads differently to you, or the rulebook has changed since this was read, ask. We answer with the section we read it from, or that we have not read it. Ask a rules question.

Every UIM class · where the rulebooks differ · checks on parts the shop sells · how this site reads a rulebook.