Engine block substitution, IHRA

IHRA answers this part for 24 of its classes. 0 of those answers permit the part outright and 2 permit it with stated conditions, leaving 22 that do not. 3 of the ones that do not are classes we read as open rather than as refusing it. 21 are inferred from how the class is written rather than quoted from a rule naming this part.

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.

What each answer below means

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.
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.

OEM part required 1 class

Must remain OEM. Four-stroke entries: Internal modifications allowed. · V.4.5

Sport GP

Not permitted, not listed in the rules 18 classes

On a closed list an unlisted modification is not permitted. · A.2.2, J.1

Legal, with conditions 1 class

1101cc+: blocks interchangeable across any OEM manufacturer EXCEPT the 1500cc Kawasaki SX-R which must use the manufacturer's block. 1100cc and below (K.3.6): donor block must be 1100cc or less. · K.2.6

Ski GP

Legal, with conditions 1 class

Must originate from a production runabout watercraft. Only limitation is the cc limit. Interchanging top decks and engines permitted. · X.2.7

Runabout GP

No rule, this is an open class 2 classes

The class states only what it bans and calls itself run what you brung, against a section DD preamble that says anything unlisted is illegal.

Drag, Stand Up Unlimited · DD.10
Drag, Unlimited · DD.6

No rule, this is an open class 1 class

No restriction in section CC, and CC.1 permits any modification it does not restrict.

Freestyle · CC

Engine block substitution: 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 IHRA class · where the rulebooks differ · checks on parts the shop sells · how this site reads a rulebook.

Engine · four stroke · legal in 2 of 24 answered, all with conditions