Not permitted, not listed in the rules 13 classes
On a closed list an unlisted modification is not permitted. · A.2.2, J.1
IHRA · Engine
Aftermarket or thicker head gasket
IHRA answers this part for 29 of its classes. 12 of those answers are permissions, with or without limits attached, and 17 are not. 17 are inferred from how the class is written rather than quoted from a rule naming this part.
Where it stands, class by class
29 classes, grouped by the answer rather than listed one by one, because an answer that holds for a class usually holds for every class that borrows its rules.
Not permitted, not listed in the rules Our reading, not a rule we can quote at you. This class requires the craft to stay stock except where the rules allow otherwise, and this part is not on the list.
No rule, this is an open class Our 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.
Legal, within limits 4 classes
Limits: thickness tolerance 20 per cent.
- Runabout Endurance · W.2.7
- Runabout GP2 · Y.2.6
- Ski 1100 Modified · R.2.6
- Ski 1500 Modified · Q.2.6
Legal, within limits 1 class
Limits: min thinner 0.002 in, max thicker 0.06 in. · AA.2.5
Legal, within limits 1 class
The source numbers are unreliable here, see KB section 9, so no tolerance is published. · N.2.6
Legal, within limits 1 class
The base gasket may not be thicker than 0.8 mm. · 502.02.03-06
Legal, within limits 1 class
UNRELIABLE NUMBERS IN SOURCE, see KB section 9, and no tolerance is to be published. · L.2.6
Legal, within limits 1 class
Above 780cc the stated tolerance is unreliable - see KB section 9. Limits: sub 780cc min thickness 0.75 mm. · O.2.6
Legal, within limits 1 class
2-stroke: cooling holes may be modified or removed; not less than 0.05 mm of OEM. 4-stroke: OEM thickness and dimensions +/-10%. · M.2.1
Legal 1 class
Two-stroke entries: Aftermarket permitted. · P.2.2
Legal 1 class
Aftermarket permitted. · S.2.6
No rule, this is an open class 3 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, 2 Stroke Unlimited · DD.12
- 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
Aftermarket or thicker head gasket: 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 · signed checks on parts the shop sells.
Engine · two and four stroke · legal outright in 12 of 29