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
IHRA answers this part for 29 of its classes. 1 of those answers permit the part outright and 11 permit it with stated conditions, leaving 17 that do not. 4 of the ones that do not are classes we read as open rather than as refusing it. 17 are inferred from how the class is written rather than quoted from a rule naming this part.
This page is IHRA only, and the bodies do not agree. The same part under UIM. Which one applies to you is decided by whoever sanctions the race you enter.
Where it stands, class by class
29 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
- 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.
- 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.
Legal, with conditions 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, with conditions 1 class
Limits: min thinner 0.002 in, max thicker 0.06 in. · AA.2.5
Legal, with conditions 1 class
The rulebook's figures are unreliable here, so no tolerance is published. · N.2.6
Legal, with conditions 1 class
The base gasket may not be thicker than 0.8 mm. · 502.02.03-06
Legal, with conditions 1 class
The rulebook's figures here are unreliable, so no tolerance is published. · L.2.6
Legal, with conditions 1 class
Above 780cc the rulebook's stated tolerance is unreliable. Limits: sub 780cc min thickness 0.75 mm. · O.2.6
Legal, with conditions 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, with conditions 1 class
Model-dependent. A four-stroke entry stays within the N.2.6 head gasket thickness band, whose figures contradict each other in the rulebook, while P.2.2 lets a two-stroke entry run a modified or aftermarket head gasket. · N.2.6, P.2.2
Legal 1 class
Engine gaskets, the head gasket included, may be modified or aftermarket, with no thickness limit stated. · 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 · checks on parts the shop sells · how this site reads a rulebook.
Engine · two and four stroke · legal in 12 of 29 answered, 11 with conditions