Not permitted, not listed in the rules 3 classes
On a closed list an unlisted modification is not permitted. · A.2.2, J.1
IHRA · Engine
Aftermarket pistons, IHRA
IHRA answers this part for 29 of its classes. 7 of those answers permit the part outright and 15 permit it with stated conditions, leaving 7 that do not. 4 of the ones that do not are classes we read as open rather than as refusing it. 7 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 IJSBA and 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 2 classes
Factory compression must be maintained.
- Drag, Yamaha Pro Stock · DD.1
- Drag, Sea-Doo Pro Stock · DD.2
Legal, with conditions 2 classes
Must retain the original compression ratio and profile, with no abrasive cleaning of the ports and piston weight within 25 per cent.
- Ski 1100 Modified · R.2.1
- Ski 1500 Modified · Q.2.1
Legal, with conditions 1 class
Must retain compression ratio, dome profile, skirt length, skirt shape, material type. Limits: max port chamfer 1 mm, max chamfer angle 30 degrees. · W.2.1
Legal, with conditions 1 class
Must retain compression ratio, dome profile, skirt length and material type. Bead blast only, with no steel wool or Scotch-Brite. · AA.2.1
Legal, with conditions 1 class
Boring is allowed provided the piston keeps its compression ratio, dome profile, skirt length and material type, and chambers are bead blasted only with no steel wool or Scotch-Brite. · N.2.1
Legal, with conditions 1 class
Must retain original compression ratio, dome profile and material type. Boring and stroke modifications within displacement limits, and ports must not be abrasively cleaned. · Y.2.1
Legal, with conditions 1 class
On four-stroke entries as Runabout GP2 at Y.2.1. On two-stroke entries the piston must keep its port timing, compression ratio, dome profile, skirt length and shape and material type, a non- conforming piston needs advance written approval, and weight stays within 25 per cent. · Z.3.1
Legal, with conditions 1 class
Must retain the original compression ratio, dome profile, skirt length and material type, and the ports must not be abrasively cleaned, with piston weight within 25 per cent. · L.2.1
Legal, with conditions 1 class
Must retain port timing, compression ratio, dome profile, skirt length and shape, material type. Non-conforming pistons need written permission. Limits: weight tolerance 25 per cent, max port chamfer 1 mm, max chamfer angle 30 degrees. · O.2.1
Legal, with conditions 1 class
Must retain original port timing, dome profile, skirt length and shape, material type. Limits: weight tolerance 5 per cent, max port chamfer 1 mm, max chamfer angle 30 degrees. · U.2.1
Legal, with conditions 1 class
Model-dependent. A four-stroke entry may be bored, with the replacement piston matching the OEM compression ratio, dome profile, skirt length and material (P.1, N.2.1). A two-stroke entry follows O.2.1, which also requires the original port timing and skirt shape, a weight within 25 per cent and port chamfers of no more than 1 mm at 30 degrees, with a non-conforming piston only by written permission. · N.2.1, O.2.1
Legal, with conditions 1 class
Four-stroke entries: Limits: weight tolerance 25 per cent. BB.2.1 Two-stroke entries: Cylinders may be machined to accommodate girdle-system heads. Limits: weight tolerance 25 per cent. · BB.3.1
Legal, with conditions 1 class
Boring and stroke modifications allowed within displacement limits. · X.2.1
Legal 3 classes
Aftermarket pistons are permitted, and unlike the Pro Stock classes factory compression need not be kept.
- Drag, Yamaha Spec · DD.3
- Drag, Sea-Doo Spec · DD.4
- Drag, Super Stock · DD.5
Legal 1 class
Ring configuration may be changed. · T.2.1
Legal 1 class
Ring number, type, and placement may be modified. · S.2.1
Legal 1 class
Pistons may be replaced or bored, aftermarket ones included, provided engine displacement stays within the class limit. · K.2.1, K.3.1
Legal 1 class
Aftermarket pistons are permitted and the engine may be bored, provided displacement stays within the class limit, and two-strokes may change the number of piston rings. · V.2.1, V.4.1
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 pistons: 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 22 of 29 answered, 15 with conditions