Class rules

Vintage Ski against Vintage Ski Limited, the same ski and opposite answers

Two IJSBA classes run the same Kawasaki 550 cases in the same hull and disagree on twenty three components. It is the clearest demonstration on this site that a part is not legal or illegal on its own, only in a class.

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Same cases, same hull, two rulebooks

Both classes require original Kawasaki 550 engine cases. Both require the hull and deck to be what the manufacturer furnished. The Limited class states its own purpose as a traditional limited performance level with access to performance parts historically available from the aftermarket. So the machines start identical and the two documents then pull in opposite directions on what may be done to them.

The engine is where the two classes really separate

In Vintage Ski the ports may be modified, with diameters and shapes changed, an aftermarket cylinder is permitted, and stroke and rod length may both be changed. In Limited the cylinder must stay OEM with original port timing and no internal modification of any kind, porting is reduced to chamfering, and the bottom end is pinned to one rod length and one stroke. Limited is also the only Vintage class carrying an explicit numeric chamfer limit. That is not a small difference of degree. It is the difference between a built motor and a rebuilt one.

Ignition is the single cleanest discriminator

Vintage Ski allows a modified or aftermarket ignition box, permits the timing to be changed, and expressly allows total loss systems. Limited requires the unit to remain OEM, with no aftermarket box, no timing change and no total loss. One component, two classes, and the answer flips completely. If you only check one thing before entering, check this one, because nothing about the part's appearance tells you which class you are in.

Induction and exhaust, where Limited earns its name

Vintage Ski takes an aftermarket carburettor and leaves the exhaust largely open, including relocating the exit. Limited permits one single carburettor and no more, allows only the manifold, head pipe and expansion chamber, refuses relocation outright, and caps the exit opening at two inches. The waterbox and the stinger insert are both absent from the Limited list, which under the closed list principle means they are not available rather than unmentioned.

The hull answers are the ones that cost money quietly

Vintage Ski permits up to four sponsons, two traditional bladed plus two convex, within a stated maximum drop. Limited permits no aftermarket sponsons at all and lets you remove the originals. Vintage Ski takes an aftermarket handlepole; Limited requires the pole itself to stay OEM and allows only the bracket to be aftermarket, with the attaching point reinforced. An aftermarket hood is named as permitted in Vintage Ski and appears nowhere in the Limited document, which makes it a refusal.

Two answers our own data flags as unresolved

The pump housing, stator and shoe are permitted in Vintage Ski and simply absent from the Limited document, so the closed list makes them OEM. Our record marks that as a commercially significant open question rather than a settled reading, because it rests on silence. Separately, titanium driveshafts and housings read as permitted in Vintage Ski where the two other vintage classes refuse them, and that one is flagged in our data as worth verifying rather than relied on. Both are printed here as open because that is what they are.

Why this matters more than any single part

A seller can tell you a component fits a 550. Whether it is legal depends entirely on which of these two classes you enter, and the two are close enough in name that the distinction is easy to miss and expensive to discover on the day. We do not stock anything for either class: the whole catalogue here is SuperJet and SX-R. This page exists because the comparison is worth publishing even where we have nothing to sell against it.

Before you act on this. Compared from the two class documents as extracted into this site's data, by nobody who has raced either class. Two of the contrasts above rest on a rulebook's silence rather than its words and are marked as open questions. Confirm with your race organiser before building to any of it.

Symptoms this addresses: Same ski, two classes, different answers · Not sure which vintage class my build fits · Told my ignition is illegal in one class but not the other · Want to know why one class is called Limited. All symptoms.