The design number
Whoever drew the component numbered it in their own register. On large two-stroke engines the firm that owns the design and the yard that assembled the machine are usually different companies.
Numbering, not naming
One component. Several numbers. All of them correct.
A buyer types a number into a search box and gets nothing, or gets three unrelated items. Neither result means the number is wrong. One marine component routinely carries four or five references at once, each valid inside the system that issued it. Wrong deliveries follow from reading a number in the wrong system.
This page explains those systems. It publishes no catalogue data, and the scheme below uses placeholder letters rather than characters anyone could copy into an order. What it offers is a way to say what kind of string you are holding, and where it came from.
Six origins
Where each of those numbers comes from.
None is a counterfeit. They are separate registers, kept by separate organisations, describing one object.
The builder's number
A yard building to another firm's design keeps its own material register. That reference appears in the parts book issued with the machine, and sometimes only its own service desk reads it.
The sub-supplier's mark
Bearings, seals, sensors, filter elements and fasteners are made by specialists. The specialist marks its own designation, often a standardised one, while the parts book carries an internal reference for the same object.
The kit number
A reference naming a set rather than an object: an overhaul set, a gasket set, a service kit. It has its own price, its own unit and a contents list.
The retired number
Withdrawn when a drawing is revised, a material changes, or two near-identical items merge into one. The machine runs on, and the documents on board keep carrying it.
The seller's reference
Marketplace listings carry the shop's internal code: a warehouse label, not an engineering identity, and the one most often copied into a request in the belief it is the maker's.
Reading the string
Shape gives a guess. Provenance gives the answer.
A register generated by one organisation tends to be uniform: same length, same separators, no meaning encoded inside. A specialist's designation usually carries meaning — letters for a series, digits for a size, a trailing character for a variant such as a clearance class. A scheme shaped like LLL-NNNN-V, where changing the last character changes the item, points to a component maker rather than an engine builder.
Kit references defeat that test: they are issued in the same format as single items, so only the page they sit on separates them. Record where each number came from, beside the number. A reference without a source is half a fact.
- Off the item. Belongs to whoever made that object, and may be a heat or batch mark.
- Off the packaging. Belongs to whoever packed it, on a box that gets reused and relabelled.
- Off the parts book. Issued against this machine, carrying a page and a position.
- Off a record. The maintenance system or store card, saying what was last fitted.
Supersession
"Replaced by" is a pointer, not an equals sign.
An old reference is not automatically wrong; often it is the strongest key you hold. A designation is withdrawn when a drawing is revised, a material or coating changes, a sub-supplier changes, or two near-identical items are consolidated. The object may be improved, or merely renumbered.
- Direction matters. Replacement can run one way only: the successor covers the retired application, but not the reverse. Other pairs are declared interchangeable both ways. Two different facts, and a fair question for any quotation.
- Chains, not steps. A retired reference can point to a successor that was itself retired later. A parts book printed at delivery stops at the first hop.
- Splits and merges. One retired reference can resolve into several successors chosen by application, and several can collapse into one. Neither has the tidy answer a search box wants to give.
- The old number still travels. It stays a correct historical identity for your machine, and is often the only key that reaches the right page. Say where you read it and let it be resolved forward.
- Then check it against the machine. A successor is resolved against the catalogue; the vessel needs it resolved against what is fitted. Where a change altered a mating face or a matched pair, the counterpart may have to change too.
Kits and items
Two orders that go wrong in opposite directions.
One buyer needs a single seal, finds it listed inside a service set and orders the set. A box arrives holding a dozen items, most of which will sit in the store. Another names the single item for a job that consumes a set: the gasket arrives, the O-rings renewed alongside it do not, and the machine stays open while a second order is raised.
The reliable tell is the unit, not the format. A set is counted in sets, kits or boxes rather than pieces, and has a contents list behind it — ask for that list before ordering. A kit may legitimately cover several machine variants, so a correct kit can hold items you will never fit. That is a superset, not an error.
The previous invoice
An old invoice describes a purchase, not the machine.
Reordering from the last invoice is the most natural thing a purchasing team does, and it is right often enough to be trusted more than it deserves. The line records what one supplier sold on one date, under the numbering current then.
It drifts four ways. The reference may have been retired. The item supplied may have been an alternative, so the number belongs to that producer. An overhaul may have changed what is fitted. And the line may have been a set, or a description typed by a salesperson. An invoice proves only that something was bought — not that it fitted, and not that it was installed.
When sources disagree
Which source wins depends on the question asked.
- "What is this object?" The mark on the item wins, because it travelled with the item — bearing in mind that it may be the specialist's designation, or a casting mark that identifies nothing orderable.
- "What do I order for this machine?" The parts book issued against this unit wins, because it is indexed to this build rather than to the type in general.
- "What was fitted last time?" The vessel's own maintenance record wins. Records of maintenance activity are kept aboard under the ISM Code, and answer what no catalogue can.
- Packaging ranks last. A box outlives its contents and gets relabelled by hand. A marked part in an unmarked box beats a labelled box holding an unmarked part.
- Do not adjudicate alone. When two sources disagree, send both, say where each came from, and photograph the marking. Two conflicting numbers with provenance beat one clean number that arrived from nowhere.
Dead ends
The number returns nothing. What else identifies the item?
A reference comes back empty when it has been retired, when it is internal to a builder or a component specialist, when it is a drawing position rather than a part identity, when it is a shop code, or when a character was misread — zero against the letter O, one against I, a hyphen that was really a space.
- Machine. Maker, type designation, serial number, the builder named on the plate, year of build.
- Position. The assembly it came out of, the part-book page and position, the system it serves.
- Object. What it seals or carries, its mating faces, its interfaces measured on board, and marks that are not the number.
- Evidence. The marking photographed in raking light, the position it came from, the nameplate, and how the failure appeared.
Stores codes
IMPA and ISSA codes answer a different question.
Both are ship-supply catalogues. Neither is a machinery parts system, and using one as though it were is a recognisable way to receive the wrong item.
What they are
The IMPA Marine Stores Guide gives each catalogued store item a six-digit code: leading digits place it in a section, the rest identify the item. The ISSA Ship Stores Catalogue is separate, with its own section-based numbering.
What they cover
Consumables and stores: cleaning materials, safety equipment, hand tools, hoses, packing, standard fasteners, protective equipment, provisions. What a ship consumes, rather than what a particular machine was built from.
What they do not
Spares tied to a maker, a type and a serial. There is no stores code for the cylinder cover of one engine, and a code that looks close describes a family rather than the item.
When one helps
On a true store item a code removes description ambiguity across languages and fixes the unit. The two systems are separate keys, so send the code with the description — never in place of maker, type and serial.
Common questions
Part numbers — short answers.
Can one item really have several correct numbers?
Yes, and it is the normal case. The firm that drew the component, the yard that built the machine under licence and the specialist who made the object each keep their own register, with a kit reference and a retired reference alongside.
My number is old. Should I still send it?
Send it, and say where you read it. A retired reference stays a correct historical identity for your machine and is often the only key that reaches the right page. It needs resolving forward, not deleting.
Is this reference a kit or a single item?
Look at the unit rather than the format. Sets are counted in sets, kits or boxes and have a contents list behind them, while the reference itself looks like any other. If the unit is not stated, ask for the list.
Is an IMPA or ISSA code enough for a machinery spare?
No. Those catalogues index ship stores, not spares tied to a type and serial number. A stores code is useful on consumables; on a technical spare it should accompany maker, type and serial data rather than replace them.
Does Vessel Core publish part number tables?
No. This site publishes no catalogue data and no cross-reference lists. A number asserted on a public page and then copied into an order is one of the ways a wrong part gets bought.
Send the numbers you have
Every reference, with the place you read it.
Send each number together with its source — the item, the box, the parts book page, the maintenance record, the old invoice — plus maker, type, serial and a photograph of the marking.
Numbering schemes on this page are illustrative placeholders and correspond to no real component. Vessel Core publishes no catalogue data, and holds no dealership, distribution or partnership agreement with any manufacturer.
