Identify your Cat engine before ordering parts: serial prefix and arrangement number

To order a part for a Cat machine or engine you need the serial number before the model name. On a machine it is the last eight characters on the identification plate; the engine has its own plate and its own number. The prefix does the first sorting, but it is not always enough: in Caterpillar’s own parts guides, machines with the same prefix take different parts depending on the engine serial number and the engine arrangement number.

Where is the serial number on a Cat machine?

The machine serial number is on the identification plate, which the US Cat dealer Warren CAT describes as “typically located on the outer frame”. The number on it is the Product Identification Number (PIN), 8 or 17 characters long depending on the machine’s age. Avesco, the Cat dealer for the Baltic states, explains the longer one:

“Starting in 2001, Cat machine models began using a 17-character Product Identification Number (PIN) on the machine plate. The serial number is the last 8 characters (prefix/sequence ID) of the PIN.”

Avesco Cat, PARTS.CAT.COM Online shopping user guide, January 2024

Warren CAT draws the line at “models built prior to 2002”: on those the 8-character number is the serial number. The prefix comes first. The cover of Caterpillar’s parts reference guide for the 320C excavator lists four three-character prefixes for that model: ANB, PAB, PAC and EAG.

Does the engine have its own serial number?

Yes. Another dealer, Boyd CAT, puts it this way: “Cat engines within a machine also have a serial number, as well as some larger systems within the equipment.” Warren CAT cautions that the label titled “Serial Number” “could be your engine serial number” and not the machine’s. On the engine plate the number is usually marked “Serial Number”, S/N or SER. NO.

An engine serial number also starts with a three-character prefix. A guide from a US engine-parts seller says: “These prefixes all correspond to a specific model, and, in most cases, to a subset of that model.” For the 3406E it lists 1LW, 2WS, 5DS, 5EK and 6TS. The same guide says the plate is often on the driver’s side of the valve cover, and adds that “not all engines are consistent”.

Why the engine number is asked for separately shows in Caterpillar’s 320C guide. For all four prefixes it gives two part numbers for the primary fuel filter element, 117-4089 and 326-1644, with the footnote: “Choice depends on engine serial number, consult your dealer”.

Older machines are different. In a heavy-equipment forum thread about a 977 track loader, a member who knows older Cats well explains that with the D333C engine the engine plate carries the tractor’s serial number: “When the 3306 was introduced engines started to carry their own individual Serial Numbers.”

What is an arrangement number, and why are you asked for it?

The arrangement number identifies the build of the engine, the set of components it left the factory with, and it is written like a part number. No Caterpillar document we could open defines it in those words; the reading comes from the table below and from a forum account. In Caterpillar’s guide for the E Series backhoe loaders, the engine parts table opens with four columns: sales model, engine power, serial prefix and “Engine Arrangement Number”. The part numbers are set out against that last column.

Source: Caterpillar, E Series Backhoe Loader Parts Reference Guide (AEXQ1873-00, February 2016). Four rows selected; the guide lists more for the 416E.

In the first two rows the model, power and prefix are identical; the arrangement number and the starter are not. The third row is another SHA machine with a different fuel pump and injector. The prefix does not tell these engines apart; the arrangement number does.

The number also leads to the engine’s factory rating. A boat owner on the same forum who posted the arrangement number from the plate of a 3208 marine engine was told: “1W-5070 Arrangement specification is 260 BHP @ 2900 RPM.” The answer assumed the engine was still in its original “as-built” condition.

Can one model carry more than one prefix?

Yes, and the parts book changes with the prefix. Caterpillar’s 320C guide listed four prefixes for one model. According to the member in the 977 thread, one reason is where the machine was built: the 70J prefix marks a 977 built in Japan, and “the US-built equivalent model carries an 11K S/N prefix”. By the same account the first 70J parts manual stops at 70J2222, and the 3306 engine arrived at 70J2223 on the Japanese-built machine but only at 11K5083 on the US-built one.

That is one model on one forum, not a general list. Our advice: look parts up by the prefix and serial number on your own plate, not by the model name or by a US-built twin.

What if the plate cannot be read?

Boyd CAT again: “The identification plate can be in hard-to-reach places and wear down over time, so it may be difficult to find or read the number.” Its order of work: inspect the machine thoroughly, turn to the owner’s manual, which “often includes diagrams or descriptions showing its location”, then ask the dealer who sold the machine. Once you have the number, keep a photo, and add the plates to your routine maintenance checks.

On an engine the number may also be stamped into the block. In the 3208 thread the advice, when the plate is missing, is to look in the centre of the vee for a stamped serial number: “you may have to take some paint remover to be able to see it”. Not every number on a block is a serial number. When the 977 owner cleaned the block and asked about the markings, the answer was that they were a casting date code and the part number of the block casting.

What should you write down before ordering?

1. The machine plate. Copy the whole number. If it has 17 characters, mark the last 8.
2. The engine plate. Take the engine serial number and the arrangement number separately. If the engine number is the same as the machine’s, note that too.
3. Search with the full number. Avesco’s guide says Caterpillar’s online parts book (SIS) also searches on the prefix alone, but returns a longer list; with all eight characters the results rest on that machine’s factory configuration data.
4. Leave the part number until last. Reading the number on the old part is covered in a separate article; it is useful once it agrees with the plates.

The 977 thread adds a warning: an engine built up from the parts of two different models “might require more than one Parts Manual”.