IMO Reference Numbers

As an independent manufacturer, supplier or service provider in the marine engine market, it is almost impossible to not be aware of IMO reference numbers for NOx-affecting components. 

Unfortunately, it can be difficult for independent companies to use these numbers on their parts. Thus, they are not always able to sell their products. The current rules make it easier for large corporations to impose monopolisation on the market. This process, coupled with the misconceptions about independently-sourced parts, creates an unfavourable position for independent companies. 

Many of our members have come to us with questions on this topic. With time, we have been able to compile our knowledge and offer some proposed solutions. 


Regulation 13 of MARPOL Annex VI prescribes certain NOx emission limits for marine engines. Related to that regulation is the NOx Technical Code (TC). The TC lays down rules regarding the measurement of NOx emissions, among other things. Out of the three methods provided, the Parameter Method is by far the most popular. 

In simple terms, the Parameter Method starts with a parent engine test performed on a test bed, usually by the engine manufacturer. If the engine complies, the surveyor can issue an Engine International Air Pollution Prevention (EIAPP) Certificate on behalf of the Flag State. The NOx-affecting components’ IMO reference numbers are written down in the Technical File (TF), which accompanies the engine during its operational life. The template for reference numbers is normally IMO XXXXXX although not all engine builders / their licensees follow this pattern.

The purpose of these IMO numbers is to help the surveyor easily and quickly check if the components, fitted to the engine during operation, correspond to those used during the parent engine test. There is no industry standard for the numbering system and each engine builder numbers them differently, based on the template. 

This is how the PM works – it is based on the assumption that, if the parts are the same as during the test, and the engine is operated within the approved parameters, the NOx emissions would be compliant (as they were on the test bed). In reality, the PM does not take into account factors such as weather, load, wear and tear of the NOx affecting items, items with an IMO number which have been “reconditioned” but to what standard (?), as well as engine operating conditions, all of which can influence NOx emissions. 

Since the 1990s, when these rules were first established, the PM has become the industry standard. However, the IMO never made it mandatory. Even today, more modern methods such as onboard direct emission monitoring remain almost unused, despite their accuracy and reliability, or perhaps because of it. 


The main issue is the claims from engine builders that IMO reference numbers can only be stamped on the components by them. Some have even claimed that the numbers are subject to their Intellectual Property rights. The IMO regulations do not state who can stamp the IMO number on the component, and the number itself, as well all reference numbers will have no IP unless it is used as a trademark.

If you stamp your parts with IMO reference numbers, you are not just making a contractual statement that your components correspond to those used on the Parent Engine test, but you also run the risk of being accused of “passing off”. In simple terms, passing off means presenting your products to your customers in a way that gives the impression that they have come from a different manufacturer.

We have compiled some tips on how to overcome this, based on our expertise and knowledge.  

The “corresponds to” symbol

One way to try to avoid problems is by using the “corresponds to” symbol in the order documentation of the components. The symbol looks like this: .  You could use it as follows: Your own part number IMO XXXXXX. It is a widely-used mathematical symbol that most engineers and surveyors would or should understand. 

The most important thing to remember is that, if you use the symbol, then you should be prepared to prove that the parts really do correspond. This is much easier to do when the components come from the original manufacturer who supplies the engine builder and supplied the parts tested on the parent engine.  In that case, the parts would be made on the same machinery, using the same manufacturing drawings and subject to the same quality standards. But if the parts come from another source, then proving that they correspond could bring high costs. 

Reverse-engineered components?

If you are manufacturing reverse-engineered components, then the situation can be much different. Unfortunately, there are no common standards for reverse-engineered components. That will make it difficult to show that the part you have made is of the same quality as the OEM one.  It is not just a question of the material specification or major dimensions. It is important to show that the tolerances are within the original tolerances. This can be achieved by measuring multiple original samples for different batches and making sure that your tolerances are within the tolerances which have been measured.


It is important that you do not make a statement which is untrue or which is likely to create confusion in the mind of the customer regarding the origin of your components. 

We have been working on eliminating the misconceptions about independently-sourced parts. We have also been pressing for the more modern methods for NOx emission monitoring such as Continuous Emission Monitoring Systems (CEMS). Such monitoring systems can be used for other purposes such as measuring other gases and recording CO2 emissions which is now a requirement, as well as for fine tuning the engine in order to optimise performance and reduce lube oil consumption.