OEM or approved replacement? The maths for a main engine

The price gap between an OEM part and an approved replacement runs 30-60 percent. But the maths only gets interesting once you add lead time, class requirements and the cost of a day in port. Let us count it honestly.

What you are actually comparing

OEM means the engine maker sells the part under its own brand - physically it often comes off the same line as the independent equivalent. A marine-grade replacement is not a bazaar copy: it carries an EN 10204 3.1 material certificate, dimensions to the manual and, for critical items, a classification society approval.

When the original is beyond discussion

• An engine under warranty or a maker's service agreement - a third-party part voids both.
• Components where class requires conformity with the type documentation: shafts, connecting rods, high-pressure fuel system parts.
• Cases where the manual calls for selective fitting - without maker data a replacement is a lottery.

When the replacement wins the maths

• Fast-wearing parts bought regularly: rings, bearings, seals, injectors for auxiliary engines.
• Older engine types where OEM keeps prices high with multi-week lead times - the replacement is often on the shelf.
• Pumps, valves and fittings, where the connection, duty point and certificate matter, not the logo on the casing.

A worked example

A set of injectors for an auxiliary engine: OEM at 38,000 euros with a 4-week lead time, or an approved replacement at 19,500 euros from stock in 48 hours. If the vessel is waiting in port and a day off-hire costs 15-25 thousand dollars, the difference in lead time is worth more than the difference in price. The same calculation at a planned overhaul can flip - then you are only comparing price and paperwork.

What to check in a replacement supplier

• A 3.1 certificate and type approval where class demands it - documents before shipment, not "attached to the invoice".
• Photos of the parts before packing and full batch traceability.
• A delivery record with shipowners, not just onshore workshops.

In practice one simple rule works best: quote both options in parallel. Every engine-part enquiry with us comes back with two columns - OEM and approved equivalent - with prices, lead times and document scope. The decision stays with the engineer, but it is made on full data.