In general, Cummins engines having a six digit serial number, like the 1013426, are very rare. A very small percentage of older engines will have 7 digit serial numbers. In these cases, the engine is rebuilding or resprayed and the second digit is 7 to represent this fact. By this logic, the (2nd block) serial number and the dataplate serial number are the most accurate indicia of originality, and one of these sources would be usually be sufficient to accurately identify most original Cummins engines of this vintage.
Wth the advent of the electronic control unit (ECU), and the treatment of many engine components as interchangeable or common, it makes little sense to try to make estimates of originality from the serial number alone. However, the serial number may be a still a very good indicator that the engine came from a pre-EEC series. I've heard of people throwing out parts from what they thought was a century old engine and locating a replacement binder that said Cummins, which was from a long-time competitor, probably a Downs.
As a general rule, the first block of a Cummins serial number always has lines 3-4-5-6-7-8, meaning those are the numbers of a regular production engine that has been installed. The second block has lines 1-2-3-4-5-6-7-8, which means the engine has now been modified and the first two numbers represent the generation of the engine. So a seven-digit serial number will always include those pairings of first and second blocks. But keep in mind that a one-digit first block can have the second block in an 8-digit serial number. If the first block is zero, this means the engine is a prototype or rebuilt. If the first block is 4, it means the engine is a rebuilt or an experimental engine. If the first block is 8, it means the engine is a rebuilt or experimental engine.
This is a way to obtain the engine serial number for C44 or C54 on a simple two-digit year. For C26, use the address and zip code of the C44 or C54 owner. Truck owners who are unsure of the engine serial number can obtain this information by visiting www.quickserve.com. Quickserve provides these serial numbers for all Cummins engines, including the heavy-duty C70. If you have access to Cummins Quickserve, you can go to add your engine and add the engine serial numbers. d2c66b5586