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Just bought a 400 small block and I'm trying to learn what it came out of with the RPO AND LAST 6 DIGITS


Other number on same pad is 


Any help would be very appreciated

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Posted (edited)



1 - Chevrolet.  First digit identifies the make.

5 - 1975.  Second digit is the model year.

R - Arlington Assembly.  Third digit identifies the vehicle's assembly plant.

494904 - Production sequence for the vehicle.




T - Tonawanda.  First digit identifies the engine's assembly plant.

04 - April.  Second and third digits identify the month of the year built.

30 - 30th.  Fourth and fifth digits identify the day of the month built.  So April 30th.

CTX - A,B.  The last three digits identify the car line the engine was used in.  CTX represents A and B body vehicles. 


The RPO code seems to be LT4.  4 barrel carb, 175hp.   


So.  GM and Wikipedia seem to disagree on some of the Chevrolet products built in the plant so I've linked both sources:


GM - https://plants.gm.com/Facilities/public/us/en/arlington/about_us.html


Wikipedia - https://en.wikipedia.org/wiki/Arlington_Assembly


Being the engine VIN is a Chevrolet, you've got an engine from of the following:


- Caprice

- Chevelle

- Monte Carlo

- El Camino


Edited by newdude

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