How can you find out if your 1965 Pontiac Bonneville with a 4 bbl Rochester carb is 389 cid or 420 cid?

The easiest way to determine if your 1965 Pontiac Bonneville has a 389 or 421 (not 420) cubic inch engine is to check the engine casting number. This number is stamped on a prominent surface of the engine block itself, usually on the side near the front. You'll likely need to clean the area to see it clearly.

Once you find the number, you can look it up in a Pontiac engine casting number database or a Pontiac parts catalog specific to 1965. These resources will definitively tell you the engine displacement.

Other less reliable methods include:

* VIN: The Vehicle Identification Number (VIN) *might* offer a clue, but it's not always definitive regarding engine size. It's best to verify with the casting number.

* Documentation: If you have original paperwork for the car (owner's manual, build sheet, etc.), that would clearly state the engine size. This is ideal if you have it.

* Visual Inspection (Less Reliable): While the 421 is generally slightly larger in physical dimensions, visually distinguishing between the two without detailed knowledge can be unreliable.

In short: Finding the engine casting number is the most accurate and reliable way to determine if your 1965 Pontiac Bonneville has a 389 or 421 cubic inch engine.