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I was told by the previous owner that the engine was a 389 with a mild cam but after a little research of my own I think I was swindled. The code is telling me it's a 326. My understanding is there is no way to know a 326 from a 389 other than if you pull the head and measure the bore size or by deciphering block code. Hoping someone more knowledgable in Pontiac Engines can confirm this for me. Below is what I think the code says. 9778840 meaning it is a 326. The rest I haven't figured out the meanings yet.

M D
B 9778840
I 745

 

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You need to check the engine ID on the left front of the block just under the left head. Will be 2 alpha character ID like ZN or YP. Your date code looks like I145 which is Sept 14, 1965.

Check at Wallace Racing's Pontiac Engine Code Search

I think you're right, looks like Block Code 9778840 was only used for 326ci.For 1965 there are about 5 different engine codes to identify which one is yours.
 

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Yes, the 9778840 is a 326 block. The engine code will be on the front of the block underneath the right head, passenger side. That will tell you if it was a 2-barrel or 4-barrel block, automatic or manual transmission. Your date code of I145, September 14, 1965, puts it as an early production '66 model year.
 
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