I've been cleaning off the engine and now that enough of the sludge and dirt has come off I can read some of the lettering around the block. For example I noticed the number 6 on the head near the spark plug is below number 3 on the intake manifold. I know the numbers on the manifold are the cylinder numbers, but are the big numbers on the head next to the spark plugs also cylinder numbers, or do they represent something else?

 

Those numbers and letters actually identify the cylinder head casting :

https://oldsjunction.classicoldsmobi...sfaq/ofhed.htm

 

Number 6 heads are from 1970 and have 64cc chambers as opposed to the nearly 80cc chambers on the original 403 heads. This is a common swap to raise compression. What is the actual casting number on that intake?

 

Sorry Joe, I’ve never seen a set of 5’s, 6’s, 7’s, or 7a’s any less than 68. I think the 64 is an NHRA spec number right?

 

4A, but there's another number that seems to represent something else, on this side it's a 6 while the other side it's a 5. Here's a picture of what im talking about.


Its on the right hand

 

Correct. 64 is the NHRA stock blueprint number. As-cast tend to be 2-6 cc over that.

 

So what does the 6 refer to on the head, the number to the right of the 4A cast number? Is that supposed to indicate cylinder number?

 

4A is the head casting ID, which is original to the 403. The 6 is likely a mold number and unrelated to casting ID. Your block and heads are matching.

 

Ok, good to know it's not pertaining to anything important. Thank you.