Spark plug question

['72 LeSabre]

I went to change the spark plugs in my 1937 Roadmaster and ran into a curious issue.  The shop manual says the original plugs were AC H9 (18mm).  I bought some modern replacements (Autolite 386, 18mm) but they're too big.  The plugs in my engine are AC-46, 14mm. 

 

I have an old Champion book that lists D16 plugs for 1929 - 1937 Buicks and RJ12C for 1938 - 1942 Buicks.  Did Buick switch to 14mm plugs in 1938?

 

The Standard Catalog of Buick says my engine serial number is late 1937.  It's got a 1937 AA-2 carb, not an AAV-2 which was used in 1938.  The car is definitely a 1937.  So my question is why do I have 14mm spark plugs?  My other question is what type of plugs are folks using in 14mm?  I want to make sure I get the correct heat range, etc.

 

Thanks

 

 

[27donb]

Different cylinder head?

['72 LeSabre]

That's what I'm thinking.  I was wondering if anyone knew of 1937 320 blocks receiving a 1938 head.  Or maybe the head was replaced years later.

[FLYER15015]

Why change out the AC-46's ?

Just clean them and reinstall.

My '40 LTD runs fine on them.

[jenz38]

I drive the AC 45 Plugs in my small 263 since years ,also had it done in my old 248 Motor,both are with raised comp ratio..,they run absolutley stressless.

The old 46 ACs are gone they told me years ago,would replaced from the 45.

 

And I think so too,they changed only your head many decades ago ,check the cast # for ID it.