In the 1929-1938 Master Parts Price List, it lists the part number for the 1938 Spark Plug as Part #1557745. In the 1929-1939 parts book it lists the spark plug as Part # 1557545 and it is a Blue Top #46. It also shows #44 and #45 as blue top spark plugs fitting 1933-1939 cars.
So is the correct spark plug for the 1938 Chevrolet an A-C Blue Top Spark Plug? And is the correct heat range either #46 or #45 for 1938?
I was asked this question at the Miami Valley Swap Meet this past weekend. A fellow VCCA member is building a 1938 and wants it to be bone stock.

dtm

The 1939 Engineering Manual says ;"Physical dimensions of the 1939 spark plugs remain practically the same as those of 1938. However, a change in the chemical composition of the porcelain makes the 1939 spark plug more durable and resistant to cracking. These new spark plugs have the same outward appearence , carry the same identifing number, and are interchangeable with the 1938 spark plugs".

In 1938 no spark plug info is mentioned.

1937 service news says K-11 spark plugs, Then June 1937 Serv News says new spark plug released for production and replacement. Its the AC #47 Blue Top.Says it has the same characteristics as the K-11 and used for the same conditions.

The Q&A section of the Dec 1838 Serv. News says "How do thee AC-46 plugs used in the 1939 engine differ from the AC-46 Blue Top plugs used in the 1938 engine?"
Answer; The material in the insulation of the new AC-46 plugs has been improved so that the plug life is greatly increased over that of the AC-46 Blue Top. The heat Characteristics of the two are the same."
That would lead me to believe the 1938 came with the AC-46 Blue Top plugs.
The heat range of the #46 would be for average driving. For high speed driving a cooler #45 could have been used. No one heat range is going to cover all driving conditions.
I find the modern AC #45 works well in all of my cars. The fact that unleaded gas keeps the lead deposits from forming on the plugs also allows for running a cooler plug.

Now it as clear as mud.

Thanks Chev Nut. I told the Miami Valley VCCA member that I thought that a #46 AC Blue Top is what he wanted, in order to be absolutely original. I have some NOS #45 Blue Tops, but no #46 Blue Tops. I will send him your information.

thanks again

dtm