C3 69 BB Spark Plugs: Follow-up

[Craig C.]

I made a rookie mistake recently and, in doing so, found something interesting I thought I would pass along. A few days ago I posted a request for spark plug suggestions for my '69 427/390 I am rebuilding. Since the cylinder heads are cast GM #3964290 I assumed - with help from other sources - that they were machined for a tapered seat, and said so in my post. Well the tapered plugs I purchased (AC R45TS) won't turn into the heads. Subsequently I have found a post by Joe Lucia from earlier this year stating that if these heads were used for '69, then they would have been machined for the gasket type spark plugs.

In the next day or two I am obviously going to go back to the auto store to purchase the AC R45N plugs. I suspect that they will work. Since I am the 4th+ owner of this car and do not have its history I cannot claim the originality of all of the engine components, but for what its worth the block and head casting dates all match up well. This is a 512 block with casting date G 2 9. The head casting dates are F19 9 and F26 9. The stamped date on the block is July 10.

My lesson in this is that doing my homework would have saved me about a week. I am just happy it did not cost me a bunch of money.

-Craig #32671

[Clem Z.]

Re: C3 69 BB Spark Plugs: Follow-up

the taper seat plugs started in 1970 and i would use AC R45XLS extended tip plugs for a street driver

[Joe L.]

Re: C3 69 BB Spark Plugs: Follow-up

Craig-----

Generally speaking, I think that the '290' heads are considered to be a 1970 head. However, it's very possible that very late 69s got them since the 1970 heads were in production for 1970 applications while 1969 Corvettes were still being produced.

If I said previously that the '290' heads used for 1969 were machined for gasketed spark plugs, then I'm not sure that I was correct when I said that. Are you sure that these heads are '290' casting number heads machined for gasketed spark plugs? If they are, then maybe I was correct if I said that the '290' heads used for 1969 were machined for gasketed spark plugs.

Whether the spark plug machining is for gasketed or tapered seat spark plugs, the plugs should be able to screw into the threads. The threads for BOTH style plugs are 14 mm and either style plug should be able to screw into the threads in the head, NOT that the plugs are interchangeable. The tapered seat or gasketed machining is easily discerned by looking at the spark plug machined seats in the head.

[Steve Antonucci]

Re: C3 69 BB Spark Plugs: Follow-up

Craig,
It's a little late, but Alan Colvin's books "Chevrolet by the Numbers" cover
this and similar topics quite well. I have found '290 heads with close casting
dates but with different spark plug seats. Remember, 1969 was an odd year with
the long sales cycle for Camaro's. And, don't forget service replacement
stock. I think Colvins books are a must have for anyone restoring Chevy's.
I even heard recently about an original 69 (late production) that was reported
to have one 3931063 head and one 3964290 head. I guess anything is possible.
Steve

[Clem Z.]

Re: C3 69 BB Spark Plugs: Follow-up

if the heads are cut for "N" series plugs there are no threads in the top half of the plug hole. "N" plugs only had threads on the bottom half of the 3/4" reach