Spark Plug Question

[sjarrett71]

Looked up some spark plugs at the auto parts store and noticed there were several recommended. I chose to go with AC Delco but there were still several different numbers to choose from. Some had washers, some had tapered seats. I went out to the parking lot and pulled one out of my 72 with a 350. It ended up being the plugs with a washer. The number on them was R46S. Since they don't make an R46S anymore, I bought the R45S's.

My question is, what is the difference between the heads that take a tapered seat versus the heads that take a washer'd spark plug. My experience with Chevy engines has been with tapered spark plugs.

[vectorit]

Pretty sure GM (Chevy) went to the tapered seat plugs in 1970, so it appears your rig has an older set of heads that utilize the crush seat type.

[sjarrett71]

Yeah, I did a little bit of research about it after posting my question and started to think they might have been replaced. My heads have the accessory bolt holes though. What year did GM start putting those in the heads? Also, where are the casting numbers on the heads that will tell me what they are?

[vectorit]

Not sure about the accessory holes when they came out, but to get the low down on the heads you have to pop the valve covers and look in the middle section for casting numbers.

There is quite a bit of data out there on head castings, so it should be pretty easy to figure out the vintage head you are running.