Spark plug won’t start
 

I have a 67 GTO head code is YX. I’m finishing up the engine and trying to put new plugs in it. I have AC delco R45S Plugs and for the life of me, I can’t get them to start in the hole, they won’t thread in. The plugs that were in it are champion 17 Y. They thread in OK. What the heck am I doing wrong?

I just noticed the champion plug is tapered. Is the spark plug hole and a YX head supposed to be tapered?

First YX is not the head code. It is a block code. You could have different heads on the block. That said those heads might require the tapered seat plugs. I tried looking for your "17Y" plugs and all the ones that I found had more letters in the designation. Look at the spark plug hole and see if the seat is tapered or flat. If it is tapered you can use an AC R45TS plug. If you can post your Head code and casting date we could be sure.

Thanks, have found I have 72 heads so I’ve installed R45TS plugs