Do these heads take a tapered index washer ???? What spark plug you using.

They made Dart BB heads that used short reach plugs and they were tapered seat.

I had a 509 with those heads and found out the hard way about what plug to use....

I bought it used from a racer friend and took it home with no plugs in it....and the owner offered to stick a set in there as we left.

When I looked up a Dart head for what plug to use it said 3/4" flat washer plugs...

They hit the pistons....by a lot...

Engine ran normal at home.....out on track NFG!

You can look at the sparkplug hole and tell if it takes a tapered seat just from where the plug seats in the head....

NGK 5673 heat range 9. Yea .460 reach.

Funny story; I call Dart kind of complaining about the 320’s not performing like my GM iron heads. After telling my story to the woman who answered the phone she interrupts and days hang on. A guy gets on and I give my results and he says of course they won’t make as much power as my iron heads unless they get a full port job. He goes on to say he designed that head for a bigger bore than I had, and what he wanted to see done to the port and especially the chamber. I ask him his name : **** Maskin he says and asks me if I have a die grinder. Yea I do but never did anything beyond port match. He says no prob and talks to me for 20 minutes on what to do. I did his mods during the winter so don’t have apples to apples comparison but felt the changes were worth over a tenth in the 1/4.

My dart 320s take a tapered seat plug. As Greg said NGK 5673 I use 7s on alcohol.