I am getting close to firing up my 396 for the first time. I looked up what spark plugs it needs in my dad's Chiltons manual. It recommended an R44N. That P/N does not exist anymore according to the new AC book. So, we looked up what they recommend now, its a R44XLS which has a flat gasket style seat. But my 1969 #290 heads have a tapered seat in them. So we got an R44LTS. It is the same long reach as the R44XLS and the same heat range, but it has a tapered seat.

My question is: What AC plugs do you guys with 290 heads run? If you don't use AC, what are you using? I would like to get an idea if I got the right plugs. Thanks.

You may have late 290 castings. 290's were made in both styles. Look in the plug hole to see if the threads come all the way to the top where the plug seats. Heads that use R44N have about a 1/2" counter bore before the threads start. Heads that are made for tapered seat plugs have threads that come all the way up to where the plugs seat. Tapered seat plug heads use R44T or R44TS type plugs. If you use R44LTS plugs in a tapered seat head you will find the plugs and pistons trying to be in the same space at the same time. Check it out before you crank it over. If you really need R44N plugs for car show judging I may have a spare set.

I have the 1/2" counterbore between the seat and the threads. So, that means I need the R44XLS plugs then? It just seems odd because the seat is not flat, its tapered. But I also have the counterbore, the threads do not come all the way up to the seat. Thank you for your help. I really don't want to mess this thing up before I even get a chance to fire it up here.

Yup, you need the long plugs with gaskets. Another thing, heads made for long reach plugs are machined to clear a 13/16 socket. The R44N plugs were only threaded about half way to the gasket. The R43XLS have threads all the way to the gasket. One more thing, factory aluminum heads have threads all the way to the gasket seat and use plugs that are threaded all the way to the gasket. They use R43XL without the "S".

Thank you so much. I will get those tomorrow. I hoped someone had some knowledge about all this way back stuff. Thank you again. Happy burnouts..... graemlins/beers.gif