318-3, J heads, Hardened seats, and a few other ????'s

-

J-MacsMopars

Shade Tree Vintage Auto on YouTube
Joined
Nov 27, 2007
Messages
2,415
Reaction score
86
Location
Utah
Hello everyone. I have a few small block questions for you.

First off, I am looking at a car that is supposed to have a 318-3 in it, is there anyway to verify just by looking at the engine?

Second; Do 73 J heads have hardened seats and how do I tell if they do or don't?

Third; Can I put 340 J heads on a stock 318 without any mods? I know they will bold on, but will it change the compression?

Fourth; How high risk is it to drive the car with modern fuel and non hardened seats?

Thanks, Jeremy
 
I think you can pull the numbers off the side of the block to determine what it is.

I do not believe 73 J heads had hardened seats in them and even if they did I would get them redone.

Bolting on those J heads to your 318 will probably drop compression a full point. They probably have 68-72cc heads. While 318 heads had 61-64cc heads from all I have measured.

I would not risk driving the car without hardened seats, I can not inform you of the risk, but I do know there are.
 
I get 68cc when measuring stock '675' open chamber 318 heads
and 64cc with '302' closed chamber 318 heads.
Stock '915' ive measured have been 72cc.

fwiw
 
hardened valve seats appeared in 73 models. The lack of lead anti-knock compounds in todays fuels means that with non hardened seats the valves will slowly recess into the head. Adjustable rockers correct the very small change that takes a very long time to occur.
 
-
Back
Top