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

J-MacsMopars

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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
 

Dustin The PcP

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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.
 
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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
 

RedFish

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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.
 
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