Another 340 J & O head thread (what do i have?)

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eduardostewart

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Hi guys,

did some googling to educate myself. Let me explain: my father and are are in the midst of starting a modest restoration of his 70 duster he bought before i was born. Going off of his memory, the J heads that were on the engine are supposed to be original to the engine and the O heads he got from a friend way back in the day.

As i was learning, it seems that the 340 was detuned from 10.x to 8.x compression in 73? And i believe these J heads are early J heads as the intake valves are 202s. Actually, both the J and O heads we have are 202 intakes valves.

i was looking at some of the numbers on the heads and i think i'm making out 360 on them. In the end, i want to make sure the heads we put back on get us to the 10.x compression before they detuned them.

Any help is most appreciated.
 

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J and O heads are identical.
 
I'm no small block expert but I'm pretty sure there was no big (if any) difference in chamber volume between either head you mention. All the detuning was done by dropping the piston down in the hole.
 
eduardostewart said:
As i was learning, it seems that the 340 was detuned from 10.x to 8.x compression in 73?.
Click to expand...

I was under the impression that compression drop was mid-71 or perhaps 72. This may have been in CA, as that is where I was (Navy) at the time.
 
67Dart273 said:
I was under the impression that compression drop was mid-71 or perhaps 72. This may have been in CA, as that is where I was (Navy) at the time.
Click to expand...

Unrelated but then again.... :D

[ame="https://www.youtube.com/watch?v=vt5O3rsu8u4"]In The Navy music video at end credits from Down Periscope - YouTube[/ame]
 
thanks guys. i could be off on the date when they detuned 'em. i've read so much that it's kind of running together. Is there any way to decode the block so i can sleep easier?
 
i was scratching my head because of the 360 casting on the heads. not that casting numbers are always directly related, but made me think it was too much of a coincidence.
 
How far are the pistons down in the hole when the piston is at top dead center?
 
hi, the 340 used 2.02 valve from 68 to 71, also had 10.5 comp ratio. in 72, comp ratio was dropped to 8.5, and the 360 int valve was used, it was 1.88. the 71 340 has 915 J head, the 68 to 70 has the 894 X head.
 
The "O" heads are right for the car. J heads were used on '71 production and later 340s and 360s. 2.02s were the intake valve size for '71 and a few '72 early models may have them. The rest came with 1.88 intakes and could be found on the 340s for 72-72, and the 360s I think until 74. Like RRR said they are pretty much the same thing, but with the car being a '70 model I'd use the "O" heads.
 
got it.

i don't know how far down the piston goes, haven't measured it. do you have the numbers?

if the heads are the same, the j's are in better condition i think, but it all has to get reworked - they would both need different valve seats to run unleaded, correct?
 
most likely this doesn't answer anything, but in a longshot... would the pistons come up that close to the edge of the block if it were the lower-compression engine? maybe some of you are gurus :)
 

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Those look like they are quite a bit in the hole.

Compared to this for instance.
 

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Those are the 10.5 .. 71 10.3 to 1 pistons. The 72 low compression pistons are easy to identify by the 4 valve relief notches on the piston and they will sit lower in the bore.
 
EL5DEMON340 said:
Those are the 10.5 .. 71 10.3 to 1 pistons. The 72 low compression pistons are easy to identify by the 4 valve relief notches on the piston and they will sit lower in the bore.
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are you talking about the valve relief cutouts? or "notches?" the pistons in the engine have two notches on the left of them
 
The only difference between the 71 heads and 72-74 heads are the intake valve got smaller to 1.88".

If you put in 2.02" intake valves, they are the same thing....
 
'72s had IIRC 8.8:1 with 915 Js. '73s had about the same compression with 587s. '73-up 360s and 340s had 587s and prior had 915s. Both '72 and '73 should be cast crank external balance with 1.88 intakes.

If you're worried about having 10:1- make sure you don't put something smaller than the stock 340 cam in it.
 
Those look more like aftermarket pistons to me. You will want to do some measurements rather than guess on the compression ratio. It may be higher or lower than you think depending on the machining done on the block previously. It's not worth it to guess, and there is no such thing as a "10.5:1" or "8.8:1" piston. There are just pistons of differring designs that get put in a block with a head gasket and head combination. Whatever that combo yields is what that piston "is". So you have to do a little measuring, do a little math, and then you know what YOU specifically have in that engine.
 
^^^^What he said regarding the compression. They kinda look like the TRW 2316 pistons to me, which should be a few thousandths out of the hole at TDC...but, I'm probably wrong.

This is courtesy of krazykuda; pulled from the tech archive

http://www.forabodiesonly.com/mopar/showthread.php?t=278215

If you have a few precision measurement tools, you can nail down your true compression ratio...if you want...
 
do any of you have measurements relative to the deck and the piston as to where the high and low compression engines had their pistons come up to?
 
eduardostewart said:
do any of you have measurements relative to the deck and the piston as to where the high and low compression engines had their pistons come up to?
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You can't just go by "the book numbers".

If you try to calculate an engine's compression from the numbers in "the books" you won't get anywhere close to what was advertised.

Also, they tend to run the heads on the "fat" side in case they need to do a clean-up cut to be able to repair a damaged surface.

also, there is no way to tell if your parts have already been machined by a third party before you got them, especially with parts that you bought individually.


The only way to tell your engine's true compression is to measure the components as in my How to check compression thread.
 
if i can tie the engine to the vin i'll be happy. i'm pretty sure my dad didn't replace anything internal like that back then.
 
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