mpaahana23
Well-Known Member
whats the difference between the j head and the x head????
I'm new
I'm new
the difference heads small block
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yellow rose
Overnight Sensation
The only difference is valve sizes on any of th 340-360 heads.
They are virtually identical other than valve size.
They are virtually identical other than valve size.
mpaahana23
Well-Known Member
which one get the 202 valves?
yellow rose
Overnight Sensation
I honestly never remember. Lol. Nothing I do gets less than a 2.02 valve. So I grab th cores and make them what I want.
X heads came with 2.02 and J heads for 70-71 340s had 2.02 valves ...all other J heads had 1.88 intake..
not sure about 1970...those could have been x or j but they were 2.02...
71 360s had J heads but they were 1.88
not sure about 1970...those could have been x or j but they were 2.02...
71 360s had J heads but they were 1.88
If you are going to buy a fresh factory head with 1.88 or 2.02 valves, the 2.02 head will flow better. If you have a good 1.88 head and someone who knows what they are doing, putting in a 2.02 with proper throat and bowl work will way out perform a 2.02 from the factory whether ported or not.
Remember too, since the J and X heads are pretty much identical other than valve sizes, the X isn't worth snot in comparison.
Maybe if you're going after a total restoration and need the X heads. But other than that, they are not worth anymore than the rest, so keep that in mind.
Maybe if you're going after a total restoration and need the X heads. But other than that, they are not worth anymore than the rest, so keep that in mind.
They changed sometime mid-year in 1970 from X to J heads...
You can have 2.02" valves put in any of them. Even if they originally had 1.88" valves, any good head guy can open them up and put 2.02" valves in...
We have a winner... give that man a prize.
Give me a 1.88 head and do a good job cutting/blending in a 2.02 valve and you have better head than any factory 2.02 x/j piece.
Infecteddog
Well-Known Member
For '70 models Xs were still common for 340s. All Xs had 2.02s- not the same as Js. Js were used on some '70 340s, on '71 340s and 360s- but with 2.02s in the 340s. '72 all Js were 1.88 intake. Technically the heads for '73 were not J heads- typically they have AIR holes that were blocked stock but the design's different than later heads. After June, a motor is most likely the next model year. Honestly Xs are a bit better, but with the big price difference your money's better spent in improving something else or stepping above stock. Otherwise- the 340/360 heads are all about the same- take your pick of what you can find, a more desirable casting cheaper means worth more when it's time to go. Some of the late '70s heads can be hard to give away machined- great when you're buying, awful when selling.
mpaahana23
Well-Known Member
you guys know what the cc's of the j heads?
I want to say 72cc, I believe.
mpaahana23
Well-Known Member
low compression ???
yellow rose
Overnight Sensation
I have measured them as high as 76 cc's, which is why you have to physically check them. They are all over the place.
Lustle
Well-Known Member
I got J heads on my 340. They were 1.88, with a cut, some polish and what not they are now 2.02 and they flow excellently and run great, best of all J head's don't break the bank. There are some guys (even on here) convinced their X heads are worth 1g. Unless it's a "date/part correct" resto, they really aren't. The X head hype is just that, hype. A J or X head with a really good port job can still flow 250+ CFM and make you more than happy in a street car.
U heads too. 
Lustle
Well-Known Member
U heads too.
Yes U are correct haha. I almost forgot about them. In reality almost any iron head that came from factory on an LA from that period has the same potentially. Some heads just get a bad rap cause of the engine they were on. If X heads cam on 8:1 compression 1975 318's, they would have a bad rap too.
I have several sets of J's and the U heads appear to be the same, with the exception of the larger 2.02/1.60 valves.
With the capabilities and pricing of the aftermarket heads now available I can't see using factory heads in anything other than stock form. Spending money for anything beyond a valve grind and new guides is a waste. The only exception being if you can do the port work yourself, in which case you can save yourself some money still.
I had a set of 308 heads totally reworked for my 340 because the pistons I used were proud of the deck on my early 340 and I needed an open chamber head for reasonable compression numbers. 2.02/1.60 valves, chamber work, new valves, spring guides, etc., and a stage II port. After everything they flowed 254 cfm @ .500". But I'd also spent over $2k.
With the aftermarket heads out there now you could have brand new heads that flow that well or better for the same, or even less, money depending on your choices. I set the 318 for my dart up for zero deck and bought a set of RHS heads for less than I put into the 308's. The capabilities aren't identical, but for my application they're more than enough.
I had a set of 308 heads totally reworked for my 340 because the pistons I used were proud of the deck on my early 340 and I needed an open chamber head for reasonable compression numbers. 2.02/1.60 valves, chamber work, new valves, spring guides, etc., and a stage II port. After everything they flowed 254 cfm @ .500". But I'd also spent over $2k.
With the aftermarket heads out there now you could have brand new heads that flow that well or better for the same, or even less, money depending on your choices. I set the 318 for my dart up for zero deck and bought a set of RHS heads for less than I put into the 308's. The capabilities aren't identical, but for my application they're more than enough.
X heads are no better than J's The J has a slightly better exhaust port in it.
I've had 2 original 70 340 cars that had an X head on one side and J on the other.
I've had 2 original 70 340 cars that had an X head on one side and J on the other.
I'm a diehard fan of the 340/360 head! and with a little work you can make good ### BUT even i have fallen from the stock head. I went with a set of RHS head with my last build. I can make a 340/360 head flow as much or more then a RHS head prepped with a 2.02. BUT for the same flow (CFM) the rhs will outperform the stock head.(better port shape,(velocity) that you can't make without finding coolant passage or so close they crack)
The short answer........If you need to put guides in the heads, building ANYthing over stock. RHS iron heads is cheaper in the long run, and more power.
The short answer........If you need to put guides in the heads, building ANYthing over stock. RHS iron heads is cheaper in the long run, and more power.
mpaahana23
Well-Known Member
thanks
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