Small Block Heads

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Bill Rossi

Bill Rossi

Plum 340
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Hi! I recently bought a '71 Dart with a '69 340 engine and found that someone had replaced one of the X heads with a U head. I know that heads are supposed to match but what about this combination? No go? Find another matched set?
Thanks!
 
Marginal diff in the intake ports, it'll work fine if the chambers are all the same cc as the x head.
Get another J head and sell the x, unless you can find one x head floating around...
 
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You’ll never know the difference.
Run it!
 
Are the valve sizes the same? X’s have 2.02 intakes: J’s are 1.88’s
 
Some J's are 2.02.

No two heads even if the same alphabet flavor would be exactly the same from the factory.
 
70 & 71 340's are 2.02, 72's are 1.88 all are J heads
68 & 69 are X heads 2.02
 
'70 T/A & AAR 340-6 heads are 915's, which were typically J, but I believe some 340's were O & U 915's. Never seen a definitive explanation of the different letters. Casting numbers are better to go by.
 
It’s all the same chit minus valve sizes
 
Bill Rossi said:
Hi! I recently bought a '71 Dart with a '69 340 engine and found that someone had replaced one of the X heads with a U head. I know that heads are supposed to match but what about this combination? No go? Find another matched set?
Thanks!
Click to expand...
 
Thanks! I think the U head will be okay especially if the intake valves are 2.02.

DSCN0997.JPG


DSCN0999.JPG
 
What is a "U" head. I thought I knew a lot about small block Mopars, and I know quite a bit about X and J heads, but I don't think I've heard of a U head.
 
my back up mill will go high 10's in Vegas without 2.02 valves.... not a huge concern imho. This is not a high tech mill by any means. KB pistons, mech cam, old crane ductile rocker arms, stock rods, Holley victor 340 intake, 750 BG carb. We did put a forged crank in after some concerning cracks showed up on our OE cast crank.
 
It has been explained to me that other than the difference in stock valve size. The x heads were designed for the bigger intake valve so the water passage isn't as close so you can get 10-12 cfm more at max porting effort than with the others. No difference otherwise.
 
I had both x's and original big valve j's and was told to bring the x's by Porter Racing Heads.
 
harrisonm said:
What is a "U" head. I thought I knew a lot about small block Mopars, and I know quite a bit about X and J heads, but I don't think I've heard of a U head.
Click to expand...


I have had 8 sets of T/A heads and most were U on top and J on the bottom.
The casting half's could be any combo of J, U, O, or a rare Z head
 
Found a couple pics of T/A heads, one was a J on top, one was a U on top, and both were a U on bottom

001.JPG


002.JPG
 
I can't see in the picture...are those two heads date coded close? Also, it's funny they say 360 on them.

If they weren't such crazy money I'd love to work with a pair.
 
The dates are 02100 and 03110 so Feb 10 70 and Mar 11 70, one month apart.
Only 68, 69 X heads have 340 on them, all other 70 & up J,U,O,Z, heads have 360 on them.
 
yellow rose said:
I can't see in the picture...are those two heads date coded close? Also, it's funny they say 360 on them.

If they weren't such crazy money I'd love to work with a pair.
Click to expand...
For what it’s worth, I’ve had several sets of O and U heads over the years. Every one of them had 2.02 intakes, and had the AAWJ 360 on them. They all performed comparably with the X heads. They definitely were a step up on the 1.88 J heads.
 
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