X-heads vs J-heads: what's the difference??

X - heads (1968-70) with factory pistons were always rated 10.5 to 1, but like others have said, lots of variation - point is if you are building an engine you can make it 10.5 to 1 by matching the original specs.

J-heads first appeared on the 1970 1/2 340 six pack motors, but had relocated push rod holes that moved the pushrod away from the intake ports allowing most of the vertical hump just inside the intake port to be removed for much better flow. These "TA" heads all had 2.02 valves and special adjustable rockers that don't fit any other factory 340s or 360s. None of the other J heads did this, but the castings are identical - it's all in the machining of the pushrod holes.

A good set of X heads or 2.02 valve J heads set up well are great, durable heads for a street 340 or 360.

If there was no difference between 2.02 valve and 1.88 valve engines do you think Edelbrock would be bothering to sell a different head for these two applications?