taken from the tech section... you might be able to gleam more infor from here.. it doesn't actually say the valve sizes. all you can do is measure them with a set of calipers to see the diam.
http://www.therangerstation.com/tech_library/40rebuild.htm
HEADS
There have been three distinctly different heads with five different casting numbers installed on the 4.0L since 1990. The combustion chamber was changed in ’95 and the exhaust ports were revised in ’98.
1990-’94 All and ‘95-’96 Aerostar
The original 90TM casting had an oval-shaped chamber that was slightly bigger in diameter on the intake side. It was replaced in ’93 by the 93TM-AA casting that was exactly the same. These heads can be identified by the letter "T" located on the top of the right rear/left front exhaust port.
1995-’97 Ranger, Explorer and ’97 Aerostar
The 95TM-AD casting that came out in ’95 had the heart-shaped, fast-burn chambers that shrouded the intake valves. It was replaced by the 97TM casting in ’97, but it was the same, so they can be used interchangeably. These castings were used on the Ranger and Explorer from ’95 through ’97 and on the Aerostar in ’97. They can be identified by the letter "U" that’s located on the top of the right rear/left front exhaust port.
1998-2000
Ford introduced another new head in ’98. The exhaust ports on the 98TM-AD were much narrower than they were on the earlier castings; they measured 1.40˝ across the port compared to 1.70˝ on the 95TM/97TM castings. According to the engineers I have talked to, the smaller ports increased the velocity of the exhaust gasses so they carried more heat down to the catalytic converter. This helped the converter “light off" sooner, so it did a better job of reducing emissions during the critical start-up and driveaway phase of the EPA emissions test. With that in mind, it’s probably not a good idea to swap these heads back and forth with any of the earlier castings.
taken from the tech section...
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