Hi guys, I have a 99 ranger w/ 3 litre flex fuel model. the block is a 98 non flex fuel model but we used the 99 injectors to make it run right. it has been diagnosed w/ a burn't valve. I have a set of already machine checked heads from a 2000 ranger. I realize they are different like 7mm valve size compared to 8. will they work on my engine& run correctly? THX! Shawn

 

Doesn't matter about valve stem sizes, but the combustion chambers are different.
Here's some info on that.
If your new machined head has the "heart" shaped combustion chambers, then you may use it in a flex fuel engine.

Ford Ranger II 3.0 liter Engines

 

Yes, 2000 heads are fine to use on 1998 block, use 1998 head bolts, 1999 and up blocks used longer head bolts, they have deeper holes

Yes, Flex Fuel computer(PCM) needs to have the higher flow Flex Fuel fuel injectors, regardless of the heads, or if you never use E85, PCM is programmed for the larger injectors

Burnt valve can be caused by a few things, but all have to do with heat build up in that cylinder.
Valves rotate as the rocker pushes them down, this prevents hot spots on the valves.
Coolant flow carries away heat from valve seats
Lean fuel mix makes a cylinder run hotter, especially when accelerating
Rich mix and Carbon build up retains heat inside cylinder.

Burnt valve starts with small gap between valve and valve seat, this can be caused by valve heating up and warping, or defect in valve seat.
When the cylinder fires the hot gases escape thru that gap and make it larger, and in very little time you lose compression so that cylinder dies, misfires