HI, I have a 2000 Expedition that quit sitting at a light. Would crank but not start. Checked spark, fuel and compression. Spark and fuel are good. Shop told us it was probably timing chain break and damaged motor and new motor will run about 3200. Husband took it apart...found timing chain slipped but didnt harm the valves. Replaced both timing chains and head gasket. Still no compression and wont start...any ideas?...its a 4x4 4.6 L

 

If the engine has no compression at all , the timing of the cams to the crank have to be off. Did he have the correct timing marks on the gears in the correct place when replacing the chains?

 

Yes, he made sure of that

 

Unless someone comes up with a better idea, under the circumstances, I would remove the easiest valve cover to remove,and look to see where the valves are when a cylinder on that side comes up to the compression stroke. If both valves are closed and still no compression , there has to be some damage in there .

 

This is NOT a freewheeling engine, and if the chain slipped more then 3 teeth (I'm not making this up), the engine probably has damage to valves or piston. Did anything look amiss when the engine was apart? Bent valves are hard to miss, but sometimes these valves can kiss the piston, bend either the valves or piston, and not be easily seen as damaged.
To make a final determination, do a good old base compression test, just to make sure you have an engine that CAN run. It sounds rough, but will save time and money in the long run.