Timing chain.
The 5.0 litre / 302 cubic inch V8 used in a 1999 Mercury Mountaineer has a timing CHAIN
The 4.0 liter SOHC , V6 , has ( 3 ) timing chains and the 5.0 liter V8 has ( 1 ) timing chain , if that's what you are asking ?
You can check on eBay.
Timing chains are designed to last the life of the engine. They are normally replaced during a complete engine overhaul. Unless the chain becomes noisy no maintenance is required.
Timing BELT
Mercury Mountaineer was created in 1997.
Timing Chains (2) with respective guides, tensioners and sprockets.
According to the ( 1999 ) Mercury Mountaineer Owner Guide : ( the length is 188.5 inches )
There are two head gaskets in the V 8 1999 mercury mountaineer
http://www.autozone.com/addVehicleId,2579302/initialAction,repairGuide/shopping/repairGuide.htm?pageId=0900c152801decc0 This is for a 2000 don't know if it's the same as a 99 but might help
looking for a diagram for 1999 Mercury Mountaineer. Need to take the fuse out for the seatbelt alarm, it will not shut off.
The 2.5 litre V6 engine in a 1999 Mercury Cougar has ( 1 timing CHAIN to each cylinder head , so there are 2 timing CHAINS )