WARNING!! its not a good idea for you to adust the timing because it very dangerous for your car, your safety. i recommend that you see a mechanic. doing this yourself requires a lot of experience in car mechanics. if you have the experience. then you observe the timing belt and the pullys. also contact the Subaru dealer and ask to be transfered to the mechanic and ask him/her for the timing configuration for your car.
in with the timing belt change them at the same time
105k is the recommended time.
Water pump is run by the timing belt. So, you have to do everything required to replace the timing belt plus a few more capscrews that hold the pump in. These should be done at the same time, 10 years or 100,000 miles, which ever comes first.
You don't. Timing is adjusted totally automatically by your ECU.
there are timing marks on both cam pullys and a timing mark on the crank pully as well. you have to have them all aligned to the marks on the inner timing cover, which is virtually strait up and down on all three. and you must also match the timing lines on the new belt up with the timing marks on the pullys. be sure not to roll any of the pullys backwards (counterclockwise) or you could cause some major valve damage.
You should change the water pump when changing the timing belt for convenience. Subaru recommended timing belt change interval at 105k miles. Here a link for DIY: http://www.iwsti.com/forums/how-install/154977-04-sti-timing-belt-change.html I wouldn't recommend DIY for noob since the work is quite extensive. I just let it to the pro for my timing belt change, although I didn't change my water pump at the time for the lack of money.
Replacing the water pump on a Legacy motor is fairly easy-- you just unbolt the old one, clean the gasket surface on the block, and bolt the new one on. Ummm, what makes this job tougher is you must remove the timing belt to access the water pump, as the timing belt drives the water pump. This then is a grand time to replace your timing belt while you are at it. Both these jobs (the timing belt and the water pump) require precision and attention to detail. Make sure to refill the radiator several times before firing the engine up, and recheck it once the thermostat has opened, then engine cooled. Legacy's make dandy air pockets in their cooling systems-- these pockets purge out when thermostat opens, then you are low on coolant.
Mostly it has a time belt 99% it has a time belt WRONG IT HAS A CHAIN !!
It has no Timing Belt, it has a Timing Chain.
Why would a timing belt jump time and cause damage to the cylinder heads ?
When you have the belt put back on correctly it will be back in time.
If it is a 4cyl then no. It has an upper and lower timing chain. If it is a 6cyl then yes. It has a timing belt, an idler, and a belt tentioner that all should be replaced at the same time.