Get a picture.
Odds are the metal, if steel is part of the guide rails. Pretty common to break.
If it is aluminum it could be a guide rail, but more likely part of the timing chain housing. If your chain snapped or messed up, either is likely.
Rails are expensive to replace all of them, the timing housing is expensive and a hassle, it involves the Profile Gasket.
If you plan to do it yourself, start ripping it apart.Write down everything you remove that needs to be replaced, other wise you will likely not get something ordered and need it later (hint, lower gasket set does NOT include the upper timing cover gaskets or water pump).
If you do replace the timing chain housing, there some other info on the site here, but basically there are 2 methods. One is remove the head and do it, this is the safest way, but a bit more difficult, it is safer too. The second is from the bottom, meaning having to drop the upper pan, meaning lifting the engine a bit and scraping it while it is lifted or you can remove the entire engine. Bmwman91 and myself went from the bottom and have had no problems. This is more difficult also because of having to install the profile gasket in a strange way, again, there is good info here on how to do it.
At any rate, pull the radiator and have at it. The work is not difficult really, just a hassle, and a lot of parts.
Either method you chose you will need another person to assist and you will need a crank bolt puller, check the specialty tool loaner thread or you can fab one up. Actually, since you are in St. Louis (WOW another 318IS in the STL area HOLY SHIT!), I can drop mine off if you want. It is just a bed rail cut up but I have one for the water pump pulley and one for the crank. You also will need a torque wrench.