While reassembling the timing gear on my E46 320td I made the silly mistake of releasing the chain tensioner before inserting the guide rails
At first I thought I was in trouble as I remembered the manual specifically saying that failure to compress the tensioner prior to disassembly would result in 'a very time-consuming operation' to correct it. While it may be easy to compress the tensioner with the cylinder head off, I didn't fancy spending another hundred quid on a new gasket and bolts. So if you find yourself in the same situation then the following trick might save you
Find a piece of wood like this and file the sides down at the end so that it fits down inside the timing cover.
The end of the tensioner is about 13-14mm diameter so drill a shallow hole of roughly the same size on the end.
Then use something heavy to hook onto the chain to hold it out of the way.
Use a telescopic magnet on the side of the plunger to lift the end upward slightly and locate the end into the drilled hole on the stick. Then its just a case of levering the plunger back into the housing until you can insert the locking pin