As others have said, if you really used 1N4001s you're lucky to still have a mains transformer. Ed is right about the reverse voltage so you will need 800V rated diodes, not 50V
If you only have 400V rated components (e.g. 1N4004s) you can use 2 in series. The first will be loaded by the forward voltage and the second by the reverse voltage.
High voltage rectifier diodes cost next to nothing, so it's worth stocking up:
http://cgi.ebay.co.uk/Diode-Rectifie...QQcmdZViewItem
Silicon diodes generally fail short circuit so it's a very bad idea to push your luck.
Best regards, Paul