Help with a John Deere 855

[altrac]

Hi there,  At work we have a John Deere 855 with yanmar 3cyl diesel engine, which has gang mowers on it.  We use it for a bit of light work cutting the grass for a summer camp site. 

At the end of last summer I started it, only to find it soon stalled.  To cut a long story short I realised it was out of oil (I know, should have checked before I started it!).  I diagnosed this problem to be down to to oil drain plug, or to be more precise, the copper washer around it.  It would appear the bottom of the sump on either side of the drain plug hole has what i would describe as 2 bumps and the copper washer had deformed to fit over the 2 bumps.  When the oil was changed in the tractor last June I didn't realise this, and so when the drain plug was put back in I just did it up tight as I could, though this wasn't enough to stop the oil gradually leaking out over the course of the summer.

I tried heating the washer on a gas ring to reshape it, this didn't work, someone else went to John Deere for a new washer, they didn't have any that fitted and didn't seem to know much about the machine.

Has anyone else had this problem?  

Any answers much appreciated

Andrew