There is a rubber seal on the front of the crankshaft where it comes out of the crankcase, it's easily changed by an Engineer and is stretched and goes over the crankshaft flange with the prop off, have a feel around the crank / crankcase join behind the starter ring, it may be that.
See
http://www.lycoming.com/Portals/0/te...il%20Seals.pdf
Picture of seal
http://www.vansairforce.com/communit...d.php?t=104566
Another place may be the end plug in the crank, ( if it's a hollow crank ) this is a domed cap that goes in the end of the hollow crank and is punched in the centre to spread it and seal the end, easy change for an Engineer as above and requires prop off. (People often add a smear around it of RTV after installation in case it does weep )
See the expansion plug info at the end of this.
http://www.lycoming.com/Portals/0/te...0Corrosion.pdf
Picture of plug in place
http://www.chrisjgood.com/photo-log/...crank-plug.gif
From
http://www.chrisjgood.com/photo-log/engine.html
Other thoughts as above is check with your engineer to make sure he hasn't lubricated the likes of the starter as a little can go along way.
Hope they help you locate the leak, but as said get your Eng to have a look see to make sure it's nothing else.
EDIT
I would add after reading the link above re the end plug on one of the sites about using a ballpein hammer then hitting that with another hammer to dislodge the plug cap.
DO NOT AND NEVER hit a hammer with a hammer, they are both case hardened and can literally shatter and explode in your face if you do this.
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