I got the LT313 trencher fixed the other day. Turns out the motor was alright. It leaked fluid real bad where sprocket connects to output from pump when trencher was running. The 1st Bobcat dealer said hydraulic motor was bad. Finally brought motor to a independent hydraulic shop they tested it and said motor was fine and it will leak a bit of fluid around shaft when running (part of the design). As it turns out the carrier the pump bolts to and the shaft connects inside of carrier to the output to sprocket of trencher has a high pressure seal that retains this fluid for bearing lubrication in the carrier. That high pressure seal was bad. I replaced high pressure seal, o ring and dust seal in carrier. Trenched a 100' 2' deep trench yesterday in real rocky ground and it worked great no leaks.
The 2nd Bobcat dealer was real helpful in figuring out the high pressure seal and that Bobcat changed the motor in newer trencher that's why the one I bought had a larger diameter shaft. You where right Wayne I should have checked the serial number.
Now I have a new hydraulic motor Bobcat part no. 7116831 (Eaton #161-0088-005) that will fit a newer trencher, angle broom, drop hammer or tiller from Bobcat. If anyone is looking for a new hydraulic motor for these let me know maybe we can figure out if this one will work. Motor new from Bobcat is $3,746.99 I will sell this one for much less then that.
Thanks for everyone's input on this site. It's helped me more then once and I enjoyed reading others posts.