There were 2 M-2's,long shoe and short shoe,if its a 77 its got the short shoe,about 6" shorter than the long.Is it gear drive or hydro.Gear drives had a zerk tapped into variable speed sheave,really helped.Probably got a 130 hp 301.I 've had both long and short shoe machines,both can be set to work well.At 2800 hrs the machine has lots of life left if maintained.Obvious would be to check cylinder bars concave,raddle chains,and straw walkers.Also check striker plates on rock door to see if the door was run loose,it wears the rod that holds the door.Check the swivel on the unloading tube to see if the ball bearings are worn.Could combine 35-50 acres per day.