I just bought a JD 5075M with the 5 cylinder 5030 Turbo engine. I have a coolant leak coming from somewhere near the thermostat housing, that I'm having a hard time with. I have tightened the hose clamp, and even added a second clamp on the upper rad hose where is meets the thermostat housing....no change. Also, there is about a 3/4" very course threaded hole on the front of the motor just under the pulley on the thermostat housing. I have coolant that is leaking from there as well. I don't know what this hole is for....?
Do you have any good pictures of the effected area where the leak is, and the hole you're referring to? I don't have and experience with a 5 cylinder John Deere, but maybe a few good pictures can help tell someone on here something to help you out with.
You don't see any cracks where the coolant holes connects to the thermostat housing do you? They're often very fine and hard to see without close examination. Obviously not a tractor, but I once had the thermostat housing on my old 1999 Mustang GT develop a hairline crack running down it. Unfortunately its thermostat housing was molded into the intake manifold so I had to buy a new intake to fix it. Lol
Sounds like you have a timing cover leaking. The original ones were sealed with sealant and sometimes leaked. There is now a paper gasket to take the place of the sealant. There is also a brass shim added on the top portion of the timing cover between a rubber gasket and the cylinder head. The timing cover is a time consuming repair. The oil pan needs to come off and the governor and oil pump are mounted to the timing cover. I don't recommend tackling it yourself unless you are very mechanically inclined. THe threaded hole you see is unused in this application. Some applications mount a fan bearing there.