i tend to check all fluids beginning of each day i will be using the tractor/machine. and if i know there is a leaky piece of equipment i will check 1/2 way through day. along with give a quick look over everything in the beginning of the day.
as far as the oil nut. it has not been the oil nut. but various bolts on equipment coming loose to busting to breaking. main reason i keep some wire and pair of pliers on tractor. so i can tie stuff up and limp back to the garage for a good repair.
have lost gas caps a few times over the years. have had oil dip stick for engine bounce loose. nuts on tires come loose. even when things were torqued. i run the machines so hard at times. and beat them to death not to mention vibrations and jacking and hee hawing. the tractors go through running over timbers and ruts and everything else. i am kinda amazed it is not seen more often of something happening.
drain plug / bolt though, would be one of them major "ouchers" ya just "cringe" thinking about it.
not even sure i would even be looking for an "oil" leak behind me. if i was bush hogging. i normally try and keep one eye forward one eye behind me. but an oil leak... not even sure if it would click.
when ya pull drain plug as it is, it does not take but a couple minutes to get all the oil to drain out as it is more so when engine and oil are warmed up to operating temperatures. not sure if a temperature gauge for radiator would even show anything and if it did i doubt i would even catch it myself if temp raised. i normally check my gauges every few minutes, but good amount of time between checking the gauges. once i had the machine up and running for 30 minutes to 1 hour pending on temperatures and what i am doing.
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if it is insured give them a call.
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hopefully you can get things into neutral so you can at least tow it / get it up on a trailer. to start tearing things apart, to diagnose what has happened. or take to a repair shop.
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you say engine will not turn over. but will the starter turn? and sort of clicking sounds? ya i would assume engine locked but.... start at the beginning, maybe someone did vandalize tractor and did more to tractor than just oil plug, or something got burned up, (wire for example) due to un wanted high temperatures.
hope for the best, vs new engine / rebuilding of engine.