I paid $500 for mine last summer.
Strong points:
*) 600cc Diesel engine is overkill for a lawn-tractor.
*) Fuel efficiency is surprising.
*) The Eaton model11 hydro unit and rear-end is WAYY beefier than any other garden tractor ever made.
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http://www.eaton.com/ecm/groups/public/@pub/@eaton/@hyd/documents/content/pll_1616.pdf
*) With chevron-tread tires the G5200 is a workhorse for pulling implements
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Incase your research has not revealed the 'weak spots' on these 1980s era G series.
MOWER DECK
seems to be manufactured out of poorly recycled steel. If not meticulously maintained (removed, cleaned and oiled underneath) it WILL rust into oblivion. Kubota ran out of replacements years ago so repair is the only option. My mower deck has a 6-inch hole in it, 100s of pinholes and many cracks. I have been spending hours wire-wheeling and prepping for welding.
STEERING GEAR
gets sloppy. Kubota seems to have realized this because it re-designed for the later models. (see parts-list for details)
COOLING SYSTEM
Keep an eye on the engine temp whilst running. The dust-shield on the radiator MUST be cleaned regularly to maintain airflow. If one installs a larger battery, it may block airflow through the radiator. Keep battery away from radiator airflow.
SAFETY SWITCH(es)
Seem to be an add-on afterthought. They have tendency to fail and disable the starter from cranking.
AIRFLOW OVER HYDRO
Remove cover between footrests to expose driveshaft, fan, and hydro-unit. If cooling-fins are dirty, degrease and use brush to clean them.
FRONT AXLE PIVOT
May 'freeze up' and be VERY hard to free-up. It is best to get some grease in the nipple and 'work' it to spread the lubricant. (lift front of tractor and the axle can be moved by hand)
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General details ==>
http://www.tractordata.com/lawn-tractors/000/3/8/381-kubota-g5200.html
Quick parts reference ==>
http://www.kubota.com/part/PartsLookup.aspx
Exploded parts lists ==>
http://www.kubota.com/part/partsList.aspx