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Is your STX 38 a black deck or yellow mower deck color?? Bob Lituri lituri54@aol.com
Remove the deck from the tractor, then remove both blades. They can be sharpened using a grinder. If you don't have a grinder, then there are shops out there that can sharpen them for you (try asking at a shop that sharpens chainsaw chains).
bladebelt diagram for john deere stx38 yellow deck
The STX-38 should have an electric clutch for the mower deck. The switch to engage it is on the right rear fender. It is a toggle switch. You will have to pull up on the switch and forward to engage the blades.
The number that you didn't list after the STX-** will tell you the width.
This depends on the condition of the mower and also what country you live in.
http://imgshack.info/images/k9bxsdwcte1kup0mijg6.jpg
From everything I see the 125 is a 42" mower with a 20HP engine. The STX-38 mowers were 38" with either a 12.5 or a 13HP engine. So, yes the 125 is bigger than the STX-38.
10w40 for the engine and hy-gard for the transmission.
The yellow deck STX38's were made from 1989 to 1993The black deck STX 38's were made from 1994 to 1997
stx 38 fly wheel
Check out crigby's John Deere deck page. It shows the belt routings for most of the JD mowers. See the link below.