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About 15 years ago, I rescued a Yazoo S20 Master Mower from the transfer station. This big wheel mower has pneumatic tires, belt-driven blade and friction self-propel (FSP). The machine had been left outside for too long and its FSP chain drive was frozen along with the differential ratchets on each wheel. It has a 4 HP Briggs & Stratton Max engine (vertical crankshaft) whose carburetor was full of white aluminum oxide. The mower deck and frame was solid but the blade spindle needed new bearings and the PTO pulley needed to be remachined to eliminate a lot of pitting corrosion on its V-belt groove.

Fast-forward 15 years and I've been cutting one-foot tall grass on a hill in the back yard. I checked the engine oil and it was very low so I added at least 8 oz. I finished mowing and checked the oil again to find it was again too low. Another 8 oz oil got added.

Today, I saw the grass had some dark patches on it and I saw some oil drips on the garage floor beneath the mower. An inspection underneath the mower revealed the engine PTO oil seal had become completely separated from the engine block! I removed the PTO pulley and was able to tap the oil seal back into the engine block using a large deep socket.

I've never seen a lawnmower engine's PTO seal come completely out of the engine. The only thing I can think of that may have contributed to this was that I tightened the blade drive belt before mowing. I didn't go crazy tightening the belt but maybe the PTO shaft spun the oil seal out of the engine block?
 

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Get it out and gently re-flare the outer edge, clean everything up squeaky clean and reseat it using a smear of gasket goo between the seal housing and the block. Unless... the seal may have been damaged during the excursion, hence replacement will inevitably be required sooner or later.

Hard to see from here but... 🍻
 
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