Hello,
After 3 hours of continuous use, my Airens riding mower w/ Briggs 11.5hp engine (28D707 model) made a banging noise then quit. Someone other than myself had been running it w/o checking oil. The engine was seized/frozen.

Following suggestions from other helpful posts :) in this forum:

1) I used some PB Blaster on it (next day after it cooled).

I tilted it up to get to the crank shaft pulley nut (b/c the fly wheel is plastic), put a 3/4 socket on it and turned the engine/shaft. As soon as I turned it, I heard a metal ping, like something had fallen out & it began to rotate freely. Nothing fell out on the ground, so it must have been internal (sounded like metal on metal).
Now it spins quite freely with no compression whatsoever.

2) To check piston movement
I removed spark plug, inserted screwdriver, and pulled the starter rope.
For one full pull of the starter rope (~36") the piston pushes the screwdriver twice and moves it 1/4 inch each time.

I'm thinking maybe the piston pin or connecting rod is broken/damaged meaning a complete engine tear down.
Other thoughts/suggestions?

Thanks

Your rod is broken and it moved on two different strokes just touching the piston enough to make it move.

Thanks MOWERMAN2468 - you confirmed my fears ;)

Now - what's the best way to remove the engine? I've stripped all external parts (tank, filter, starter, etc). The engine shaft goes through the body and has a large nut with a pulley attached. If I turn the nut, then everything turns.

If I remove the starter clutch and flywheel, will I be able to lock/stop shaft from turning so that I can remove the pulley nut? Are there special tools I'll need?

I'm trying to do this myself as a learning project for my son & I, plus all the small engine shops are backed up for 1-2 months.