2006 craftsman briggs & stratton 18.5 engine ran fine , I shut off for 10 min coould not get it started again. Starter is good, oil level is good. Took cover off top of engine can only move engine 1/4 turn & it stops like it is jamed on something. The machine has very few hours of use & was always well serviced.

1st take plug(s) out and see if it still jams.

If it doesn't, adjust valves.

I did remove the plug,no change.
Will try valves tomorrow.
Could the valves mess up just all of a sudden with no warning, no poor performance, no smoke , always easy to start, ran perfectly. Would there have been signs of any type?
When I move the engine the quarter turn (either direction) it stops with a "thunk"

Thanks
Al

You missed my point. With the spark plug removed, it has to be something else besides valve adjustment.

It actually sounds like you threw a rod, but could be a bent valve. Anyway, it's internal.

Rods break when you run an engine without oil. I know. I did it when I was about 12. Dad said: "Your fixing it".

Funny story
I helped a neighbor whose new lawnmower (just taken out of the box) did exactly that. The handle protrudes into the blade compartment and moves with thumbnuts. He didn't want to read the directions. It was easy to call me over from his back yard.

Like kiss said sounds like you threw a rod,