Small Engines (Lawn Mowers, etc.): 17.5 briggs clunk no start, briggs and stratton, spark plug wire

Question
Hi I have a 2003 lowes tractor with a 17.5 Briggs and Stratton. The tractor was running, made a loud clunk and will not start now. No compression and when you turn it by hand sometimes there is a clunk that sounds like it is coming from the cylinder area. Sometimes it will spin and no start or spin and clunk. The tractor was backfiring when shut off before.  

Answer
Sounds like you threw a rod. That's bad. That means blown engine.
Is there any oil on the dipstick?
Pull the spark plug out and have someone crank the engine while you hold your finger tightly over the spark plug hole,
The engine should blow your thumb of the hole.
Use caution while doing this, stay atleast 4"-5" from the end of the spark plug wire. Trust me here, if you don't follow my advice you won't get hurt but the shock will make you wish you were dead, for about 1 second.