The other day I got stuck in some mud and was given her hell when motor started diggin and killed the motor started it back up and nothin was happenin I look back and my prop is gone. Got my prop back and it just snapped right outside the housing just on the last thread. It's mad mud motor frame and a 16 hp duromax engine. I bought the frame used and had to weld up something different for the bigger motor. Any thing like this happen ever to anyone? I never hit any rocks or nothin. I thought about putting shear pin in it so it couldn't happen again anyone try that ever?

Shear pins wont work. You'd need a bucket full of em, every time you bumped something solid you'd be replacing it. Just by the sounds of it the motor may not have built correctly or something wasn't quite right. Not much you can do but speculate, people run these motors hard and the previous owner could have done something that really hurt the integrity of the frame on accident... I don't own any of the plans but from I've heard the instructions are a good start, but not quite spot on... I've also heard the best part about the plans is that some sort of parts list is included...

just like the donkey said, there is definetly room for improvements and modifications. I have modified mine from the mad mud motor plans that i bought. The parts list is awesome. I am going to have a guy on here turn me some bearing housings. Should send the thrust to the top bearing and the housing. I think that swamp donkey is right on about the stress that the previous owner may have put on the shaft. I would forget about it and spent $25 and get a new shaft.

It started off as a 6.5 hp an then I put a 16 on it the shaft had some rattle about mid throttle. How bad is that? What grade of drive shaft stock is the best bang for your buck?

cold rolled or stainless. You might have had a bent shaft and it wore out the center bushing causing the shaft to whip in the center, resulting in a broken shaft.

Been there done that twice 2 different GDs 23hp, 25hp got one prop back davey jones locker for the other.