I have a 4.0 and the rear timing cassette one of the chain tensioners or guides come apart. Was making a very loud rattling just loud noise when motor is running due to the chain being loose. I pulled the motor out. And when turning the crank by hand with a breaker bar there's a hard spot in the same place on every rotation. None of the valves look bent from looking at them through the intake. All the logs on the cams look fine no bad spots or anything. One of the rod bearings has a little bit of play in it just a little bit and when turning over the motor when you hit the hard spot it's a little clunk to get over it like going over hump then rotates just fine all the way around to that same spot. Do I have a bad rod bearing?could there be anything else it may be causing it.

 

I have mine apart for the first time, 200k miles on it and it does the same thing. No idea why though. I don't plan on tearing into the bottom end.

 

does it do it on a 360 rotation not once in 2 times around? if you have the motor out you could drop the pan and see what cylinder it could be?

 

Hey, I got to looking into mine a little deeper and on the number three piston there’s discoloration on the connecting rod around the wrist pin. I could see the resistance happened when that piston was at the bottom of the stroke getting ready to go back up.

 

are the heads off?

 

I took the piston out where the head gasket blew and the wrist pin had overheated and marred up the piston where the wrist pin is installed and smeared some aluminum over the oil inlet hole.

 

so is the rod ok? you just need a piston? might want to think of main and rod bearings and rings?

 

The rod seems fine, but yeah now I’m going to redo the pistons and bearings with standard size hardware. I expected the bearings to be worn, but it all looks good to my untrained eye. I can still see crosshatching in the cylinders so I don’t it needs to be bored out.