Hey everybodyhow ya doin.
I have a 302 in a f250 with low oil pressure. So, what I'm trying to figure out is exactly what needs to be replaced. Symptoms include a knock when at high rpm's. Not when pulling to speed only when I begin to let off of the gas,or at curbside if I hold it at high idle. I'm not sure what the difference in sound would be between a spun bearing or valve tapping . Any imput would be helpful.

thanks

 

Well with low oil psi you know its a worn bearing or you can be pretty sure. If its a knock, not a tick then you can be almost certain its a bearing, either main or rod.


My spun main bearing would knock at 2000+ rpm consistently, under that rpm no sound. good luck with fixing it, later

 

IFyou spin a bearing you will know it immmediately and thats about as far as you'll go because a rod comes through the side of your block next. If it's a con rod you'll hear a double knock quite high pitched while a main bearing will be a single deeper sound somewhat muffled. I assume your statement about a spun bearing is a figure of speech because an actual spun bearing occurs with some attempted seizing by the motor and you already have con rod knocking ,substantial damage occurs.

 

i'm now replacing rod bearing on a inline 6. check this out while the engine is running crawl under the truck and see if the knocking is any louder. if it is then most likely its the bearings. good luck

 

Your problem sounds just like the one I have with my 400, which is a wrist pin. The sound changes or even goes away, depending on accelerator position.

 

Thanks to everyone your replie's are very helpful. Obviously I'm inexperienced, so everything may not be explained correctly. However, sounds more like what Scroob said plus worn bearings and low oil pressure certianly could not help. Having said that would it be possible to just change bearings and fix possibly bad wristpin on bottom end with out removing engine. Is it critical to have crank redone or would new bearings alone fix my oil pressure problem.

Thanks in advance
Jerky38

 

you have to pull your engine out as the wrist pin is at the top of the con rod and pistons only come out the top,after the head is off. It is very difficult to identify the offending wrist pin without high quality measuring instruments and as for the crank and bearings ,usually if the bearings are quite worn the crank has grooves or wear spots and needs to be refinished(turned), also if you have a knock then the rods should be resized.

 

If the knock is loud enough to hear reliably, then there is a way to find out which cylinder it is. You pull one plug wire at a time, and drive it. When the knock goes away, you've found the offending cylinder.

A very involved, time consuming process, yes.

And yes, it is possible to fix the bad wrist pin without pulling the motor. The question is: is it worth it? If one wrist pin is bad, how many more are almost ready to go?

Pull the head and the oil pan. Use a ridge reamer to remove the ridge at the top of the cylinder. Release the rod, making sure not to mix up the bearing shells. They need to go back exactly as they were.

Now, you can remove the piston/rod assembly, and replace it with a new one.

You have to make sure you have exactly the same piston, or you will change the compression ratio in that one cylinder, and the engine will be out of balance.

It's also possible that the wrist pin has moved, and has scored the cylinder wall.

Now, you see why I said "Is it worth it?"

It may run a long time after this, or may not. It's a crap shoot. With worn bearings and low oil pressure, all the signs are there that the motor is worn out. You could take a chance and replace all the bearings while the pan is off. I know people who did this and it worked. I also know people who did this, and got a knock almost immediately.