I've had my 70 F250 with a 360 and auto trans as my daily driver for almost two years now. I've just about finished up my new project truck (79 250 4x4 w. 400) so my buddy Tom is inheriting the old 70 since I'm moving to L.A. then (hopefully) heading to Austin and I just can't afford to drive both the gas guzzlers.
As long as I've owned the truck it's had a low end knock, no repitition, just sounds like random clanking. Goes quiet when the oil is circulated, but comes back when the motor heats up after a while and the oil thins out. When I got the truck I knew very little about repair, and an oil change (20w50 with a can of Restore) quieted it down plenty (enough to drive to L.A. and back with no problems).
Now that my friend is getting the truck and he's just learning about auto repair, and it's going to be his work truck, and his wife and kids will be in it, so I want to fix this problem before I leave town.
After a few friends going over it, we've figured it's a connecting rod bearing on cylinder 7. I've found the part online for around six bucks. (www.partsamerica.com, searched for "rod bearing")

I know how to install the part (I'm going to drop the oil pan to make sure it IS the problem without a doubt). But I'm curious about why it says the housing bore is resized .002" larger than stock. Any ideas? I have about a week to get out of here, so I want to get this taken care of. Any help would be great! Thanks.

 

Usually it's not recommended to just replace a bearing without doing more work. It's a quick fix, but usually not what I call a long term fix. It's more apt to just tear the new bearing up.

The "housing bore" they are referring to is the rod end. They manufactured the bearing to fit into a slightly(.002) larger bore. Thus if you try to install it in a stock rod, it will crush the bearing too tight and more likely destroy itself when you start it. There is certain amount of "crush" allowed when you torque the bearing in the rod. Too much and the bearing will try to squeeze out somewhere....usually at the mating line between the rod and cap. You might think that .002 is not alot, but in this type of application it's MILES!!!!

That all said....you can try to just replace the one bearing(if that's the problem), but in my honest opinion.....it's a short lived fix that might leave you somewhere along the highway.

I know that's not what you want to hear.

 

Now I'm wondering if I was looking at the correct part or not...
I'm looking for the bearing that connects the piston arm to the crankshaft...(too many different bearings, can't keep names straight).
If that's not the connecting rod bearing, than what is it called?
And if that IS the right part, what procedure do I need to go through to get it running safely?

 

The part you are talking about is the connecting rod bearing. The clearance between the crankshaft and the bearing inner side, where the oil squeezes through, is about .0015" That is about 1.5 thousandths of an inch. Some a little more some a little less. When you have a knocking bearing, you likely have several more that are very close to knocking, too.
The bearing inserts are available in slightly 'tighter' versions than originally fitted at the factory. If you replace a standard size with an undersize, it would be a good idea to use a product called. Wow, I can't remember. Oh, plastigage. It is wax that is extruded to a specific diameter. Place a bit across the bearing insert, replace and torque the bolts. Remove. Use the package to determine the clearance in your bearing. The plastigage will get squished flat.. the flatter, the more wide the squish and the closer your bearing fit.
Most people won't just replace 1 connecting rod bearing. You probably can't buy just one. If you want it to last, you'd have to measure the journals on the crankshaft to see if they were egg-shaped, determine if they were too small for refitting new standard sized bearing inserts, polish or grind the crank to the proper diameter, etc.
If you are pressed for cash, you can replace all the bearings, all the main bearings, and probably be OK for a while. If it knocks enough that you hear it, it is probably going to fail when you least want it to.
tom

 

You may want to use the plasti gage on the old bearing to see what clearence you do have. My reason for suggesting this is you say it is a random knock, usually a bad bearing is there all the time.

 

The truck's been running like a champ for the last couple of years even with the knock. I'll get some Plastigage and get the oil pan off and see how it all looks in there and update accordingly. Thanks.

 

Another thing, if you tighten up the bottom. Likely the top will want to give out soon after. Rather than the stress be distributed 50/50, it's 100% upstairs. Usually not a good idea.

Madmike33

 

So what CAN I do? I keep getting told what won't work, but what will? I know the knock will shut up after I do an oil change, but I'd rather find a more permanent fix.

 

Following your last post I think we better go thru some basic knock diagnosis.
If you race the engine to maybe 1500 rpms quickly (like snap the throttle) do you hear the noise? Connecting rod bearings knock when the engine is decellerating . If you don't hear it then you don't have a loose rod. How is the oil pressure? If you don't have a noise and it developes I more inclined to believe it is a lifter collapsing or something else, Can you pull a plug wire or ground it out does it leave. That tells you it is a rod brg. or something in that cylinder. Main bearing are noticed on the hard pulls a deep knock. Now you tell us all what your hearing and we'll all try and help you. An engine which runs well as you have stated and only has a noise from time to time needs a bit more thought then to go opening it up and not
be sure what your looking for.

 

My Vulcan 3.0 has a quarter million miles on it. It too will get a little noise when the engine warms up. Doesn't knock as far as I think the word knock applies, but has a little bit of rattle.
If I were to do anything to tighten up the bottom end, such as replace the rod bearings and maybe the mains, I don't think that would affect anything else in the engine except to increase the overall oil pressure in the system. Perhaps more would flow to the lifters & rocker arms, but so what. It is not the same as doing a valve job and not touching the rings & bottom end. This IS the bottom end, and you don't add any load to the upper end by replacing the worn parts.
IMHO you should drop the pan. Plastigage the rods and mains, and if the journals look clean and don't have any scratches or grooves, throw a set of bearings in. The cost for parts should not be terribly expensive, especially if you realize that this is not a race engine, and you are not looking for performance. Lowest price bearings are the choice here. Take a look at the timing chain while you have it that far apart, and get a timing set if the slack excessive.
If you replace the bearings, use plastigage to see what your clearance is. You probably have a worn crankshaft(picture oval cross section), but... but if you put a cheapo set of rod bearings in, you will close up the tolerance, and maybe get another few thousand miles before it becomes necessary to do a complete overhaul, maybe tens of thousands, as you've gotten a bunch with it being noisy.
Be aware that you may not get those miles, this is just a bandaid fix with no guarantee of success.
tom

 

Bandaid fix is all we're looking for right now, until we can find a replacement FE motor. With the gas prices the way they are, people just give them away (that's why this one's going to my buddy) Thanks, I'll find some plastigage in a couple of days and get to work on it.

 

If all you're looking for is a bandaid fix, then go ahead and change all the rod bearings rather than just one. You mentioned that the bearings weren't so expensive then I would change them all if I were you.

Good Luck and let us know what you decide.

Madmike33

 

9x out of 10 if you've got a rod knocking you have hurt the crankshaft is what these guys have been trying to tell you in a nice way. Would you hit a polished surface with a hammer? ...NO well thats basically what happens when a rod bearing goes bad , it beats the crankshaft to death bearing surfaces have to be smooth and pefectly round or they will ruin the new bearing right away and knock again the connecting rod big end gets pounded out of round ect. ect so to be blunt if it is a rod knock you need a new crank or remanufactured crank at least a new rod you could band-aid it but the odds are if you leave it alone it'll last just as long as if you change the rod bearing and use the same pounded crank and rod

 

Good point...

Well as it's turned out, my friend that wanted the truck changed his mind because all the work seemed too overwhelming to him. It's already in pieces, so I get to put it back together tomorrow and park it at my friend's place when I go to LA. I figure I'll do an oil change with a fresh can of Restore, and just look around for somebody getting rid of a decent FE block I can toss in... Thanks guys.