The Stovebolt Site Forums The Shop Area Engine Shop Thrown Bearing problem.

Short version is I had trouble turning my Chevy 1957 235 engine over by hand (it's never been started because the starter couldnt even turn it). I pulled the crank and had it turned 10 under. Installed the new 10 under bearings, plastigage checked out, and I started slowly torquing each cap to find the problem.

Turns out it was the 3rd rod. But, it just got really tight, and I have a bar I welded that allows me to turn my harmonic with quite a lot of torque. So I turned it, thinking that would free it up. Stayed very tight, but spun. so I took the cap off the 3rd rod, and I had spun one of the bearings. Completely shorn the tab off, and it had a pretty good nick down the center where it touches the crank. I'm hoping that a flake of metal just got in and caused the problem. I don't want to pull everything to get that rod checked at a machine shop.

Anyone ran into a similar problem and know what could have caused it?

And better yet, I've heard a lot of guys will just throw a standard bearing in there, mixed with the other 10 under that's still good. Some guys say it's a big no no, some guys say it doesn't really hurt. I was thinking I could just torque it a tiny bit more than the required 35-ft-lb to offset it. Of even find something to shim it with.

Any definitive opinions or experiences you guys can offer?

Also: I just pulled a 10 under from one of the other rod caps, and put it on the piston side of the 3rd rod (the bearing that got thrown). Torqued it, and it won't spin at all, so somethings off. Tried a standard / 10 under mix on the 3rd and the cranks spins freely with about the tightness you'd expect turning it by hand.

Did you have the rods reconditioned when you had the crank reground? There's no point in trying to match up a nice freshly-ground crankshaft with rods that are egg-shaped. The rod bearing will assume whatever shape the big end of the rod has when it's torqued down. Grinding the crank was only half the job- - - -now tear everything back down and do it right. This time, either Plastigauge everything or use an inside micrometer or dial bore gauge on the rods with the bearings in them before you assemble the engine.

I check every bearing on our race engines with a dial bore gauge before they get assembled over the crank, both rods and mains. That's the only way to be sure they're round and straight, and have the proper oil clearance. Plastigauge is OK for street engines, but not round track stuff.
Jerry

Thanks Jerry, that's what I've been reading about just now. Luckily this is just a street engine, but I'd still like to do it right. My only problem is the only local shop that does Crank and Rod clearances and grinding is a helluva headache. But you're right, it's definitely the rod being out of round, after I checked it a few different ways.

Pretty sure I even know what caused it. My father misread the manual when we first assembled it, and torqued that rod to 100 ft -lb instead of 35-45. I'm guessing that might have did it.

I also did some research on mixing bearings, and a lot of hot rodders will do it to get their clearance down to splitting by .0005. Pretty impressive stuff.

Thanks for the info, I'll check it with a micrometer and see just how bad it is.

I have measured individual bearing shells by the dozen, and matched up the ones that give me an exact oil clearance, but I've NEVER used two mismatched undersizes!

If you know you've got a questionable rod, just swap it out. Most decent parts stores should be able to order one reconditioned rod for you.
Jerry

Thats one tough rod bolt, to withstand 100 ft. lbs. of torque and not break!

I'm wondering if a rod cap has been swapped with the wrong rod?

A cap on the wrong way could act like that.

I know, right? One of them did get stripped though. Pretty stupid on my part, I had no idea what I was doing at the time.



Unfortunately, I etched numbers and directions on each rod and cap before I ever tore it down so I wouldn't get it mixed up. I wish it had been that simple.

At the moment I have a .001 undersize and a standard in the number 3 rod. I checked all my other rods, and their clearance is sitting right around .0015 (within the range of .001-.003). The number 3 rod is at less than .001 with two undersizes which explains why it's too tight but it goes right into the .0015 range if I use the standard + .001 mix. I'm worried about it, but because the tolerances are so close, I think it'll be alright.

I know it's unorthodox, but I think I'll try it like this. If it starts knocking, I'll know I made the wrong decision.

Here's an article that details mixing .001s and standards, which makes me think it's ok: http://www.carcraft.com/techarticle...e_engines_bearing_clearance/viewall.html

Gonna try starting it in the next two days, wish me luck!

Wait a minute- - - - -you said in your original post that the crankshaft has been reground to .010" undersize. What in the name of sense are you doing using std/.001" bearing shells on a crank that's supposed to be .010" undersize? Have you used a micrometer, or even a dial caliper on the shaft to see what the diameter actually is? Either the machine shop forgot to grind one journal, or you've got a rod that's so egg-shaped a hen would be proud of it! Please find out what's actually going on before you totally screw up a good engine. I see visions of a rod sticking out the side of the block unless you do some detective work right now!
Jerry

I agree with Jerry. .010 ain't even in the ball park with .001 and standard.

Sounds like one journal didn't get turned.

I would have the machine shop that turned the crank double check it.

And I never mix sizes on bearings.

Maybe the problem isn't the rod after all. I would sure take a closer look at what that crank really measures. If one is not turned, what shape would the others be?

I agree with Jerry and Roy. No one is perfect and if the machine shop didn't check behind themselves you could have more trouble than you want to deal with. Larry

Actually, this is my fault in semantics and terminology, I mis-spoke. I originally typed .010 because that's what I was told. But then I read somewhere that when cranks are ground, the bearings for mains are .010 and the rods are technically .001 even though they label them as 10's. Relooking at it I'm guessing that information was false, and they're all just .010's.

But for the record, the crank was turned right, and the bearings are the right size. It's just that rod that's out of round. But with the bearings mixed on rod #3, it puts my clearance at .015 for every rod. It's risky, but a lot of the guys I've talked to said they've done it and had no problems. The clearances check out and the engine turns by wrench at about the torque I'd expect.

Be sure and get a commitment from the friends who are saying the bearing fit is alright.So they can help you with the work and replacement parts when it fails.
I think it won't run until the "water gets hot" with an out-of-round rod bore.Swapping bearing shells until you get enough clearance to get it started is a ticket to disaster and you will be out the cost and work of bearings and crank grind--maybe the rod won't break and come out the side.
The only acceptable shaft journal is round, straight, smooth and sized for proper oil clearance.
The bearing shells will take the shape of the hole (rod) they are clamped into.
You should measure the rod bore without bearing to evaluate it and if out of specs, change it or resize it.Also when a bearing has spun in the rod the important smoothness is gone.
Make it right now or you will regret it quickly.
Oh yes, you did mean the "clearance for every rod" was .0015 "fifteen ten-thousandths" didn't you and not .015 (fifteen thousandths".That one little zero makes a lot of difference.

Guys, we've done our best to prevent a disaster. It's beginning to look like we're wasting our time. Walmart has BIG plastic tubs to pick up the pieces of that engine about two blocks down the road from the beginning of the first test drive.

"Some people learn by listening to other people, some learn by watching other people. A few stubborn ones just have to learn for themselves what happens when they pee on an electric fence!"

Good luck!
Jerry

Yeah, definitely replaced it because it got spun. No way I'm running it without that key holding it in place. And yeah, I checked it for .0015. .015 would be so far out of clearance I'd just replace the whole rod!



That's sort of rude, but I guess thanks anyways. I took your advice, I checked it with a micrometer instead of plastigage, and it checked out. It's one thing to say "I've tried that and it won't work" versus "I've never tried that, but I don't think it will work". Won't know until I try it. And of course I'm stubborn! I don't think anyone would survive a frame off restoration if they weren't a bit stubborn!

Thanks for the input fellas.

Are you saying you are going to try using different size bearings on the one rod?

I hope not.

If the crank is turned to .010 then it shouldn't require mismatching bearings to try and make them fit.

That's a lot of expense and work to go through and have that rod toss through the side of the block.

If that one rod locks down when you use the correct bearing then either the rod is out of round, or the crank is out of round or not turned to the correct dimensions.

Either problem needs to be corrected, not putting a band aid on and hope it holds.

There are some things you don't have to try to know they are not a good idea. Mismatched bearings on a crank is a disaster waiting to happen.

The "key" on the bearing shell is not all that keeps the shell in place in the rod.
A properly sized/finished rod big end bore is smaller than the OD of the bearing shells resulting in "crush" of the shells when the cap is installed and tightened--this also REDUCES the bearing ID to the proper size for the selected journal size. When crush is reduced the bearing/journal fit is made looser-also the shell is not secure in the bore and will shear the key and spin the shell destroying the journal because the oil supply is cut off.
A bearing that has spun in the bore destroys the dimension as well as the smoothness of the bore, and since the soft part of the shell takes the shape of the steel back and supporting bore,now the part contacting the journal is not uniform.
Even small particles trapped behind the shell at assembly cause a high spot in the soft layer contacting the journal and can cause premature failure.
The size/finish of the rod big end bore is CRITICAL to bearing life.
Question : why are you so opposed to pulling the head/piston/rod to make this one right?

"Rude"? I prefer to call my comments straight to the point. You're headed for a wrecked engine unless you correct the obviously wrong fit on that connecting rod. You have the advantage of drawing on several hundred years' worth of engine building experience by people on this site who have taken their time to give you good advice, and you spend your time arguing about why your way of cobbling up the situation is better. Go ahead and do it your way. The engine will fail, and then maybe you will be more willing to accept good advice.

When I ran the connecting rod reconditioning department at a big automotive machine shop in Nashville the maximum out-of round we would accept on the big end of a rod was .0002". That's 2/10,000 of an inch. The diameter also had to be right to the nearest .0005" and we usually held it to much closer tolerances. Conservatively, I'd estimate I have reconditioned at least 10,000 connecting rods.

Maybe your way of dealing with your problem is better than mine. I remain unconvinced.
Jerry

That's a good explanation of it, thank you. The answer to your question is pretty much the only reason I'm even trying this. I've had to take the engine apart so many times, and I'm on a deadline now. There is only one engine shop in town, and I had a lot of trouble with them turning my crank. They kept telling me it would be ready 'tomorrow' and every time it wasn't. I even went on a week long trip to NYC and when I came back it still wasn't done. All in all it took them three weeks to grind it. And since they're the only place in town that does it, I just can't wait that long to have them do what might end up being sub-par work. Plus I'd have to wait for gaskets to be delivered on that head gasket. So I guess that's the main reason why. To have it done right, we're talking weeks of time and possibly sending it out of town to another machine shop. But that may be what I have to do.



Jerry: Hey, sorry if I came off as arguing. The advice you guys have given has been great, and correct. I never disputed that fact, I just wanted to make sure I got all the info I could to make sure I was getting the whole picture since I don't know what I'm doing. I'm still unconvinced my way will work at this point, but because I have to get this truck rebuilt and off this property in such a small time frame, it's putting me between a rock and a hard place. I'm gonna stop and take a thinker, and maybe do some more research before I do anything drastic. Maybe I'll get lucky and find another machine shop in the area that isn't listed.