Well after getting about 500 miles on my forged motor, I started having some serious vibration problems. I thought the tranny (M6) had a bad input shaft bearing, so I ended up doing an auto swap in the car, only to find out that the vibration was still there. I drove it another 50 miles or so and I lost oil pressure. It has sat in the garage for the last 2 months, and I finally got to ripping it apart today.

It looks like something went seriously wrong with my 4 bolt conversion. I used SBC 2 bolt conversion nodular Iron 4 bolt caps, and had my machine shop builder drill and tap the holes, as well as register the caps and do an align bore of the main caps. All caps were labled as to their location. All 3 of the 4 bolt main bearings showed evidence of spinning within the caps. They broke off the little tangs on the bearings and wore grooves into the caps mostly, the block didn't have a shoulder where the bearings were spinning like the caps did. Well all this metal floating in the oil eventually caused the oil pump to lock up which caused the oil pump drive to break off some teeth. The outer 2 main bearings look like crap as well. Some scars are evident on the crank, but can probably be machined out.

I asked when picking up the block if I was supposed to use standard bearings, and was told "Yes".

Is this block trashed, can you get bearings with thicker outer races? This SUX!!

 

Not sure on your question but I've felt your pain before and it sucks.

Good luck getting it back together.

Don't give up, nothing like hauling the mail in an LT-1

Marc

 

What were the main bearing oil clearances measured as and who put it together?

 

Main clearances were in spec, I put it together, the only thing that struck me as odd was how tight the caps sat in the registers. maybe the lack of clearance caused a problem when everything heated up?

Marc,

I guess the question I had is if the mains get aligned bored again to true up the main bearing OD, if the circumfrance is larger than a standard bearing is there something that can be done to take up the slack, or am I better off starting with a fresh block. I know that undersized bearings can be ordered if the crank is ground, but that is making the ID of the bearing smaller. Looks like I will need to make the OD of the bearing larger if I keep this block.

Bill

 

sorry to hear about that man. if you end up needing another block let me know i gota buddy with a few laying around.

 

Well then what did the guy who did the machining have to say about it? Sounds like he screwed up.

 

I feel your pain, when I trashed my motor this summer I had a main bearing problem too, trashed the block, but I think the crank can be fixed. Atleast you have all winter to get it fixed.

 

You said you have a forged crank but used "standard" bearings? Did you chamfer the bearings to clear the larger fillet on the crank? If not, that is probably what caused the bearings to spin. The little tangs are not designed to hold the bearing from spinning, they just locate the bearings on center.

 

I used Federal Mogul bearings for an LT1, got them from summit. I was using an Eagle 4340 crank 3.48 stroke. No I did not chamfer the bearings, I installed them as is. I was not aware that this was required, what is involved, so next time I can get it right

 

It is usually the worst on the rod journals, but it bears checking on the mains. If you look at the sides of each journal, where it mates with the throws, you will see a fillet radius, a nice little curve. With the crank out where you can get a good look, hold one half of the bearing on the journal, so you can see the mating surface between the crank and bearing. The part you are looking at is either edge of the bearing, where it meets the throw. If the bearing doesn't have enough chamfer/bevel, the edges dig into the fillet and it will spin.
I just use an old pocket knife to shave the bearing edges at a nice angle, trial and error till it clears. The race style bearings are narrower, and also have a larger bevel on them to clear it. But you still have to check them.
I don't know how well I am explaining this without visual aids, but once you look at it you should understand?

I would also torque the mains down and check the ID of the holes, without the bearings, to verify it was machined correctly. The machinist should have the numbers. If not I can get them from my guy. So far as the block, you said the caps had grooves worn, but not the block? Should be easy enough to fix. Take some off the base of the caps, then re-align hone it. Just make sure he sets up to cut on the caps and leave the block alone. You don't want to cut the block too much, or your timing chain starts getting sloppy, as well as moving all the pistons up the bore a few thou.

Also, look at the rod, on the bearing surface of the big end. Sometimes they will discolor if you have been getting detonation. That will also hammer the bearings.

 

If he plastic-gauged his mains before final assembly....wouldn't that show that the bearing isn't sitting properly? Also, if I had 10 bearing halves that were grinding against the crank, wouldn't you have felt it while rotating the crank? If you need a six-foot breaker bar to turn the crank while setting the valve lash, something is wrong. I have a 4340 crank and standard bearings....didn't have to chamfer the edge. I also bought all my stuff from the machine shop... he's usually one step ahead of my questions anyways. I guess it's a possiblity though.

 

Yes and no. Depends how close they were; metal expands as it gets warm. Just about every 4340 crank I have ever messed with required a larger than stock chamfer on the rod bearings, and some needed it on the mains. Not all of them. I usually run the Clevite race bearing, which is a little narrower, and has a larger chamfer.
And if you have a larger side clearance on the rods, it won't show up until the motor runs, if you are real close.

Just trying to point out things to look at; I find it very hard to diagnose stuff over the intraweb!

 

my 383 i have an eagle 4340 crank with clevite race bearings that come chamfered and they have a burnt look to them not silver like standard bearings i as well did the 4 bolt conversion and i have to be very carful with assembly my guess is the same the chamfer was not right and dug into the crank
sorry to here that man
hope u get it going again
the joy of racing is dumping tons of money into aftermarket parts and then 1 componet doesn't work well with the others and there goes your 6k
good luck