Trying to keep the story short, a few months ago with about 700 miles on my built engine, I did an oil change after running the car hard to get some logs and found an oil squirter tube in the pan which I posted a thread about. I had also noticed the engine had a buzz around 3000rpm that day.
So I left the car in my shop while I was out of state for 2 months. After dropping the pan to replace the oil squirter, I decided to removed the main girdle to make everything easier. All the bearings looked good but noticed the center bearing was still on the crank. I took some rocking from side to side to break it loose but here's what I found.







The top and bottom bearings are different brands as the ACL bottom bearing had a scratch all the way across so I used a good, unused Cosworth bearing in its place. So the bottom Cosworth bearing has no wear at all, only a layer of copper on both ends and lost its shape as it didn't sit correctly in the girdle due to the smaller diameter. I'm not sure if the bearing moved when I assembled the girdle to the block or what but all the other bearings including the rod bearings look good.

 

I'm assuming you are running a 100mm crank in a stroker or 4G64 block?

Not enough oil clearance or revving in excess of 9000rpm will overheat the center main on the OEM 100mm crank.

What main bearing oil clearances were you using and how high were you revving it? Also, what oil brand/weight were you running?

 

It's a 100mm crank in a 63 block. Main clearances were .0025 and was never revved past 6800rpm. I think the big mistake was running BP 10W-40 but I already had a case of BP 20W-50 to use but it's too late.

I ordered new ACL mains and thrust washers yesterday to install and since the girdle will have the bearings face up during install this time, they won't come loose.

 

What was your oil pressure?

 

I would think you would be taking this apart to be machined? Sounds like you are going to try bearings first? If the bearing is out of round and it sure looks like it the crankk is scored and needs to be at the very least polished and more likely turned.

 

can you take a bigger pic of the bearing with a macro setting? looks interesting enough.