Main bearing again?

[64dartgt]

My car came with its original 318.  After changing the carb it started but ran like crap.  I changed mechanics (back to my regular one) and had a new carb and manifold put on.  Now the car ran but had low oil pressure.  In an attempt to help that the mechanic put a quart of kerosene in the engine, ran it for a little while then changed the oil.  Still no good and the engine developed a knock that he said was a bad bearing.  I bought a 340, had it rebuilt and put in a couple of years ago.  It only has about 1000 miles on it since the rebuild and it seems to have developed same thudding knock (sounds like someone banging on the bottom of a five gallon bucket) at idle again after the oil warms up.  The engine was assembled with cam lube and run only with Valvoline VR1 racing oil.  The oil was changed recently by the mechanic, and to be frank it looked like he overfilled it a little.  He marked the stick when he put the oil in after the rebuild originally.

I am taking it to my mechanic next week, but am just heartbroken.  I assume that, worst case, the guy that did the original rebuild (big mopar guy with lots of high dollar cars of his own) can tear it down, rebuild just the lower end (pull the crank, clean it all up and put new bearings in as required) and reassemble it, but can someone confirm?  The top end was all redone and but the motor is 0.060 already and can't be bored.  I suppose he can hone the walls, but it really shouldn't need it unless metal went through from the bearings and screwed something up.

Thanks,

Bob

[StockMan]

Hi, what is your oil pressure now at idle (at operating temp)?  If the sound is coming from the bottom end theres really only a couple of things it can be.
How did the oil look when it was changed out the first time after the rebuild?

How did your oil pressure read after the rebuild.  If it was a bit high, the tolerances on the rod/main journals to bearing may have been to tight.
Because its likely going to need to come apart anyway, I would pull the pan and take a closer look.
If its a single rod bearing gone, check the tolerances on the rest of them with some plastic gauge, you should find around 2thou or so and it should be consistent.
Check the mains as well.  You never know, you might get lucky and find a loose rod bolt!

The bearings will tell you a lot about the conditions before the failure as well (bent rod/crank, oil starvation, etc).  Make sure to keep track of the bearings and their correct place.  These are the things you'll need to know in order to decide whether the engine has to come out or if you can just install some new bearings.  
However, theres usually a reason that they crap out though.

Was the engine running at a reasonable temp?  Any overheading during the first 1000 miles?
What was the compression ratio or the 340, did you hear any detonation while running it?  Any audible detonation could have resulted in the failure of a bearing as well/.
After the pan is off and the rod bearings off, you should be able to check for any wrist pin clearance as well.

Again, you'll learn more after pulling the pan.

[64dartgt]

The engine was never overheated.  The motor is a 71 and was rebuilt to stock, which should be about 10.5 to 1, depending on the gaskets.  Only ran super unleaded and never any detonation.  Runs perfectly except for the noise.  I only have the factory gauges which lack an oil pressure gauge (non rallye).

[StockMan]

The quickest way to determine if you have a rod or main bearing problem is to check your oil pressure.  I would purchase a cheap gauge you can hook up under the hood.  No oil pressure usually means bad bearing(s), or, a cap that's come loose.  If you've got good oil pressure then you might be looking at a bad wrist pin, considering that you've got a fairly substantial noise going on.  Let the motor run long enough to get a pressure reading.  If it jumps up to 45-55lbs at idle, cold, then your bearings are probably ok.  Either way you look at it the pan will need to come off so you can take a closer look.  The fact that the engine runs good is a good sign, meaning you should be spared the cost of rebuilding the upper portion of the engine.  Good luck!

[charger_fan_4ever]

For sure get a gauge on it and check the pressure before tearing into it.

[64dartgt]

Thank you for your responses.  Charger had to take a backseat for a few days due to rain and the fact that I picked up a nail in the one new tire on my pickup (picked up a nail last month at the edge of the tread).  Truck got 3 more new tires today and the fourth got plugged.