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07-25-2007 #1Registered User
- Join Date
- Apr 2006
- Posts
- 17
406 SBC eating main bearings! Help!
I am at a loss. my fresh 406 ate the #4 and 5 main bearings out within an hour of on road runtime....twice now!
The first time, the motor was primed and started, timed etc as normal [solid roller so no cam break-in] run in the garage for half hour or so, oil and filter changed. maintaining 55-60lbs at idle. I went for a easy cruise to town and back, about 30 miles total. On the way back i leaned onit a bit, nothing major, half throttle shifting at 5k and the oil pressure drops. limp it home and dump the oil and its full of bearing material. Out comes the engine for teardown and inspection finding the 4 and 5 mains destroyed. Crank was not scored but copper was transfered on the journals. Checked the crank and measured 5 thou runout!! i could not find anything else wrong, all bearing clearances were checked via mic and bore gauge on assembly. 2 thou rods, 2.5 thou mains.
Engine machine shop was able to straighten the crank and polish the journals. I thouroughly cleaned all the parts and checked for restriction to the mains thinking it was a possible issue. everything seemed good.
I reassembled the engine with fresh bearings, the crank turned smooth in the block and everything went back together smoothly. I went through the startup procedure oil dump the same as above.
Same thing happened again! oil pressure dropped so i limped home and dumped the oil and cut the filter open. Sure enough bearing material was in there.
I have not torn it down yet but i can only assume the mains are out again.
I don't know were to go from here. I have built many SBC and this is a first for me.
Heres my combo
400 4 bolt, studded. fully machined including align honed.
solid roller cam
edelbrock victor jr heads
super victor intake
KB hyper pistons
eagle rods
eagle cast steel crank
fluid damper
fully balanced rotating assy with a balance plate instead of a 400 flywheel.
any ideas??? my first thought is the cast crank flexing under load, but im lost and tired of seeing the inside of this engine.
Please Help
Chris