main bearing issue

General engine tech -- Drag Racing to Circle Track

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bullheaded
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main bearing issue

Post by bullheaded » Mon Oct 23, 2017 12:21 pm

BBChevy , 454 +.030 stock cast crank , stock rods, srp pistons-small dome build sheet says 10.5:1 but I don't believe it, hydraulic flat tappet cam comp XE284, PBM aluminum heads, professional products single plane intake, 830 holley. Engine was in a '80 Malibu that a buddy bought his kid to start drag racing. Car and engine look like something that was built to drive to car shows and not spend a lot of time at the track. With some weight reduction and some gear they were able to get this thing to go 11.20-30. Ran it a whole season at the track then sold the engine to another friend that brought it to me to be freshened. I tore it apart and found the rear main bearing is in bad shape,down to the copper,lower is just straight copper upper has some babbit left along the front edge.. Number three has some scuffing but not bad. Others are fine other than trash going through them. Rear main now measures .007-.009 clearance but the crank looks good and is dead on size. Thrust is also not worn. Any ideas on what might have caused this?
The more I know,the harder it gets.

mag2555
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Re: main bearing issue

Post by mag2555 » Mon Oct 23, 2017 12:38 pm

If you pull the rod Bearings out are the transference of the numbers on the back of the Bearings clear in the caps and big ends!

If it's fuzzy or smeard then the motor was detonating.

Sweet P
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Re: main bearing issue

Post by Sweet P » Mon Oct 23, 2017 1:39 pm

In my experience, #5 problems that show up in that bearing are caused by whats behind it. As in clutch/flywheel/flexplate/converter/etc.

Make for darn sure this is all balanced correctly and parts have endplay after its all bolted together.

mark_follweiler
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Re: main bearing issue

Post by mark_follweiler » Mon Oct 23, 2017 2:22 pm

contamination.

pamotorman
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Re: main bearing issue

Post by pamotorman » Mon Oct 23, 2017 3:26 pm

the rear main gets oil first and if you have a oil filter bypass valve you get unfiltered oil till the filter by pass valve closes with equal pressure on each side of the disk. every engine I ever tore down that still had the filter by pass valve showed rear main bearing scuffing from debris

cjperformance
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Re: main bearing issue

Post by cjperformance » Mon Oct 23, 2017 8:52 pm

I have seen the thrust bearing on many occasions distorted/damaged due to someone belting a harmonic balancer on with a huge mallet! The thrust face that sees clutch or convertor thrust typically looks fine and its usually the front side of the main bearing that looks the worst from being deformed.
Craig.

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