[FC] Theory Behind Main Bearing Thump

[Rad Davis]

If you mean the gentle once/rev thump at low RPM under no load with a warm 
engine, I think GM decided that it was no big deal, except that it bothered 
people, and came up with the "out of line" bearing package to make it go 
away.  This is discussed in SAE Paper 140C if I remember right.

That said, periodic thumps aren't usually a good thing.  Another question 
worth asking:  Are you sure it's a main?  If the drive end cam bores are 
worn enough, the cam will thump back and forth in the bore as valve events 
occur.  If you're at this state, the next step is a stripped cam gear and 
bent valves.

I'm also assuming that you've got good oil pressure at idle and the like...

--Rad Davis



At 08:07 AM 10/9/2005 -0400, you wrote:
>What is the current theory behind a Main Bearing Thump and does it do any 
>damage?
>
>Norm Helmkay
>
>
>
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