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Re: only 76K miles (archive)

[ Follow Ups ] [ 5-series (E34) Message Board ] [ Msg. Board FAQ ]

Posted by MJKhan on April 09, 1999 at 16:15:41:

In Reply to: Re: No history of over heating???? posted by EM on April 09, 1999 at 16:12:29:

: Sometimes they just go. Mileage usually does play a part in that low milage cars don't usually pop the gasket. You didn't mention your mileage.

: Failure is really related to the "marriage" between the head and the block and the factors that influence that fit.

: Generally, here's some factors that influence failure...

: Thermal expansion. If you have an alloy head and a cast iron block, the two metals have different thermal expansion rates. i.e., the metals expand at different rates. This causes the two parts to move slightly in ralation to one another each time the motor changes temperature.

: Head bolts. Again, a thermal expansion issue influenced by whether or not the bolts are torqued properly to begin with and how well they hold the head in place in relation to the block.

: The head gasket material. Its ability (or lack thereof over time) to compensate for the different rates of thermal expansion.

: Heat cycles. The more times you get the motor to operating temperature, the more times these components have to work together to compensate for the differences.

: Eventually something wears out.

: Pop, blown head gasket.

: The reason I asked about whether there was any overheating in the car's history is because overheating puts *extreme* pressure on these components. And, as in any "chain", the weak link goes first -- usually the head gasket. While that sounds unfortunate, it is usually the least expensive to replace. You don't want to have to replace a cracked head or cracked block if you can help it. They're relatively expensive compared to the head gasket.

: comprende? Good luck.

: EM

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