Some time past in a moment of perceived lucidity, I thought I read something that recommended drilling out the intake manifold bolt holes by .100" to improve the ability of the intake and exhaust manifolds to slide relative to one another as the engine heats and cools.

Dan says he has never heard of this and due to my advance chronological state, I can't remember where I read it.

Anyone have a theory on this

Why would you need to do that? Though the materials may vary slightly, they should all expand at *about* the same rate. Therefore the stock mounting holes should increase in size as the bolts increase in diameter, etc. The steel will heat up pretty consistantly, thus everything expanding and contracting in unison.

This is a very carefully calculated number when engineering anything that will see temperature extremes (bridges, buildings, etc).

Ok Ill bite.. And if your intake is Aluminium and cast iron exhaust?
Frank

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The factory used aluminum intakes w/iron exhausts in '60, '76, and '79-'84 without the holes needing to be enlarged. *shrug*

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Too many people who were born on third base actually believe they've hit a triple.

For example, the coefficients of linear thermal expansion for different metals:

All numbers in (in/in *F x 10^-6) where * is degree and ^ is to the power of
Aluminum - 12.3
Steel - 7.3
Cast Iron - 5.9

As you can see, cast iron shows the least expansion and aluminum the most. If you have a cast iron head, steel bolts, and an aluminum intake, the holes in the aluminum will expand faster than the bolts and the head, the bolts will expand only minutely faster than the holes drilled into the head. However, it is such a minor difference between the steel bolt and the iron head that there would have to be extreme tolerances in order to cause any type of problem.

The components in question do not see the same temperatures or heat transfer, nor do they have consistent temperatures throughout themselves. It would take quite a computer simulation to accurately predict dimensional changes in the various run cycles.

Chrysler engineers did a good job of allowing for differential expansion on the longer parts. That's why installing the manifolds should be done correctly. See Doug and Dan's article:

http://www.slantsix.org/articles/manifo ... unting.htm

Intakes are cooled in the plenum and ports by the air/fuel mixture passing through, and are heated by the exhaust gases hitting the base (stock slant manifolds and Offys, IIRC), by radiation from the exhaust manifolds, and by some conduction through the gaskets.

Exhaust manifolds are the hottest, of course, with convection from the exhaust gases.

Heads see combustion heat in the chamber and exhaust ports, cooling from the air/fuel mix, heat transfer to the coolant and oil ... A pretty complicated thermodynamic situation.

Thermographs: White is hottest, black is coldest

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