On 03-Oct-17 11:36 PM, Dave Plowman (News) wrote:
In article ,
Vir Campestris wrote:
On 02/10/2017 11:15, Dave Plowman (News) wrote:
In article ,
Bill wrote:
I have always wondered whether this is a standard get-rid-of-him phrase
taught as part of the Car Mechanics PhD (Hons) course, or whether alloy
does actually start to leak.

Just the usual meja ********. If alloy really did go porous, your engine
would soon be scrap. Given pistons are made of alloy and have been for
many a year as is much else on a modern engine. And the pressure inside a
cylinder is far higher than in a tyre.

If my pistons lose 0.1% of the gas on every stroke I won't notice. They
are typically running about 25 power strokes a second. If my tyre lost
0.1% of its contents every 25th of a second I think I'd notice...

How about the cooling system often made of alloy these days. If that were
porous, it wouldn't hold pressure.


The tubes of an alloy radiator are wrought and not cast. The process
that makes the tubes compacts the alloy, distorts the crystal grain
structure and removes (most) porosity.

Casting process can entrain gas and that results porosity. So cast
blocks and heads can be porous but drawn tubes and rolled sheet won't be
porous. In part the thicker walls of castings prevent porosity as all
the holes have to line up and join up for it to leak.

Aero grade alloys for large cast structures (jet engine intercase) are
vacuum melted to remove gas and vacuum cast. They can still suffer from
strain induced porosity, where voids open up between the crystal grain.
Unlike a crack that will go though a grain.