On 01/10/2017 19:06, Bill wrote:
I may be about to buy, from a scrapyard, a set of 5 replacement wheels
for the recently purchased car to get a spare wheel, proper sized tyres
and wheels that fit the wheel nuts.

In the past, I've often been told that the reason for a slow puncture
was that the alloy wheels had gone porous.
More recently, I've been told that the cylinder head on a certain
vehicle had gone porous and that was why it chuffed when cold.

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.

Does anyone know? Is there any sort of standard test, like wheel
tapping, that can detect this sort of rot?

I think the cylinder problem and the wheel problem are different. Alloy
wheels are normally varnished or painted to prevent corrosion. But
flexing of the tyre against the seat area can remove the coating. The
crevice between the tyre and the rim is then more liable to corrosion
from water and, especially, salt during the winter. The alloy becomes
pitted, leaving a leak path. The greasy lubricant used when new tyres
are fitted can help prevent this, but once you have pits the only real
solution is to to remove them by abrasion, and to "paint" the metal
surface to prevent further corrosion before the tyre is re-fitted.
Decent tyre suppliers should be able to sort this out.