On 01-Oct-17 7:06 PM, 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?

Porosity is something that will have been there from manufacture. Over
time the pores can open up and then it leaks.

Cylinder head would have to be removed and pressure tested. Though just
running something like steel seal will cure most pinholes.

Wheels you will never find the pore, just paint the inside of the rim.
It seems that epoxy is not longer considered to be best but some sort of
acrylic.

Royal Enfield couldn't cast a non-porous crankcase with integral oil
tank for the 250 Crusader to save their lives. They all had yellow paint
on the inside to stop the oil leaking out.