Alloy porosity

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?
--
Bill

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I have found them to leak and the seal goes at the rim ......
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On 01/10/2017 19:23, Retro Futurist ... wrote:

That's just happened to me, 57 plate Zafira with 61k miles. NSR very slow leak, taking a couple of weeks to lose more than a few psi. Fixed at the local tyre place by cleaning the rim and applying fresh sealer. Two months ago; so far, so good.
--
Peter

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On 01/10/2017 19:33, Ramsman wrote:

The annoying thing is that you can't fit an inner tube to the things.
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On 01/10/2017 20:22, Ade wrote:

Why not?
--
Michael Chare

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Michael Chare wrote:
[...]

You can't use inner tubes with most tyres now, regardless of wheel type.
The inside of the tyre is not smooth enough, plus the tube shape isn't designed for the low profiles currently in vogue.
Chris
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On 01/10/2017 19:33, Ramsman wrote:

+1. I have a very good local tyre place and he knew by the wheel (2007 Audi) that the inner rim had likely corroded slightly and leaked. Cleaned it 6 months ago and it's needed no air in that time.
--
Cheers, Rob

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On Sunday, 1 October 2017 19:23:23 UTC+1, Retro Futurist ... wrote:

I had mine resealed they just spun them on a jig then applied some magic to the milled surface and they were excellent afterwards. 20 quid.
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They seem to use some form of sealer these days. I was having two new tyres fitted to the old car when the fitter took a phone call after applying it to one wheel. And was on the phone for ages. He didn't apply more, and that tyre leaked. The other OK. Had it re-done and it was fine.
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On 01/10/2017 19:06, Bill wrote:

Paint them well before the tyres are fitted, they will probably be fine. Testing would cost more than a new wheel.
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When you say paint, what with and inside or outside the wheel? Or is this referring to painting the edges, or more, with sealant.
Sorry if this is a stupid question, but I'm just learning how little I know about wheels and tyres.
--
Bill

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Bill laid this down on his screen :

No point to adding it to the outside, it needs to be inside. The problem with alloy rims, is that the salt and water corrodes the alloy, causing a poor seal to develop between tyre and wheel rim. The corrosion needs to be cleaned off down to bare metal, then the bare metal painted to protect it.
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On 01/10/2017 20:15, Harry Bloomfield wrote:

There are plenty of places that will refurbish wheels to better than new, but it is not cheap and takes a few days or so. One of my customers had his Lexus wheels refurbed because they leaked extensively and they were fine afterwards, iirc it was about 200 for all five.
For my wheels I strip the tyre, clean the inner rim up, and use a belt sander on the important seal area around the edge, then spray them with etch primer, then some silver wheel paint, works fine, even if you miss out the etch primer.
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On 01/10/2017 20:37, MrCheerful wrote:

Mine were done by Toyota on a recall when the car was a year or so old. They lasted over 10 years, and then started to leak on the rims. I had them redone, and they are bubbling in places only 4 years later.
Andy
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On 01/10/2017 20:37, MrCheerful wrote:

+1
When I was at university, my father used to edit the magazine "Paint Technology". I used to write all my exam revision notes on his galley proofs (fan-fold paper, about 5 inches wide, very convenient for summarising key points). Towards the end of my professional career, I found myself approving contracts placed with the Paint Research Association at Teddington, which had been one of his very regular stamping grounds.
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On 01/10/2017 20:37, MrCheerful wrote:

Can you see the corrosion if you take the tyre off. i.e. is it obvious to repair.
--
Michael Chare

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On 08/10/2017 20:33, Michael Chare wrote:

yes, but I would usually put detergent around the tyre first to confirm the leaking side/area.
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On Sunday, 1 October 2017 20:15:20 UTC+1, Harry Bloomfield wrote:

I doubt that is the problem as the tryes need to be leaking to allow ingress.
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On 01-Oct-17 7:06 PM, Bill wrote:

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.
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On 01/10/2017 19:06, Bill wrote:

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.
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