I am trying to negotiate up the price of a trade-in BMW 318iSE E46 model which has done 78000 miles and today the salesman set his line in the sand price by saying that "he could only go £50 higher because of its high mileage, according to the mechanics, it will need a major service soon and a head change as it is well known than BMWs have porous heads".
Has anyone else heard this ridiculous assertion? I know its a load of old tosh since BMW engines can do more than 200K with no bother but I just thought I'd run it past everyone just for a laugh and to see if there was any tiny incling of truth to it.
Perhaps the salesman thinks I have a "porous head".
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To quote from the Car By Car breakdown for the E46
"Fairly close to perfection....... All have steel bore liners eliminating problems of previous Nickasil-lined all-alloy blocks."
Just point out to the poor overworked chap that he's clearly confused his E36 with his E46 and as he has such trouble with numbers, perhaps he should have another look at the ones he wrote down for the P-Ex.
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My '98 M51 head needs skimming (when I have a free week) the place I'll take it to told me that they may not be able to do it if it's porous, and that they're notorious for going pourous. This was the company:
www.tandlengineering.co.uk/
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Yes, they have had problems with porous head castings. This is quite different to the Nikasil issue.
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Oh dear, so what would make metal porous? Corrosion? I don't really see how it can go porous!
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BMW heads have been known to crack but I've never come across any mention of porosity before. I agree with ST that the metal shouldn't "go porous".
Cast auminium often suffers from porosity but I would have expected BMW to pressure test for this. I suppose there could be internal porosity which only shows itself when corrosion takes place later in the car's life, perhaps due to inadequate corrosion inhibiting.
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I would assume it is a casting problem or perhaps impurities in the alloy used. Citroen had a similar problem with the diesel engine fitted to the CX.
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Aluminium is a very difficult material to work with; its has a low melting point and cools quickly, a high viscosity when liquid (flows poorly) and generally difficult metallurgy. For these reasons 'aluminium' cylinder heads are actually made of quite complex alloys. Getting the composition of the alloy correct is not simple, that's why there have been so many problems over the years with alloy heads.
The 'pores' come from included gas (usually given off by the casting mold material, and also impurities in the metal).
BMW have long had a problem with porous heads and still seem to today (interesting they also have the same problems with their motorcycles engines - e.g. the F650 is infamous for porous castings).
The M30 engine heads were notorious for being almost impossible to skim; if you do a skim you run into the pores and the head is scrap.
Obviously the heads are pressure tested at the factory, but with use the included pores can 'open' due to thermal cycling of the head (constant expansion and contraction and shearing against the head gasket). I have seen even minor cracks appearing between included pores (due to stresses in the casting).
Obviously the problem will be worse is corrosion is added to the equation.
In short, it is a problem and the salesman at the dealership was not talking rubbish.
A lot of other manufacturers have also had problems with porous heads and some have even had porous blocks (eg some Peugeots a few years back had water ingress into the cyclinders through pores).
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Aprilia, that's a useful and interesting answer.
I agree about the difficulties of selecting an appropriate alloy, having been involved in the material specification of engine components. We didn't use aluminium for our cylinder heads but it was specified for quite a lot of other components. One of the problems was that LM24 or LM27 alloy machined quite well but was poor on corrosion resistance, whereas LM6 resisted corrosion but was relatively difficult to machine.
We also needed to be aware of porosity and I was once caught out on this. A gravity-casting supplier was fitting four water outlet elbows in a mould, flange up, and asked if he could place them differently so as to get six in the mould. I was party to the decision to allow this. What we failed to realise was that, with four in a mould, porosity in the upper (flanged) surface would be machined off, whereas with the elbows a different way up in order to get six in the mould, the porosity was in an unmachined part and this caused embarrassing coolant seepage problems. That must have been getting on for 30 years ago but was a lesson not to be forgotten!
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Marty,
Are you sure he was not referring to the drivers heads?
Matt35.
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Marty,
Oops - just reread your post in full - I'll help NoDosh with his coat!
Matt35.
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