Just a note from a thread on Seloc, an unprompted comment…
Dave,
Interested at your suggestion that the head has annealed Dave. Do you mean that the aluminium is softer now than it once was, or has it just deformed because elevated temperatures have reduced the yield strength of the material.
As far as I am aware, the Al that is used on cylinder heads isn’t heat treated in any way, so I can’t quite understand why it would ‘go soft’. If anything, I would have expected the area at the gasket contact to be harder than elsewhere.
When I was working on the EU-4 K-series upgrade we needed to do a far bit of testing at ‘top-limit’ CR, which meant that test engines required skimming. One of the problems we often encountered was micro-porosity just below the original head surface, especially on the VVC heads . It seemed that the deeper you skimmed the head, the more likely you had the problem causing me to suspect that the aluminium is more like a sponge under the surface. This might be caused by the casting freezing in a classic dendritic alloy fashion, where the porosity rate increases as the casting cools and progressively freezes towards the casting section centroids.
One failure mode we saw more than once on our re-worked heads was the porosity collapsing locally at the fire-ring and causing seal failure . Strangely enough, this never seemed to be a big problem on the directly ex-factory engines - only our top-limit CR engines.
From this, I suspect (postulate?) that what actually has happened here is the metal temperature has got hot enough for the metal to yield and collapse/compact the porous metal under the surface. (Could be a bollox theory though…)
I wonder if you could skim that head (assuming you still had enough clearance volume) that you wouldn’t have a more reliable cylinder head since now it’s been beaten into proper shape. Has anyone tried to salvage a failed head like this before, with any success?
Andrew
My response
Andy,
The head is quenched out of the mold to harden it, the hardening isnt very and on some it is removed with skimming. I have performed hardness tests on 100s of K series heads, with the equipment I use the average hardness is around 60 (unknown units). On a head like Robs, on the exhaust side it be 20-25, in between cylinders it can be as low as 15-20, on the cool side of the head it will be in the 50s with 40-45 between the bores.
Elise_s1s advice is spot on.
Even of you skim the head it will simply recur and much more quickly, you cant save it with a shim either, the material is way too soft and it will just dig in. It occurs when the engine is turned off, the heat from the exhaust valves and exhaust manifold soak back into the head, there is a low volume of stationary coolant around this area of the head and the heat causes local boiling and a very high heat gradient across the head. Letting the engine idle for a short while before turning off will soak more of the excess heat into the coolant where it belongs. A post turn off pump would also allow the coolant to circulate after the engine is turned off, effectively reducing the heat gradient, this is a common addition on some of the ‘warmer’ 16v engined cars.
The sort of compression you describe does happen but in a much more limited and patchy manner. Most heads I remove had sporadic pot hole type indentations immediately under the fire-ring where the material has collapsed into small underlying vacuoles. In most cases this isn’t significant enough to cause a seal failure but is very noticeable. If the head is linshed to remove these, then they will likely not re-appear since the compression of the underlying material is already done, if the head is skimmed more than a few thou they will generally re-appear.
The porosity problem on the fire ring I have encountered myself, it is a different mode of failure, the head retains its hardness but just collapses under the fire ring, it is very localised. The sort of damage Rob has is very common and happens over time, with a porosity collapse its pretty instantaneous failure,
Dave
What more can I say?
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