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Cylinder head and corrosion

Author: Mike Sharp

The photo below shows the cylinder head from a box saloon. The water outlet manifold has been removed along with the securing bolt and washer. There is obvious evidence of corrosion, particularly to the securing bolt and the aluminium manifold itself.

The water manifold of the later Ruby is different in having two securing bolts, but it is also made of aluminium (or an aluminium alloy?) Corrosion is inevitable with dissimilar metals connected together in an aqueous environment ie the aluminium of the manifold bolted to the cast iron of the cylinder head.

Electrolytic corrosion is accelerated by the high temperature of the thermo-siphoning cooling water, as it enters the radiator header tank. I suspect that many of us, because we use our A7s mainly in the summer, do not add antifreeze. Antifreeze, of course also contains benzoate and other corrosion inhibitors, but there's no possibility of a protective effect when just using plain water.

Another surprising feature, clearly shown in the photograph, is the large amount of debris sitting in the waterway. Much of this can be removed with a strong vacuum cleaner plus nozzle, and wiggling a wire. The purist would remove the head to extract the crud and perhaps clean the waterways with a de-scaler: phosphoric acid or something similar.

The head, as new, has a crosspiece with a threaded hole. The aluminium manifold was then held in place using a set bolt, washer and fibre washer, screwed into this crosspiece. I expect one also liberally applied a sealant to the gasket, too.

But the threaded crosspiece, after many decades, becomes so corroded as to not hold the manifold tightly, and the thread may simply strip. I know that stepped bolts are available from A7 component suppliers, and I presume these are meant to be used with an enlarged and re-threaded hole in the crosspiece. But if the crosspiece has been significantly thinned by corrosion I would prefer not to remove still more metal from it, in case the whole thing collapses.

I prefer, as you can see if you look carefully at the photo, to slide a nut beneath the crosspiece, and then attach a threaded bar which passes through the manifold piece, and is secured by a nut and washer on top.

You can see the extreme corrosion to the lower nut and the bottom of the threaded rod. The rod, incidentally, was just a suitable bolt with the head cut off. Component suppliers also supply stainless steel set-bolts which would be expected to resist corrosion, but if you use my method the bolts provided might not be long enough, and should also need their head removing - locating a headed bolt in the internal nut, when the manifold is in position, and the sealant is setting, could be a nightmare. And one would be left wondering whether the bold had adequately entered the now invisible nut!

I don't use a gasket between the manifold and head. The face of the head may well be pitted and would need lashings of gasket goo. Neither is it essential that the portion of bolt passing through the manifold piece is plain ie not threaded. Simply use that wonderful stuff…silicone sealant. It will fill all the irregularities, and is a strong adhesive as well as a sealant! The lower half of the bolt, immersed in the hot cooling water, can corrode away, but the silicone will still hold the assembly in position!

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Cylinder head and corrosion

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