I've a radiator beside no cap on the valve, so how do I know which one is lockshield and which the wheelhead? ...

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Answers:    Turn on the heating and test the temp of the both the pipes the hotter one is the wheelhead and the other is the lockshield.
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The wheelhead is on the feed side; the lockshield is on the return side. When the radiator is running, the nurture pipe should be a couple of degrees hotter than the return pipe.